Life on Planet Earth

Intelligent Life

[Astrobiology] [Uniqueness of Earth] [Creation of Life]
[Plate Tectonics] [Unique Earth - Sun Relationship] [Moon and Jupiter]  
[Odds for Life] [Mankind]

Most thoughtful people consider the existence of life elsewhere in the universe obvious today.  The massive Search for Extraterrestrial Intelligence (SETI) project that looks for observable signs of life through the electromagnetic noise their civilization would admit.  It is thought by these scientists that most intelligent life forms would eventually discover radio and thereby emit beacons of electromagnetic radiation into space that inadvertently advertise their existence.  Thus far, no such signals have been found; however, it is assumed that we might not have been looking in the right spot or just may not have recognized this beacon.  We have become accustomed as a civilization to the concept of extraterrestrial life through countless television shows such as Star Trek, Lost in Space, and even blockbuster movies such as the Star Wars series.  Even though not a hint of the existence of any such civilization exists, we still are certain as a culture that such civilizations are “out there” because it seems like they must be.  Surely, since there are countless billions of galaxies each of which has billions of stars, the odds of intelligent life must be favorable if not overwhelmingly positive.  Recently, astronomers have begun to find other planets orbiting relatively near stars; never mind these extrasolar planets are vastly different from those present in our own solar system.  Surely, the presence of extrasolar planetary systems must indicate that planets similar to our earth must also be common.  Finally, the logic goes, if planets similar to our earth exist, then life – indeed, intelligent life – must also be common.

The bias toward intelligent life elsewhere in our galaxy comes from such considerations.  Carl Sagan and Frank Drake published a now-famous set of assumptions codified in the Drake equation.  This formula was based on educated guesses concerning the number of planets in the galaxy, the percentage of these planets that might harbor life, and the percentage of planets on which life not only could exist but which may have advanced to intelligence.  In 1974 using the best assumptions possible at that time, these two astronomers came to the startling conclusion that life should be common and widespread throughout our own galaxy.  Indeed, they estimated that a million civilizations may exist in our galaxy alone and since our galaxy is but one of billions of other galaxies in the universe, the number of intelligent species – even species vastly more intelligent that us – should be enormous.

The idea that intelligent life is common in our own galaxy has become a part of our Western culture that to think otherwise is almost to admit scientific ignorance.  However, the solution to the Drake equation includes many assumptions which were only very vaguely understood when the original equation was developed.   We now need need to be more carefully investigate the constituent components of the Drake equation using the greater understanding of planetary life that we now possess.  Additionally, there are both implicit and explicit assumptions contained with the Drake equation.  An explicit assumption might be the number of stars similar to our Sun which exist in the universe that might harbor a life-sustaining planet.  An implicit assumption would be that once life originated on a planet it evolves toward even higher complexity culminating in the development of animal intelligence and culture.  Most scientists assume that natural evolution  is the process by which life first appeared on this planet, and through which progressively more complicated life developed.  Life originated here about four billion years ago and then somehow evolved from single-celled organisms to multicellular creatures with tissues and organs culminating in animals of such complexity that they possessed intelligence.  But are the assumptions necessarily contained in this series of events valid; namely, is the progression toward increasing complexity an inevitable result of the development of life or even a common one.  Maybe the development of life - and certainly the development of intelligent life, a very rare event indeed.


[Planetary Habitable Zone] [Continuously Habitable Zone] 
[Temporal Habitable Zone] [Galactic Habitable Zone]

Astrobiology is the field of biology that encompasses life beyond our own planet.  This new science forces us to consider the presence of life on our own planet as but a single example as to how life might work rather than the only possible example.  Astrobiology forces us to consider entire planets as ecological systems, requiring an understanding of fossil history, planetary evolution, and geology.  New findings from diverse scientific disciplines are being brought to bear on the central question of astrobiology; namely, the likelihood of life elsewhere in the universe.  Much of the revitalization of astrobiology that life on Earth occurs in much more hostile environments than was previously thought.  The discovery that some microbes could live in searing temperatures and crushing pressures deep within the sea and deep beneath the surface of our planet led many to feel that life could similarly exist in the hostile environments elsewhere in the solar system.

However, just knowing that life can stand extreme environmental conditions is not enough to indicate that life actually exists elsewhere.  Not only must life be able to exist in those environments, but also it must have originated there.  Unless it can be shown that life can form in hostile environments there is little hope that life is widespread elsewhere.  But biologists counter that those organisms which do exist in harsh environments on Earth are genetically the most primitive forms of life indicating to some that life on Earth may have originated under conditions of great heat, pressure, and lack of oxygen.  No one has yet developed even a very simple life form from chemicals in the laboratory using the most sophisticated molecular biology techniques under very controlled conditions let alone explain how sophisticated life forms might develop under very harsh early Earth conditions.

The fossil record is also revealing.  This record indicates that life originated about as soon as environmental conditions allowed its survival.  Chemical traces in the most ancient rocks on the Earth’s surface give strong evidence that life was present nearly 4 billion years ago.  This seems to imply that life forms rather easily and that perhaps life may originate on any planet as soon as temperatures cool to the point where amino acids and proteins can form and adhere to one another.  Similarly, new ways of more accurately dating evolutionary advances recognized in the Earth’s record indicate that animal life – complex life – originated more suddenly than we previously had expected.  Thus, life does not progress toward complexity in a linear manner but rather in sudden jumps.  The formation of animal life is much more recent – and much more fragile – than the formation of early bacterial life.  In other words, attaining complex multicellular life forms is one thing – but maintaining these forms is quite another.  In the case of our own earth, there were countless planetary disasters that produce mass extinction events that destroy complex life while sparing the earlier simpler forms.  Astrobiologists propose that on other extrasolar planets, life may form – and perhaps even complex life in some cases – only to be wiped out by one of these countless planetary disasters that seem so common. 

We have only one example of a planetary system in which life has evolved; namely, our own.  Our planetary system is unique in many respects that seem to encourage the existence of life and promulgation of complex life forms.  The earth has been orbiting a star with a relatively constant energy output for billions of years.  While life may exist on other planets orbiting other stars, complex life requires benign conditions that must be stable for great lengths of time.  Animal life requires oxygen, but it took about 2 billion years for early plant life to produce enough oxygen to allow early animal life to exist.  Had the Sun’s energy output varied too much during that long period of development, there would have been little chance that we would be here today contemplating life elsewhere.  It is difficult, for example, for complex life to exist on planets in a multiple star system, or a variable star with its energy unstable energy output cooking or freezing any primitive life.  If life were to evolve on planets in such an unstable system, it would be difficult for it to survive for any appreciable time to permit evolution to more complex life forms.

Not only must the star around which a planet orbits be stable, the planet itself must be stable for eons of time.  An animal inhabited planet must be a suitable distance from its star to maintain water in a liquid state, surely a prerequisite for animal life as we know it.  Most planets are either too close or too far from their star to permit water to exist in a liquid form.  Another factor implicated in the emergence of complex life is our relatively low asteroid or comet impact rate.  The collision of asteroids and comets with a planet can cause mass extinctions as we have previously noted.  This impact rate is determined by the amount of material that is left over in a planetary system after formation of the planets.  The most left-over material, the greater the likelihood of such material crossing planetary orbits and eventually crashing into the planet with an unbelievable release of energy frequently sterilizing the planet of any early life forms.  The types of planets that exist in an early planetary system also determine the impact rate upon a life-harboring planet.  For Earth, there is evidence that the giant planet Jupiter acted as a comet catcher – a gravity sink sweeping the solar system of cosmic debris which otherwise might have impacted the earth with dire consequences for our own existence.  Jupiter may have reduced the rate of mass extinctions here and may be a prime reason why higher life was able to form on this planet and then maintain itself long enough for intelligence to become possible. 

The earth is the only planet (other than Pluto) with a moon of such appreciable size compared to the planet it orbits, and is the only planet with plate tectonics which causes the phenomenon of continental drift.   Both of these factors also may be crucial to the emergence of animal life. 

Even the position of the star within its galaxy probably plays a significant role in advanced life development.  In the dense, star packed interiors of galaxies, there are more frequent supernovae and close encounters with other stars that life once formed would frequently become extinct.  The outer regions of galaxies may have too little of the heavier elements required to build rocky planets and fuel the warmth of planetary interiors.  Furthermore, our sun and its planets move through the Milky Way galaxy largely within the plane of the galaxy as a whole with little movement through the spiral arms.  Even the mass of a particular galaxy might affect of odds of complex life evolving as galactic size might affect the odds of complex life developing as galactic size correlates with its metal content.  Some galaxies, therefore, might be far more amenable toward the development of life than would others.  Finally, our star – and our solar system in general – are unusual n their high metal content.  Perhaps even our own galaxy is unusual as well.

Ever since the days of Copernicus, the earth has been moved from a position of supreme dominance to its current position of being trivialized.  We have become an ordinary planet orbiting an ordinary star in an ordinary galaxy – a view formalized by the so-called Principle of Mediocrity.  However, it may very well be that the position of the earth may be much more unique than previously thought.  Indeed, we may be unique.  What if the Earth, with its cargo of advanced, intelligent animals (or so we like to think!) is virtually unique in this quadrant of the galaxy – the most diverse planet say, in the nearest 10,000 light years.  The Earth may be totally unique; the only planet with animals in this galaxy or even in the visible Universe carrying a load of animal life amid a sea of plants and microbes.  When conceptualized in this manner, the extinction of any animal species or plant on this planet brought about by the poor stewardship of man becomes an even greater loss to the Universe.

Planetary Habitable Zone

Most of the universe is clearly very inhospitable to life.  The vast reaches of empty space between galaxies and between stars, the interiors of stars, gas clouds, the surface of gaseous planets such as Jupiter – all of these locations must be lifeless.  While we cannot know for certain the absolute parameters within which life might exist, we must assume that it cannot exist under the most extreme parameters.  The Earth is seemingly an ideal distance from the sun to sustain and nourish life. The region or distance from the star whereby the formation of life is considered favorable is defined as the “habitable zone’ or HZ.  The concept of HZ has been widely accepted and the subject of several major scientific conferences including one held by Carl Sagan near the end of his career.

The HZ is the region where heating from the central star provides a planetary surface temperature at which water oceans neither freezes nor boils.  Our closest neighbors in the solar system provide mute testimony to what might happen to planets that are just outside of this HZ.  Venus is an example of a planet that gets too hot; if Venus ever had liquid water it has long since evaporated and has been lost into space.  Alternatively, Mars is frozen to many kilometers below its surface and are therefore both outside of the HZ for intelligent life to arise under current conditions.  Similarly, if the Earth were to become cooler either by the Sun decreasing its energy output or the Earth to move outward in its orbit, the planet would soon become ice-covered.  Indeed, the Earth has probably become ice covered in its current orbit in times past.  Eventually, carbon dioxide would freeze to form reflective clouds of “dry ice” particles that would cool the Earth even further.  Eventually, carbon dioxide would freeze on the polar caps.

Michael Hart, an astro-physicist, performed detailed calculations in 1978 to reach several stunning conclusions regarding the HZ.  It is known that the Sun is becoming slightly brighter over time.  About 4 billion years ago, the Sun was about 30% fainter than at present.  Naturally, as the Sun brightened, the HZ moved progressively outward.  Hart then calculated the small orbital zone wherein the Earth would remain within the HZ over the entire age of the solar system the “continuously habitable zone,” or CHZ.  His calculations indicated that the earth would have experienced runaway glaciation if it had formed 1% farther from the sun and would have experienced runaway greenhouse heating if it had formed 5% closer to the sun – both of these effects are considered irreversible.  Once totally frozen or heated, there would have been no turning back.  Alternatively, if the Earth’s orbit had been more elliptical these limits would have been even smaller.  Hart’s work implied that the CHZ was astonishingly thin for the Sun and that for stars of smaller mass it did not even exist!  These findings suggested that Earthlike planets with oceans and life were probably rare indeed.

It is now felt that Hart’s CHZ is probably too narrow because of several effects which he did not take into account.  One of these is the carbon dioxide-silicate cycle that, on earth, acts as a regulating thermostat to keep the planetary temperature within habitable zones.  Carbon dioxide is a trace gas that constitutes only 350 parts per million of the atmosphere but is very important as a “greenhouse” gas.  Carbon dioxide has infrared absorbing properties that retard the escape of heat from the earth into space.  This greenhouse effect warms the earth’s surface about 40 degrees C above the temperature it would otherwise have!  As we will see later, the thermostatic control of the carbon dioxide-silicate cycle occurs because of the effect of weathering.  If a planet warms, increased weathering removes carbon dioxide from the atmosphere reducing the greenhouse effect resulting in cooling.  When the Earth is too cool, weathering and carbon dioxide removal decrease, resulting in build-up of carbon dioxide in the atmosphere and an increased greenhouse effect leading to warming.  This remarkable negative feedback system widens the CHZ and also complicates efforts to determine its boundaries precisely.  Using this new information, astrobiologist James Kasting has not defined the HZ as “the region around a star in which an Earth-like planet (of comparable size) and having an atmosphere containing nitrogen, water, and carbon dioxide is climatically suitable for surface-dwelling, water-dependent life.’  In 1993, they estimated the CHZ as being between 15% greater or 5% less than the earth’s current orbit – much wider than Hart’s estimate but still quite narrow – especially for more elliptical orbits.

The concept of a habitable zone is now considered more a requirement for advanced, animal life rather than microbial life.  The recent discovery of extremophiles – bacteria that can live under extreme cold or hot conditions – requires that the concept of a HZ take on a more restricted perspective than a few years previously.  Furthermore, extremophile organisms may life under the extreme conditions that exist on other planets and moons within our own solar system – the most interesting is Europe – one of the larger moons of Jupiter.  This moon is now thought to probably have a subterranean ocean that might provide a habitat for extremophile organisms.  However, there are two considerations when considering life under such extreme conditions.  First, nothing other than very simple organisms with simple requirements could exist under such extreme conditions.  But most importantly, the “evolution” of such life forms in the extreme conditions occurring in such environments would be difficult if not impossible to imagine at best.

Not only must the concept of habitable zone be considered in terms of distance, but it also needs to be considered in terms of time.  For example, the earth will exist in a temporal habitable zone for approximately 5 to 8 billion years until the Sun’s continued brightening eventually bakes our planet into conditions unsuitable for any life.  However, for more massive stars, stellar evolution is much faster.  For example, the life times of stars 50% more massive than the sun would be too short for the leisurely pace at which life presumably evolved on our own planet.  Stars which are less massive than the sun (which includes the vast majority of all stars) burn too slowly and are not as hot as our sun.  Planets would then need to be closer to these stars to sustain life, but this proximity would make these planets more susceptible to minor variations in stellar burning rate and tidal forces.  Not only must the star be of the appropriate type to allow for life to develop, there must also be sufficient time for the elements making up that life to formLife requires many elements that came into existence after the Big Bang (which essentially produced only helium and hydrogen).  Indeed, twenty-six elements (including carbon, oxygen, nitrogen, phosphorus, potassium, sodium, iron, and copper) play a significant role in the development of advanced life, and many others (including the heavy elements such as uranium) play an important secondary role in supporting life by creating heat deep within our planet.  All of these elements were created within the centers of star – often in exploding stars or supernovae – rather than in the Big Bang itself.  This means that these elements were not initially present after the Big Bang, but rather had to be created within the first generation of stars and so were not present for perhaps the first 2 billion years or more of the Universe’s existence.  Thus, the habitable zones need to be considered not only in terms of distance from the central star, but also in terms of time that such a habitable zone would exist given the known life history of the central star.

Ejection of Planets out of the Habitable ZonePlanets might experience other fates once they start to develop life.  Perhaps the most dramatic of these fates is to be torn from the central star and ejected out of their solar system and hurtled into the darkness of space.  The most common sources of such ejections are interactions between giant planets.  The orbits of the planets in our solar system have not changed appreciably for billions of years; however, the planets do interact with each other and the shape of their orbits do vary somewhat.  In general, planetary systems are not stable for the time period of billions of years for there are interactions between the larger planets which adversely affect the orbits of smaller planets. For example, if Saturn were closer to Jupiter or if it were more massive, then the long-term interaction of these two planets could lead to ejection of one or the other of these planets causing it to leave our solar system into the interstellar void.  If Saturn were to be lost, Jupiter would stay trapped in its orbit, but the orbit would become oddly elliptical.  Indeed, some of the large planets orbiting other stars have highly elliptical orbits and in the distant past ejections of a long-lost planet might have been the cause.  Such ejection of planets from solar systems is probably even more likely in double or multiple star systems where the planetary orbits become more unstable due to the varying gravitational pull of the multiple stars over the orbital path.

Interestingly, although a planet ejected from a solar system would have little ability to sustain life due to the extraordinary cold of interstellar space, the situation might be different on any moon orbiting such a planet.  Consider, for example, the situation that might arise were Jupiter ejected from our solar system.  Jupiter itself does emit more energy than it receives from the sun, so Jupiter might indeed retain some warmth even in interstellar space.  But because of the extraordinary intense gravity of Jupiter upon Europe producing flexing of the moon generating heat – enough heat to perhaps produce liquid water deep within its subterranean oceans.  Thus, even though Europe were ejected into interstellar space with its huge planet, the heat generated by gravitational distortion deep within the moon might produce enough heat to sustain simple life.

Habitable Zones in Other Solar SystemsThe brightness of a star determines the location of the habitable zone but brightness depends upon the star’s type, age, and type.  Stars more massive than our sun have a shorter life along with a more rapid outward migration of the HZ.  For example, the Sun will be stable for nearly 10 billion years, whereas a star 50% more massive than the Sun enters the giant stage after only 2 billion years.  When a star becomes a red giant, its brightness increases considerable – maybe on the order of a thousand times – and the HZ increases greatly beyond its original bounds.  Furthermore, stars that have 50% more mass than the sun would not be around long enough for intelligent life to form at the same rate as it did upon our Earth.  Additionally, stars that are more massive are much hotter and radiate substantially more ultraviolet light than the Sun.  Ultraviolet light also can be disastrous for atmosphere such as the Earth as it causes heating of its upper atmosphere producing progressive loss.  Atmospheric loss may prevent planets similar to our earth from forming oceans and atmosphere.  This atmosphere problems poses additional problems along with the shorter lifetime of massive stars making the formation of intelligent life even more difficult around more massive planets.

It is often said that our Sun is a typical star but in fact this is not true.  Indeed, 95% of all stars are less massive than the Sun making our planetary system quite rare.  For the great majority of stars which are less massive than the Sun, the HZ is located farther inward. The most common planets in our galaxy are classified as M stars – they have only 10% of the mass of our Sun.  Such stars are far less luminous than our Sun, and any planets which might orbit such a star would have to be located correspondingly closer to the Sun to stay warm enough to allow the existence of liquid water on the surface.  However, this closeness of a planet to its star poses additional problems as the gravitational tidal effects from the star produce synchronous rotation wherein the planet spins on its axis only once each time it orbits the star.  Thus, the same side of the planet always faces the star as it orbits; this is similar to the more familiar tidal effect whereby the moon always presents its same face to the earth.  The result of synchronous rotation of a planet about a star would produce extreme heat on the side continuously facing the star and extreme cold on the portion of the planet always facing away from the star.

With this information, we can examine other stars in our Milky Way and determine whether they might be appropriate places for the formation of life.  Planets orbiting binary stars (or even worse – multiple star systems) would be in unstable orbits with extreme variations in local conditions such as heat and radiation energy.  Additionally, a planet forming in such a system would have to deal with the stellar evolution of more than one Sun.  With two or more suns undergoing the same evolutionary process, we might expect habitable zones to migrate and change even faster over time making it even more difficult for intelligent life to form. This understanding is extremely relevant as about two-thirds of solar-type stars in the solar neighborhood are members of binary or multiple star systems.  Other types of stellar systems would be even less amenable to the formation of life.  Variable stars (those that exhibit rapidly changing solar energy output) are certainly poor candidates for producing planets habitable by animals.  Unusual stellar entities such as neutron stars and white dwarf stars are probably uninhabitable by any form of life.

Stellar environment also plays a role in a planet’s ability to sustain life; for example, in regions where stellar volume is very high, such as open star clusters and globular clusters.  Open clusters are unlikely to harbor life as they are too young and are composed mostly of relatively young stars where life would not yet have a chance to develop.  Additionally, the density of stars in such a cluster would affect the orbits of each other’s planets to such a degree that it would destabilize planetary environments making life difficult.  Additionally, the large density of stars in such clusters makes it likely that one of the stars might explode (become a nova or even a supernova) exposing nearby space to exceedingly high doses of life sterilizing radiation.  Stellar density is particular high in globular clusters.  These clusters might have as many as 100,000 stars packed into a space only several light-years across.  The nearest star (other than our own Sun) to us – Proxima Centauri – is 4.2 light years away.  There are a total of 23 known stars which are 13 light-years away from us, whereas the M13 globular cluster has 30,000 stars packed into a space only 28 light-years across!  Finally, most globular clusters are composed of old, heavy-element poor stars, all of which are about the same age.  The low abundance of “heavy elements” such as carbon, silicon, and iron makes it difficult that any Earth-like planets could evolve, as they are required for any form of life.

The understanding that globular clusters are very poor candidates for containing life – let along intelligent life – represents a significant advance in our understanding of the universe.  It was only several decades ago that Drake – one of the authors of the famous Drake equation that predicted the probability of finding life in the universe – directed a radio signal toward the globular cluster M13 hoping that alien radio astronomers in that cluster might receive the message and respond.  Now, we know that there is no chance anybody will be there to receive the message when this radio signal arrives at M13 some 24,000 years from now. 

Galactic Habitable Zones

The concept of a habitable zone can be applied to a galaxy just as it can to individual stars.  Our own galaxy is a spiral galaxy – the other types being elliptical and irregular forms).  In most galaxies, the concentrations of stars is highest at the enters and then gradually decreases toward the peripheral zones.  Furthermore, spiral galaxies are dish-shaped (round, but flat if viewed from the side), with branching arms when viewed from the top.  Our own galaxy has an estimated diameter of 85,000 light years, and our own Sun is about 25,000 light years from the center.  It is located between spiral arms where stellar density is low compared to the density found within an arm, and we slowly orbit the center of the galaxy.  Our star is located in the habitable zone of the galaxy.  The high density of stars, the probability of dangerous life sterilizing supernovae and other energy sources, determines the inner portion of the zone.  Alternatively, the abundance of life sustaining heavy elements determines the location of the outer portion.  Outward from the center of the galaxy, the abundance of elements heavier than helium decreases substantially.  Our planet has a solid/liquid metal core with some radioactive material which gives off hear; both attributes may be necessary for the development of advanced life.  The metal core provides a magnetic field that protects the surface of the planet from significant ionizing radiation from space, while the radioactive heat from the core, mantle, crust fuels plate tectonics which is probably necessary for the development of advanced life. 

Newer thought concerning the concept of a galactic habitable zone has emerged in recent years.  Out galaxy is composed of four distinct components, each of which contains distinct solar populations.  The region where the sun is located is known as the thin disk – a flat pancake about 600 light years thick – inside of which star formation is still quite active, and young, metal-rich stars are being formed.  The second component is the thick disk which is about three times thicker than the thin disk and holds older containing fewer metals.  The bulge that resides at the center of the galaxy is the third component, containing a mixture of both young and new stars.  Finally, there is a halo of spherical star clusters around the disk.  These clusters contain the oldest stars and most metal poor stars in our galaxy with little if any star producing nebulae.

Star metal content correlates with position and time within the galaxy.  Planets form at the same time as their parent stars and thus planetary composition also depend on where and when they form within the galaxy.  Old stars formed from ancient material very poor in metals produce low mass terrestrial planets.  Conversely, those stars of more recent ancestry formed from relatively metal rich nebular, should harbor comparatively high-mass terrestrial planets. 

The stars in the halo and the thick disk are probably simply too old to hold sufficient quantities of metals for any earthlike planets to form.  The small terrestrial worlds formed from metal poor nebulae would cool quickly, develop thick crusts, and lack enough gravity to hold substantial atmosphere.  A thin atmosphere would not be able to shield the planet from intense ultraviolet radiation.  Also, since these small terrestrial planets would tend to have a thicker crust, plate tectonics would be curtailed and the climate – whose long-term stability depends upon the removal of carbon dioxide form the atmosphere by subduction of surface rocks driven by plate tectonics movement – might become too unbalanced.  Those planets, which might form near the galactic center, would be exposed to a greater concentration of stars where they might undergo regular gravitational perturbations.  These perturbations could produce serious impact hazards for planets orbiting stars that are surrounded by cometary clouds like our sun.  But even more important, being in an environment filled with other stars tremendously increases the danger in the form of radiation from stellar winds, supernovae, or anti-matter and gamma-ray bursts emitted by unstable stars – or even by the immense black hold at our galaxy’s center.

This leaves only the thin disk – but not all of it.  As we move along the disk toward the central bulge, star formation rates increase and metals become correspondingly more abundant as there are higher rates of star formation and recycling by nucleosynthesis reactions.  These stars would be composed of higher metal contents and form terrestrial planets that would be too massive.  These more massive planets would have higher concentrations of water and become a water world.  Astronomers feel that a world that is too wet inhibits the development of complex life because the interaction of land and sea is a crucial factor in developing life.

Conversely, as we move further out into the fringes of the galaxy along the thin disk, stellar densities drop and planets which might form would have too little metal content and be too small to support; an atmosphere.  It seems as though there is a critical distance form the galaxy’s center where life might form – about 15,000 to 40,000 light years.

There is some observational data to suggest the validity of the galactic habitable zone. These data note that,

  • All the stars so far found to have planets, almost none has less than 40% of the sun’s metal content.
  • Stars in globular cluster 47 Tucanae (presumably outside of the galactic habitable zone) do not appear to have any planets orbiting them.  These stars have a metal content that is about 25% that of the sun and so would not be expected to have sufficient metal content to form dense planets.

Not only is the Earth in a rare position in its galaxy, it may also be fortunate in being in a spiral rather than in an elliptical or irregular galaxy.  Elliptical galaxies are regions with little dust to encourage new star formation; most of the stars in elliptical galaxies are first generation stars that are nearly as old as the universe.  The Hubble Space Telescope has also enlarged our understanding of the early Universe.  For ten days during December 1995, the telescope took images of a portion of the Universe only about 3% as small as the full moon.  Examination of these images was startling – even revolutionary.  The images showed galaxies that were 3 to 15 times fainter than those ever seen before – and proportionally more distant.  There were more than 1500 individual galaxies catalogued in this very small window of space.  These galaxies are so distant that the light coming from them started its journey long before our own galaxy even existed, and indeed date to within 1-2 billion years from the Big Bang itself.  Thus, stars within these galaxies do not have earth-like planets as no heavy elements would have been formed; nor would life be present on any planets which might orbit these stars as none of the required elements for life had yet been formed either.  Another insight from these images was that galaxies formed during the earliest times of our Universe seemed to have been more irregular than the newer galaxies.  From 30% to 40% of these galaxies are unusual or deformed compared to those nearer to our own galaxy that were formed much later.  As discussed previously, these irregular galaxies are less likely to encourage life formation and development, as they are less stable.

Earth as a Charmed Planet

[Creation of the Elements] [Earth Construction] 
[The Earth's Atmosphere]

The earth may be unique among planets in the galaxy and perhaps in the entire Universe.  Additionally, several of the earth’s neighbors in the solar system have played a highly significant role in its ability to sustain and nurture life.  The nearly ideal nature of the Earth as a cradle of life are noted in its prehistory, its origin, its chemical constitution, and its early evolution.  The Earth has the following characteristics that make life sustainable,

  • At least trace amounts of carbon and other life-forming elements,
  • Water on or near the surface,
  • Appropriate atmosphere,
  • A very long period of stability during which the mean surface temperature has allowed liquid water to exist on its surface,
  • A rich abundance of heavy elements in its core and throughout the crust and mantle regions

The path toward both primitive and then later intelligent life included element formation in the Big Bang (mostly hydrogen and helium with trace amounts of lithium) and later in stars, explosion of stars with the formation of interstellar clouds from which solar systems and especially the Earth formed.  Additionally, the Earth itself had a complex evolution of its interior, surface, oceans, and atmosphere.  What is most important to realize is that it is unlikely that the Earth could be every fully replicated elsewhere in the universe – there are so many unique characteristics that are all present in our planet that replication elsewhere seems highly unlikely.  While similar planets may exist elsewhere, it is highly unlikely that everything necessary for advanced life to flourish is present elsewhere.  The factors endless developmental pathways that led to the formation and development of the Earth require nearly irreproducible circumstances making similar development elsewhere statistically impossible.  To see why this is possible, we will start at the beginning by constructing the base elements required for a planet.

Creation of the Elements

The Earth is very old – over four billion years.  Still, there was a tremendous history prior to the formation of the planet for the base elements first needed to be manufactured.  It is not considered one of the most established facts of astronomy that every atom of our bodies resided inside several different stars before the formation of our Sun, and has perhaps been part of the bodies of millions of different organisms since the Earth itself formed.  Planets, stars, and organisms come and go, but the elements which form these objects have been recycled since the origin of the Universe and are essentially eternal existing for perhaps 15 billion years.  All but a very small portion of the atoms in the planet Earth and in our own bodies were produced long ago by an intricate set of astrophysical processes.  The processes of element formation were universal and they provided similar starting materials for other planets existing throughout the solar system, our galaxy, and the Universe.  By looking at this history before our planet formed, we can gain a certain insight into the range of possible planets and life habitats that might be present throughout the Universe.

Everything that exists throughout the Universe – including time itself – started with the Big Bang.  While it was once thought that the Universe is truly eternal with no beginning – and with no end – it is now certain that there was indeed a beginning.  We go into why we believe in the Big Bang, and why it is so important to our theological understanding of God and His plans elsewhere.  Suffice it to say here that there is so much independent evidence pointing toward the Big Bang that it has become almost universally accepted.  In the Big Bang, the entire Universe was born in an instant in an incredible context of heat and energy.  The subsequence expansion led to rapid cooling of the primitive universe leading to the formation of matter out of this tremendous energy utilizing concepts formalized by Einstein in his famous equation equating energy and matter.  During the first half hour, conditions existed that produced much of the atoms that are still the major building blocks of stars – mainly hydrogen and helium.  These two atoms make up about 99% of the entire normal (visible) matter of the Universe.  By itself, the Big Bang produced very little in the way of chemical diversity and gave us little or nothing beyond very simple atoms; hydrogen, helium, and lithium.  No oxygen, magnesium, silicon, iron, sulfur, or any of the more familiar substances that compose 96% of our planet were formed.  Nevertheless, the Big Bang did produce sufficient hydrogen from which all the heavier and more interesting elements formed. 

The temperature of the Universe during the Big Bang was tremendous – far beyond anything that can be produced on our planet using the most advanced technologies we currently possess.  During the first half-hour, the temperature was about 50 million degrees Celsius – at this temperature positively charged protons (the nuclei of hydrogen) could occasionally collide with enough energy to overwhelm the electrostatically repulsive charge of another positively charged proton to fuse together and form helium.  This fusion is the process by which stars produce energy – and is the reason why the night sky is not dark.  Indeed, it is the energy produced in the Sun when two protons unite producing fusion nuclear energy that warms the Earth’s surface.  Nuclear fusion energy (as opposed to nuclear fission energy which powers our current nuclear reactors) may some day provide virtually endless, clean energy from water, it may also annihilate all life on earth in hydrogen bombs.  Mankind has yet to make that decision.

The fusion of two hydrogen nuclei to form hydrogen was as far as nuclear fusion could go at the time of the Big Bang.  While the temperature was high enough for helium nuclei to form other elements, the density was not.  The average density of the very early universe allows us to determine the ultimate fate of the universe as we discuss elsewhere.  Since only hydrogen and helium were available to the early Universe, planets similar to the earth could not form.  The Universe during the first two billion years of its existence was unable to have terrestrial planets as it did not have the elements to produce them. <![endif]>

Carbon is one of the most important elements for life, and it could not be formed in the early Universe because the density of the expanding mass was simply too low.  Carbon formation had to await the creation of giant red stars where dense interiors are massive enough to allow such collisions.  Because larger stars become red giants only during the last 10% of their lifetimes (when they have burnt up most of their hydrogen), there was no carbon in the universe for the first several billion years after the Big Bang.  Carbon formation requires three helium atoms to collide as essentially exactly the same time – a three-way collision.  First, two helium atoms collide to form a beryllium-8 isotope, and then with a tenth of a femtosecond later (1/ second) before the highly radioactive nucleus dissolves, it must collide with and react with a third helium atom to produce carbon.  A carbon nucleus has six neutrons and six protons – the combination of three helium nuclei each of which has two neutrons and two protons.  Once carbon is produced in aging red giant stars, heavier elements come much more easily.  The production of heavier and heavier elements occurs in the cores of stars where temperatures range from 10 million to 100 million degrees Celsius.  The sun, for example, is currently only producing helium from the fusion of two hydrogen nuclei.  However, during the last 10% of our sun’s lifetime, it will run out of hydrogen and start to cool slightly.  As the sun cools, gravity will cause the sun to shrink producing higher temperatures in its core eventually allowing the nuclear fusion of three helium atoms to form carbon (see above) as well as other fusion reactions.  Eventually, it will produce all of the elements from helium to bismuth, the heaviest non-radioactive element in nature.  Elements heavier than bismuth are all radioactive and are produced by the decay of uranium and thorium.  These radioactive elements will not be produced in our sun because it is simply not massive enough to produce the heat necessary to create them.  Radioactive elements are made in the cores of stars ten times more massive than the sun that undergo supernova explosions – tremendous explosions in which a star brightens by a factor of 100 billion over the course of a few days.  It is amazing to think that the uranium that powers our nuclear fission reactions and which heats the interior of our earth came from the supernova explosion of a relatively nearby star.

The elements that are produced by the Big Bang and in the interiors of stars are the building blocks necessary for the formation of Earth, any other terrestrial planet, and life itself.  The production of elements within stars along with continued recycling between stars produced a relative proportion of these different elements known as the “cosmic abundance” – the composition of the sun and most common stars.  The composition is approximately 90% hydrogen, 10% helium, with about 0.1% each of carbon, nitrogen and oxygen, and 0.01% each of magnesium, iron, and silicon.  The Earth exhibits similar relative abundances of iron, magnesium, and silicon, some oxygen, but only trace amounts of the other cosmically abundant elements.

The dominant atoms that formed the Earth were silicon, magnesium, and iron with sufficient oxygen to completely oxidize most of the silicon and magnesium and part of the iron.  Other elements have played a critical role in the emergence of life on Earth despite their rarity.  Carbon is a trace element in the Earth, but it is a key element for life – and probably for any alien life as well.  Hydrogen is also a rare element on earth – but it is present in water, the essential fluid of terrestrial life.  The radioactive trace elements uranium and thorium – which arose in the supernova explosion of stars – heats the earth’s interior and supplies the furnace that drives volcanism – the vertical movement of matter within the earth – as well as the drift of continents on its surface.

The Sun is similar in composition to other nearby stars but with a major difference; it contains about 25% more heavy elements than typical nearby stars of similar mass.  Extremely old stars contain about a thousandth of the concentration of heavy metals compared to the Sun.  The abundance of heavy metals is roughly correlated with a star’s age.  As time passes, the heavy element content of the Universe gradually increases so that newly formed stars are on average “enriched” with more heavy elements than older stars.  Furthermore, stars in the central portion of our galaxy are more enriched with heavy metals than those stars on the periphery.

The heavy metal content of stars is important to consider when evaluating whether a star might have an Earth-like planet.  Planets form from a ring of degree orbiting a star early in its life history.  Those stars that have higher metal contents also have annular rings with higher metal content that in turn form planets with higher metal contents.  If the Earth had formed around a star with lower, more average metal content, then it would have been smaller since there would have been less metal from which it had to form.  Being smaller, the Earth would then have lower gravity and would have retained less atmosphere, less volcanism, less plate tectonics, and less of a magnetic field – all of which adversely affect the ability of a planet to support advanced life as well shall see.  Additionally, if the Earth were further from the center of the galaxy, or even if it were a typical one-solar mass star, the Earth would probably be smaller. 

Of all the properties of the Earth, perhaps the least appreciated and the most curious is that it is so rich in metals.  These metals are necessary to important organic molecules in animals (such as copper, iron, and magnesium) – but they are so rare elsewhere.  How we got such a treasure of metals is a wonderful story.

Earth Construction

Stars are the recycling plants of the Universe.  Over billions of years, they have taken the raw materials left over from the Big Bang – namely, hydrogen and helium, and form new matter through nuclear fusion.  Like biological entities, stars are born, live their lives (only over billions of years), and eventually die.  In the process of their death, they become compact objects such as white dwarfs, neutron stars, or even black holes.  On their evolutionary pathways to these compact entities, they eject matter back into space where it is recycled and further enriched with heavy elements.  New stars rise from the ashes of older stars and in turn produce more heavy elements that they in turn eject out into space as their life ends.  That is why each of the individual atoms in Earth and in all of us have occupied the interior of at least a few different stars.  Just before the Sun was born, the atoms that would form the Earth and the other planets in our solar system existed in the form of interstellar dust and debris.  Gradually over the course of eons, the interstellar matter through gravitational attraction formed a nebular cloud, which then eventually further condensed into the Sun, the plants, and their moons.  The condensation process began when the mass of interstellar material became dense and cool enough to gravitationally collapse upon itself and form a flattened, rotating cloud – the solar nebula.  As this nebula gradually evolved, it quickly assumed the shape of a disk formed of gas, dust, and rocks orbiting the proto-sun – a short-lived juvenile state of the Sun when it was larger, cooler, and less massive as it was still gathering mass.  The planets also formed within this nebula, even though the nebula itself existed for only about 10 million years before the majority of its dust either had formed large bodies or were ejected from the solar system.

Ground-based telescopes, and the Hubble Space Telescope orbiting the Earth, have been able to reveal several lines of evidence suggesting that disks surround several nearby stars.  Among this evidence is a spectacular phenomenon that has only recently begun to be understood.  Young stars show jets of material radiating out from them.  These “bipolar nebulae” are gaseous objects resembling two giant turnips, each with its apex pointing toward the star.  As stars form, they paradoxically also eject matter back out into space.  The presence of a disk in the equatorial plane of a star forces the ejected material into jets along the polar axes of the spinning system of star and disk.

In our own solar nebula, 99% of the mass was gas (mostly hydrogen and helium), with the heavier elements that could exist as solids making up the remaining 1%.  The gas played a major role in the formation of the gaseous planets Jupiter, Saturn, Uranus, and Neptune.  Dust, rocks, and larger solar bodies separated from the gas and became highly concentrated, forming a disk-like sheet in the mid-plane of the solar nebula.

Accretion is the process responsible for the unique and very important elements of the Earth.  Earth formed within the habitable zone of the Sun.  The paradox of terrestrial planets is that if they form close enough to the star to be in its habitable zone, they typically end up with very little water and a dearth of primary life forming elements such as nitrogen and carbon, compared to bodies that formed in the outer solar system.  In other words, the planets that are in the right place and have warm surfaces contain only minor amounts of the right ingredients for life.  At the Earth’s distance from the center of the solar nebula, the temperature was too high for abundant carbon, nitrogen, or water to be bound to solid materials that could accrete to form planets.  Ice and carbon/nitrogen rich solids were too volatile (evaporated too easily) and had no means of efficiently forming solids in the warm inner regions of the nebula.  The Earth has only trace amounts of these volatile components compared to bodies that formed farther out from the Sun.  For example, carbonaceous meteorites, thought to be samples of typical asteroids formed between Mars and Jupiter contain up to 20% water (in hydrous minerals similar to talc) and up to 4% carbon.  The bulk of Earth by comparison, is only 0.1% water and 0.05% carbon.  Had the Earth formed from materials similar to those found in the asteroid belt farther from the Sun, its oceans would have been hundreds of miles deep and the carbon content would have been higher by maybe 1000-fold.  However, both the increased water and increased carbon content would have resulted in a planet totally covered by water with vast amounts of carbon dioxide in the atmosphere which would have produced a greenhouse heating effect.   The resulting temperatures on the surface of the Earth would then be many hundreds of degrees rather than the balmy temperatures we now enjoy.  The Earth would have been totally covered with water with only twice as much, and very few nutrients would have been available in the water to nurture any life.

The reason why the Earth is so carbon poor is that much of the carbon in the inner parts of the solar nebular out of which the Earth formed was in the form of carbon monoxide gas.  If there had been a way to solidify the carbon within this gas, then carbon would have been the dominant Earth element.  A genuinely carbon-rich planet would be very different form the Earth.  The planet would have graphite on its surface, with diamond and silicon carbide in its interior.  These forms of carbon would no allow either volcanism or even chemical weathering, both of which are critically important for animal life (see below).

The solar nebula out of which the Earth formed was relatively poor in biogenic elements; they were present in the outer realms of the nebula.  Although much of these materials stayed in the outer solar system, some would ultimately have reached Earth by scattering.  When they passed near the outer planets, their orbits would have been significantly altered, sometimes sending them toward the sun where they might collide with terrestrial planets such as the Earth.  Gravitational effects from encounters with the larger planets could also cause asteroid and comet debris, rich in light biogenic elements, to assume earth crossing orbits.  This “cross-talk” caused some degree of mixing between different zones of the solar nebula providing a means of bringing the building blocks of life to what otherwise would have been a lifeless planet lacking in many biogenic elements because it formed too close to the Sun.  Even today, the Earth is bombarded by material from the outer solar system in the form of comets and asteroids.  These materials carry carbon, nitrogen, and water – but even more interesting they also carry organic materials such as amino acids as were discovered in the Murchison meteorite that fell in Australia in 1969.

The constant influx of material to the Earth brings with it life sustaining elements; however, there is also a darker and more ominous side.  The Earth receives about 40,000 tons per year of outer solar material mostly in the form of small and even microscopic particles.  However, occasionally larger objects also collide with the Earth.  An outer solar system object 1 kilometer in diameter randomly strikes the Earth every 300,000 years.  Collision of the Earth with a body this size traveling at a speed of well over 10 kilometers per second results in a very energetic impact event.  Every 100 million years on average, a 10 kilometer diameter objects strikes the Earth.  This size object would produce a rater tens of kilometers deep and over 200 kilometers in diameter, producing huge fires, and global climate change.  Paleontologists believe the dinosaurs were made extinct by such an impact on Earth 65 million years ago.  Thus, while debris from the outer portions of the solar system can sustain and nurture life, it is also capable of destroying life on a vast, planetary scale.

Larger asteroids and meteors more frequently struck the early Earth because more were present.  The giant impacts essentially ended 3.9 billion years ago because other planets had swept up most of the larger rocks, or they were ejected from the solar system, or are now present in stable distant orbits.  The continuous collision of large bodies with the Earth played a role in determining the initial tilt of the Earth’s spin axis – now at about 23.5 degrees to the vertical, the length of the Earth’s day, the direction of its spin, and the thermal state of the interior.  Furthermore, it is now believed that the impact of a Mars-sized body was responsible for the formation of the Moon – a large satellite relative to the size of its mother planet.

The final composition of the Earth required several crucial events 

  • First, there had to be enough metal present in the earthly Earth to allow for the formation of a liquid iron and nickel rich core.  The rotation of this metal core permits the formation of a magnetic field – a valuable property for a planet to sustain life,
  • Second, there had to be enough radioactive materials such as uranium and thorium to enable a prolonged period of heating of the inner regions of the planet.   This property gave the earth a long-lived furnace that made possible a prolonged period of mountain building and plate tectonics.  Plate tectonics and volcanism are necessary for maintaining a suitable habitat for animals as will be seen later on.
  • Finally, the early Earth had to have a very thin outer core of low-density material that also permits plate tectonics to occur – no plate, no tectonics. 

The production and stability of the Earth’s core, mantle, and crust could only have come about through the fortuitous assemblage of the correct elemental building blocks at the right time. 

There is no direct evidence of the composition of the Earth during its earliest periods as those rocks have not survived.  However, it is most likely that this early period included high-impact collisions which produced great violence; the greater impacts would have heated and resurfaced the entire surface of the planet.  Some of these impacts might have vaporized large amounts of water and liberated huge quantities of carbon dioxide form surface rocks that led to phenomenal greenhouse effects.  The atmosphere would be heated by retention of infrared energy by the liberated carbon dioxide producing surface temperatures hot enough to melt surface rocks.

The final composition of water and carbon dioxide has had a tremendous influence upon the Earth’s history as a life-bearing planet.  Had the Earth been composed of just a little bit more water, then there would have been no land surface; if there had been just a little bit more carbon dioxide, the Earth would probably have remained too hot to host life – much like Venus.

The Earth's Atmosphere

The amount and composition of an atmosphere is of critical importance for the future life hosting ability of a planet.  Today, the composition of the atmosphere is primarily due to life, and it differs greatly from those of the other terrestrial planets which range from essentially no atmosphere (Mercury) to a carbon dioxide atmosphere a hundred times denser (Venus) to a carbon dioxide atmosphere a hundred times less dense (Mars).  The Earth’s atmosphere is composed of nitrogen, oxygen, water vapor, and carbon dioxide (in descending order), and it is not a life that would be produced by chemistry alone.  Indeed, if a space alien ascertained the composition of the Earth’s atmosphere, he could then deduce life must be present.  Without life, oxygen would rapidly diminish in the atmosphere.  Some oxygen would oxidize surface materials, while others would react with nitrogen producing nitrogen dioxide and nitric acid.  Similarly, the concentration of carbon dioxide would probably rist, resulting in the production of a nitrogen and carbon dioxide atmosphere <![endif]>

The Earth’s earliest atmosphere was produced by outgassing of volatile molecules from the interior.  These volatile molecules were probably carried to the earth by meteors and impacting comets.  The oceans are a by-product of outgassing and the formation of the atmosphere.  When the atmosphere was very hot, a great deal of it was composed of steam.  Gradually, as the Earth cooled down, the steam condensed into water and formed the vast oceans with which we are familiar.  The oceans became salty through chemical interactions with the Earth’s crust.

The presence of land is crucially important for animal life.  If the Earth were smooth, the oceans contain enough water to cover it to a depth of 4,000 meters.  Thus, if the Earth varied by only a few kilometers in elevation it would be devoid of land.  The formation of land has occurred by two principle means: simple volcanism creating mountains, and more complex processes related to plate tectonics.  Volcanism leads to the formation of islands such as Hawaii and the Galapagos archipelagos.  Unfortunately, the early island raising out of the water through volcanism had no life; they were bleak and desert-like sterile surfaces constantly bombarded by intense ultraviolet radiation from the Sun unfiltered by the Earth’s primitive atmosphere.  Most of these islands would eventually have collapsed back into the sea due to constant water erosion form the oceans around them.  Continents were able to form on the earth that could endure for billions of years.  These continents required the formation of land masses made of relatively lightweight materials that could permanently “float” on the denser underlying mantle. 

The early landmasses may have resulted when the impact of large comets and asteroids melted an outer region of the Earth to form a “magma ocean” - a layer of molten rock.  Similar processes happened on the Moon, where the magma oceans cooled, and many small crystals of a mineral called plagioclase feldspar formed and floated upward to create a low-density crust nearly 100 kilometers thick.  The ancient crust resulting from these impacts produces the highlands that can be seen with the on the Moon naked eye.  In the case of the Earth, the magma ocean may have led to the formation of the first continents.  The initial landmasses on the Earth were small, and it was not until half way through its history land covered more than 10% of the Earth’s surface.  The fortunate combination of surface land surrounded by water was probably very important in producing a planet that could sustain life.

Creation of Life

[Prebiotic Soup] [Hope in RNA] [Error Handling] [Snowball Earth] 
[The Cambrian Explosion] [The Eukaryotic Cell] [Atmospheric Oxygen] 

            The origin of life on this planet is a subject sure to bring controversy to any discussion.  We know more about life now than we have ever known throughout history; mostly, we know it is more complex and difficult to understand than we could ever have imagined.  For example, last century, the internal structure of a bacterium was expected to be a homogeneous mixture or protoplasm without any internal structure.  Today, we know that the simple bacterium is extraordinarily complicated producing hundreds of proteins, countless chemical reactions all tightly controlled by intricate facilitating or inhibitory reactions.  Such chemical reactions and controlling mechanisms literally cover the wall of a biochemist’s office.  Remember, these are the simplest organisms – the prokaryotes – and not the vastly more complicated eukaryotic cells.  Remember too that this does not even begin to touch upon the vastly even more complicated multicellular organisms with specialized tissues all interacting with other specialized tissues.  Life has become vastly more complicated and even with the explosion of knowledge today regarding medicine and the treatment of disease, we are only just beginning to understand the biochemistry of life.

            One of the assumptions held by those who promote naturalistic evolution is that there was plenty of time for the emergence and evolution of life; after all, there was about 3.5 billion years since the emergence of bacteria until today.  However, discoveries about the universe and the solar system have shattered that assumption.  What we now realize is that life originated on Earth very quickly.  Fully formed cells first appear in the fossil records as far back as 3.5 billion years ago, and limestone rock which was formed from the remains of organisms, dates back 3.8 billion years.  The ratio of Carbon-12 to Carbon-13 found in ancient sedimentary rock also indicates a plentitude of life on Earth for the era of about 3.8 to 3.5 billion years ago.  From 4.25 until 3.8 billion years ago, the bombardment of Earth was so intense that no life could possibly exist.  From 3.8 until 3.5 million years ago, the bombardment gradually decreased to a comparatively low level; however, it has been estimated that during those 300 million years at least thirty life extinguishing impacts must have occurred.  The importance of these facts is that life sprang up on Earth in what could be called geologic instants, periods of ten-million years or less (between the devastating impacts).  Thus, we do not indeed have the billions of years for life to occur on this planet; rather, we have perhaps only several million years.

Prebiotic Soup

Evolutionary theory posits that life came forth spontaneously out of chemical reactions aided by heat and perhaps electricity in a “prebiotic soup” of chemicals.  However, attempts in the laboratory to demonstrate that life can and does spontaneously come together on its own have resulted in failure.  Even under highly favorable conditions of a laboratory, these soups have failed to produce anything even remotely resembling life.  One problem is that they produce only a random distribution of left and right-sided prebiotic molecules, whereas life chemistry demands that all of the molecules be eight right or left handed.  With all our learning and technology we cannot even come close to bringing life together in the highly controlled environment of the laboratory.  How can we expect life to arise spontaneously in the chaotic environment of an ancient earth?

            Other evolutionists recognizing these difficulties have posited that life came from extraterrestrial bombardment.  Forgetting about the destructive nature of these planetary collisions, they hypothesize a possibly beneficial effect.  It is proposed that perhaps this bombardment may have assisted the formation of life by delivering concentrated doses of prebiotic molecules.  Comets, meteorites, and space dust in general are partly composed of carbon and interplanetary dust particles can carry some prebiotic molecules, they carry far too few to make a real difference.  In fact, with every potentially helpful molecule that they might bring to the planet, they bring several more that would get in the way – useless molecules that would substitute for the needed ones – again, that left and right-handed problem again.

            Carl Sagan – one of the great scientists in the evolutionary movement (and who has just recently died from cancer), clung on to yet another chance whereby life might come to this earth.  He suggested that perhaps the atmospheric conditions 3.8 billion years ago when life first appeared were not too unfavorable for life – perhaps the conditions were just neutral.  An “oxidizing” atmosphere would be unfavorable as the atoms and molecules would bond with oxygen and removed from being used for life molecules.  Alternatively, a favorable atmosphere would be one which would be a “reducing” atmosphere in which molecules bind with hydrogen rather than oxygen.  But even these tentative hopes were dashed about five years ago when atmospheric scientists established that the Earth’s atmosphere has been fully oxidizing for at least the past four billion years.  Under these conditions, processes producing amino acids (which build into proteins) and nucleotides (which are used to produce DNA and RNA) would operate 30 million times less efficiently than they would under reducing conditions.  Natural primordial soups would thereby contain far too few prebiotic molecules to overcome this inefficiency – not to mention the destructive chemical reactions would be tremendously increased.  Finally, the small amount of amino acids that would be produced would consist mostly of glycine – vanishingly little of the other amino acids would be produced. <![endif]>

            Some have proposed that perhaps we are expecting too much and the life that was formed spontaneously 3.8 billion years ago was much simpler than that which exists today.  However, the minimum complexity of an organism has to be such that it can reproduce independently, and the simplest life forms currently in existence today is about at that level.  Furthermore, if very simple life were able to be produced in a prebiotic soup, then that very simple life should be easy to create in the laboratory.  Such however, is not the case; indeed, as has been previously noted, nothing resembling a biologic life form has thus far been created. 

Hope in RNA?

            Several papers published in the prestigious journal Science recently gave some hope to evolutionists by proposing what seemed to be a possible way around some of the complications of the complexities of life.  For life to occur, you need DNA that holds the blueprint for life, RNA which are molecules that carry information form the DNA molecule to specific proteins, and then proteins themselves.  Furthermore, any one of these three basic ingredients is insufficient by themselves; rather, you need all three of them together spontaneously.  Even the most optimistic researcher would agree that the chance appearance of these incredibly complex molecules at exactly the right time and place for the initial production of life was beyond the realm of natural possibility.

However, in 1987 a research group demonstrated that one kind of RNA can act as an enzyme or catalyst, and investigators have proposed that perhaps the earliest life form utilized a chemical that acted like DNA, RNA, and protein – performing all three functions.  However, this still does not help the evolutionist at all because this purported intermediate chemical would still have to carry all the information of those chemicals it is replacing.  In other words, the task of assembling such an incredibly versatile molecule (which has not been synthesized in the laboratory) would be as difficult as assembling the three different kinds of molecules it would be replacing. 

The catch in these notions is that RNA is more easily assembled that DNA.  Indeed, for twenty years, it was taught in evolution textbooks that RNA had been synthesized under prebiotic conditions.  However, this myth was exploded by Robert Shapiro in a meeting of the International Society for the Study of the Origin of Life held at Berkeley in 1986.  Furthermore, Shapiro demonstrated how the synthesis of RNA under prebiotic conditions is essentially impossible.  Shapiro then went on to publish his research, an his proposals remain unchallenged to this day.

Error Handling

Humans constantly make errors; we live in an imperfect world and there are errors constantly occurring with our genetic material.  It is now thought that some of these errors may be the primary cause for cancer and for many other chronic illnesses.  Furthermore, radiation causes errors to occur in the genetic material – but radiation is important in the stability and life sustaining capacity of this Earth.  Therefore, it is important that life molecules be so designed so that they can repair errors which might happen within them over time.  A useful analogy for visualizing the error handling capacity of the genetic material is to consider a computer program with a few million lines of code.  In spite of random destruction of ten thousand lines of code, the program still performs its intended function.  Up until now, believe me – nobody has written such code but such a code exists in the genetic material.  There are many areas in the genetic code where random destruction or changes would have little or no effect upon the protein produced.  Life molecules are designed so that they can function even after limited, random destruction.

Snowball Earth

The majority of astrobiologists believe that the temperature of the Earth from the time of the emergency of simplest life – about 3.8 billion years ago – until the origin of more complicated organisms – about 2.5 billion years ago – was high.  The concentration of oxygen in the early atmosphere was vanishingly low, far too low to support animal life.  Gradually, the greenhouse gases in the atmosphere dissipated and the Earth’s temperature declined.  There is evidence to suggest that there have been as many as four major episodes of glaciation on the Earth on a scale far exceeding anything we might imagine. 

The first Snowball Earth period began about 2.45 billion years ago, and a second siege of several such glaciations occurred between 800 and 600 million years ago.  These two snowball periods are of great interest to astrobiologists because they are signal events in biological history.  Around 2.5 billion years ago around the first great glaciation, the first eukaryotic – or animal like - cells appeared.  Furthermore, the fossil records indicate that about 550 million years ago, diverse and abundant animal life blossomed to such a great extent that it is known as the Cambrian Explosion.  These two explosions of new life forms occurred immediately after the two most severe episodes of glaciation and ice cover in the Earth’s history. 

Trillites are deposits of angular rock fragments that glaciers deposit as they move across the landscape.  Recent ice ages have left many of such deposits in the Northern and Southern hemispheres.  Interestingly, these deposits are recovered from virtually all latitudinal regions of the globe which shows that the glaciations must have extended to near equatorial latitudes.  These “snowball” glaciations were much greater than the recent Ice Age in which glaciers only extended to mid-latitudes.  The “Snowball” Earth periods were times when the planet teetered dangerously close to becoming too cold for any life to survive.  The Snowball Earth theory received greater status in astrobiology circles when Harvard geologist Paul Hoffman’s study, published in a 1998 issue of Science, demonstrated that ice extended to near equatorial latitudes in the late Precambrian Era – about 700 million years ago.  What has been recognized is that unlike the more recent Ice Ages, the Snow Ball earth all of the oceans (in addition to the land) was covered with ice to considerable depths.  Only the deepest portions of the oceans remained liquid while the ocean was covered with ice to a depth of perhaps 1500 meters.  During these times, the Earth would have been extraordinarily cold with the average surface temperatures varying between –20 and –50 degrees C. 

These extremely cold temperatures would have had a tremendous influence upon the surface of our planet.  For example, continental weathering would have been slowed or even stopped; there would have been no or little exposure of land to the elements for weathering to occur.  Furthermore, the presence of an ice cover over the oceans would have acted as a lid separating water from the atmosphere.  Little water could evaporate out of the oceans for water would have been uncoupled from the atmosphere.  Volcanism would have continued, but there could be less release of toxic gases into the atmosphere and more into the ocean from underwater volcanic activity.  Instead, these gases would become dissolved in the water and created a very toxic environment indeed.  These condition would continue for extended periods of time during a Snowball Earth – as long as 30 millions years.

Astronomers previously had believed that such an “icehouse” or “snowball” earth would be irreversible reasoning that as a planet gets more and more thickly covered with ice, the fraction of light reflected back into space increases and solar heating of the surface of the planet decreases.  A planet covered with ice would reflect most sunlight back into space causing the planet to become even cooler. Yet, it appears as though the Earth was able to escape the grip of a deep freeze several times.  The means of this escape is thought to be through volcanism whereby greenhouse gases such as carbon dioxide are released into the atmosphere producing a “greenhouse effect” that would tend to absorb and retain heat.  Gradually, the oceans would again melt and the Earth would undergo spectacular changes.  Kirschvink described these events as follows,

“Escape from this “icehouse” condition was only accomplished by the buildup of volcanic gases, particularly carbon dioxide, mostly from undersea volcanic activity.  Deglaciation during the end of these glacial events must have been spectacular, with nearly 30 million years of carbon dioxide, ferrous iron, and long buried nutrients suddenly being exposed to fresh air and sunlight.  Hundreds of meters of carbonate rock are preserved capping the glacial sediments, at all latitudes, on all continents, as a result of wild photosynthetic activity.  For a brief time, the Earth’s oceans would have been as green as Irish clover, and the sudden oxygen spikes may have sparked early animal evolution."

            Phytoplankton represents the most important source of biological productivity in the oceans.  These tiny one-celled plants produce considerable amounts of oxygen and are responsible for a large portion of the oxygen we animals use every day.  The growth of these plants is limited by the availability of nutrients and iron.  If iron is dropped into the oceans of today, a great bloom of phytoplankton will result.  Soon after the end of the snowball earth, the stored iron and magnesium in the oceans would have acted as fertilizer tremendously stimulating growth of the blue-green “algae” – really photosynthesizing bacteria known as cyanobacteria.  Huge amounts of oxygen would have been released by the ocean bloom that preceded the appearance of new life.

            These events would have tremendous effects upon the Earth’s geology as well.  The sudden rush of oxygen produced by plankton into the sea would have caused the iron and manganese rich oceans to precipitate out iron and manganese oxides.  Evidence of this precipitation is seen in South Africa where the world’s largest land based deposit of manganese minerals has been dated at 2.4 billion years old.  This deposit sits on top of sedimentary deposits that were laid down during the 2.5 billion year old first snowball Earth, and appear to be a direct consequence of the oxygen bloom that occurred when this snowball melted.

            The oxygen that was released into the atmosphere initially was a poison to many forms of life.  Having developed during a time when there was little or no oxygen in the atmosphere, the sudden appearance of oxygen would have been a disaster.  However, the newfound oxygen apparently stimulated some life forms and for them it was a powerful spur toward further growth and development.  Organisms had only one of two possibilities left to them; die, or adapt to the newly produced oxygen flooding the planet.  Organisms would have to develop two major adaptations.

  • First, enzymes needed to develop which would help the cell avoid the ravages of dissolved oxygen and chemicals called hydroxyl radicals.  These chemical oxidize critical components of cellular structure and over the period of years can seriously damage an organism.  Today, mankind is still trying to develop means for neutralizing these oxygen radicals for they are felt to be a major factor in the development of dementia, heart disease, and even certain forms of lung disease.  We take vitamins - such as Vitamin C and E - to reduce the effect of these compounds.

  • Second, the huge amount of oxygen liberated by the growth of plankton and other plants oxidized iron and other metals dissolved in water making them harder to nourish life.  After having been surrounded by high-iron solution since the first formation of life, proteins within cells had to be reengineered for life in an environement which was iron poor.

DNA sequencing techniques have shown that several enzymes found in ancient bacteria are left over from this event which occurred some 2.5 billion years ago – no such enzymes occur in more ancient bacteria.  The implications of this are profound suggesting that the oldest bacteria developed before the Snowball Earth as they do not have the enzymes required to deal with dissolved oxygen radicals or iron rarity.  Rather, the Archeae and Eucarya developed after this seminal event as they do have the required enzymes. Indeed, the record of the oldest eucaryan – a creature discovered in 1992 known as Grypania – are found in rocks about 2.1 billion years old – far older than the bacteria which preceded them.  The Grypania are the oldest organisms that had attained the eukaryotic style of organization with internal organelles and structure.  They are found in iron deposits located in Michigan and are in chains as much as 90 millimeters long.  This discovery indicates that the first eukaryotic cell occurred during the banded-iron formation process when there was little free oxygen in the sea and atmosphere.  The Grypania may have been very rare as other eukaryotes do not occur in the fossil record for another 500 million years.

            Thus, the original Snowball earth that occurred about 2.5 billion years ago was extremely important for the development of intelligent life for the following reasons,

  • First, the Snowball produced the largest mass extinction in our planet’s history due to extremely cold temperatures which existed down to the equator regions of the earth.  All water was removed from the surfaces of continents due to the extreme cold.

  • Second, the Earth’s release from the Snowball effect would bring about a new catastrophe as nutrient rich water became exposed to warmth and plankton life bloomed releasing vast amounts of oxygen into the atmosphere.  For most bacteria and simple plants, this oxygen would prove toxic and produce immediate death.  Some life managed to survive by developing means to deal with the oxygen and thrive; this life would then expand to fill the ecological niches left from that life which did not survive.

  • Third, the first eukaryotic cells were produced about 2.2 billion years ago, laying the foundation for more complicated, diverse eukaryotic cells that would form about 1.6 billion years ago. 

The Second Global Glaciation

            Another great period of life extinction occurred about 800 million years ago.  As with the earlier deep freeze, the earth was once again locked into a global icehouse.  All life had to retreat to sources of heat such as around volcanoes and hydrothermal vents – or die.  After the first great snowball earth, there was a great increase in the diversity of bacterial, single celled life with the emergence of eukaryotic life – such as the cells which make up our body.  The second global glaciation similarly produced mass extinction of most life on earth – all life that could not somehow escape the cold was destroyed. Both of the two major episodes of Snowball Earth nearly ended all life on the planet, but each ultimately opened up new opportunities for life to develop.  The end of the last Snowball Earth event ended the Precambrian era; soon thereafter, abundant skeletons of larger animals begin to fill the sea.  New life forms suddenly appeared in what has been called the Cambrian Explosion – an “explosion” of new life. 

            The Earth’s surface temperature seems to have a determinant effect upon the form of life which might develop.  Simpler organisms such as the bacteria can have very large tolerances for temperature variations.  Indeed, most bacteria can live in temperatures ranging from the freezing point of water to the boiling point – and some can survive in even greater temperatures (the methanogens and thermophiles).  However, more complex cells such as the eucharyotic cells which make up our body, have much narrow temperature tolerances.  Eucharyotic cells have multiple small organs (organelles) which life within them such as the mitochondrion.  This organelle is responsible for producing energy for the cell and is therefore critically important.  However, the mitochondria have an upper temperature tolerance of about 60 degrees C, much lower than the boiling point of water (100 degrees C). 

The Cambrian Explosion

Earth was without life for the first 3.5 billion years of its existence, and was without animals that were large enough to leave a visible fossil record for nearly 4 billion years.  But about 550 million years ago, there was a sizable and diverse animal life that burst into the oceans – the Cambrian Explosion.  Over a relatively short period, all of the animal phyla either evolved or first appear in the fossil record.  No matter where on Earth we look, there are no skeletons of animals in rocks older than 600 million years! Yet, these animals are plentiful and diverse in rocks dating from 500 million years ago indicating that they all must have developed during this relatively short period of 100 million years.  Thus, it would seem that our planet went from a place without animals that could be seen with the unaided eye to a planet teeming with invertebrate marine life equal in size to that observed today!  The rate of production of new species has never been equaled since the Cambrian Explosion.  Prior to this event, there were a very few species of animal, each growing to only a very small size.  During the Explosion, however, there were huge numbers of new species many with completely new and previously unseen body parts! 

            Trilobites were ancient animals that came forth during the early portions of the Cambrian Explosion.  They look like nothing that is alive today although they resemble the horseshoe crabs and pill bugs.  The trilobite fossils range in size from the microscope to nearly 3 feet in length.  They have numerous spines, great helmet-like heads, and a variety of peculiar eyes.  They are quite complicated fossils left over from complicated creatures.  At many localities throughout the world with sedimentary rocks of approximately 550 million years of age, the first obvious fossils are trilobites perched on top of thick sequences of rock apparently devoid of fossils.  This observation seems to indicate that complex life appeared upon the earth without precursors; as though the orchestra began to play without any warm up.

An English geologist Adam Sedgewick is credited with naming the geologic unit of time the Cambrian era, defined by a thick layer of sedimentary rock found in Wales which contained a characteristic group of fossils including the trilobites.  With modern dating techniques, this layer of Cambrian era sedimentary rock containing evidence of the Cambrian explosion is dated between 540 and 490 million years ago.  The earliest portions of Cambrian era rock is defined as that portion where the first trilobite fossils could be found – not only in Wales – but throughout the world.   This definition prevailed for over a hundred years and has only recently been changed.  Today, the start of the Cambrian era is where the first trace fossil - the fossilized record of animal behavior rather than the animal itself – is located.  The Cambrian explosion has certainly been a difficult observation for evolutionists to explain, and was especially worrisome for Charles Darwin.  In his seminal work, On the Origin of Species, Darwin speculated that the era before the Cambrian era must have “swarmed with living creatures.” However – where were the fossils of these “swarms?”  If Darwin were correct, then a long period of evolutionary change with simpler precursors should be evident in the fossils to eventually through evolutionary change produce the complex creatures collected by Sedgewick.  Darwin was never able to refute this most stringent criticism of his theory, but instead rallied against the imperfections of the fossil record, believing that there must be a missing interval of strata just beneath the trilobite layer.

Darwin was at least partially right, but science had to develop further.  Modern radiometric dating techniques now date the Precambrian/Cambrian boundary to 543 million years ago.  Interestingly, the “Middle Cambrian is dated at 510 million years ago, whereas the oldest trilobites are no more than 522 million years old.  Therefore, the bulk of Cambrian time was “pre-trilobite.”  It was in this pre-trilobite era that animals developed which were tiny and lacked skeletons so they rarely left obvious traces in the fossil record.  They are indeed hard to detect unless special processing techniques are used to extract them from their entombing matrix.  But, what we do see is the rapid, amazing development of complicated, animal life with fully developed skeletons developing from unicellular life over the time period of only about 12 million years – a phenomenally short time as even the staunchest evolutionists admit

The Eukaryotic Cell

            There were three major jumps in complexity which occurred in relatively short time with few if any precursors; seemingly new creations.  These three major jumps are the formation of the first eukaryotic cell, the first multicellular organism, and the Cambrian explosion of life.

            The first life forms appeared on this Earth about 3.5 billion years ago as the bacteria.  From what we can tell from the records these ancient life forms have left behind, they are structurally very similar to bacteria in existence today.  Their biochemistry his now more diversified as some are more specialized to live in environments without oxygen while others can only life in an environment rich in oxygen; some might live better in cold environments while others crave warmth.  However, the basic structure of bacteria has remained essentially unchanged over the billions of years since their first appearance. 

            Eukaryotic cells then burst on the horizon about 1.5 billion years after the first bacteria.  While the simple bacteria (prokaryotic cells) did not change in form over the billions of years since their appearance, exactly the opposite is true concerning the eukaryotic cells.  The eukaryotic cell is the basic prerequisite for the formation of complex metazoan (multicellular) life from which arose more complicated life forms including plants, fungi, various protists forms (protozoans, ciliates), and ultimately animals life dogs, cats, and finally humans.  The eukaryotic cell is fundamentally different from the earlier prokaryotic cells.  Evolutionary biology has a difficult time accounting for the arrival of this new life form as different from the prokaryotic cell as a human is from a sponge.  There are no intermediaries seen in the fossil record; rather, this fundamentally new life form suddenly appears.

            The eukaryotic cell has seven major characteristics that differentiate them from their more primitive cousin – the prokaryote,

  • Eukaryotes can perform sexual reproduction (prokaryotes primarily use asexual reproduction)

  • Eukaryotes have more flexible walls that allows them to engulf food particles through a process known as phagocytosis

  • Eukaryotes have an internal structure system composed of tiny protein threads that allow them to control the location of internal organ systems.  This structural system also helps the cell duplicate their DNA into two identical copies during cellular division.  This process is more precise than the simple splitting of DNA that occurs in the prokaryote cell

  • Eukaryote cells are much larger than prokaryotes; they usually have cell volumes that are at least 10,000 times that of the average prokaryotic cell.  The eukaryotic cell has an internal architecture and salt balance regulating systems that are far more advanced and precise than that found in the prokaryotic cell making this larger structure possible.

  • Eukaryotic cells have much more DNA than prokaryotic cells – usually 1000 times as much!  Furthermore, the DNA found in eukaryotic cells is stored in strands or chromosomes, and it is found in multiple copies in every cell,

  • Eukaryotes have many other enclosed organs within their cells including the mitochondria (which produce energy), and chloroplasts (with chlorophyll to produce energy in plants)

  • In eukaryotes, DNA is contained within a membrane-bound organelle, the nucleus,

The first multicellular organisms were most likely formed from prokaryotic cells.  Cellular slime molds are multicellular, as are some cyanobacteria.  However, development of these prokaryotic cell forms did not seem to advance any further; this was an evolutionary dead end.  These multicellular life forms have existed on the Earth for billions of years with little if any further development or differentiation. 

The first jump from single-celled organisms to multiple celled organisms seems to also have happened suddenly.  Single celled eukaryotic organisms have a cell wall which helps to insulate them against the environment.  This cell wall was shed in order for cells to aggregate and communicate amongst themselves.  Furthermore, cellular differentiation had to occur, such that some cells might become gut cells, while others carry on nervous system functions.  Also, while there are a few very simple animals with only two tissue type plan (ectoderm and endoderm), most multicellular animals have three tissue types – ectoderm, endoderm, and mesoderm.  Finally, these multicellular animals had to develop a means of reproduction, locomotion, nutrition, and defense.

The largest and most primitive of higher eukaryotic organisms are the sponges.  These creatures have different cell types which perform specialized tasks, but there is very little in the way of communication and interaction among cells.  There is no intestinal cavity for processing food, and there is no nervous system.  New organisms with similar structure, and others with vastly more complex structures sprang suddenly into existence at the time of the Cambrian explosion.  There were at least 50 to75 new phyla or large groupings of animals that suddenly appeared; many of which have since become extinct; however, since this Cambrian time period, no new phyla have come forth.  It is as if the creative processes were briefly turned on to produce vast new and complex forms of life in the period of only several million years only to be suddenly turned off so that no new animal phyla are created for the next several hundred million years.  Scientists perhaps for obvious personal bias primarily study animal life and development, but similar sudden creations of new plant phyla also appeared but are much less studied.

Naturally, the problem of the Cambrian explosion poses a difficult challenge to conventional evolutionary theory on several levels. 

  • First, there are no intermediate forms showing, for example, how a sponge of mollusk might develop into a “higher” animal such as a fish.  The problem of intermediate forms is present for the entire evolutionary theory as a whole – not just for the Cambrian explosion. 

  • Next, it is difficult to explain (although attempts have been made (see below) how evolution was briefly turned on to such an extent as to create vast quantities of hugely different organisms – only to be turned off again. 

  • Finally, the essential problem of evolution – exactly how macroevolution occurs – is still present.  It is one thing to say that there were factors favorable for life shortly after a snowball earth or during time periods when new landforms arise, but it is quite another to develop a theory as to exactly how macroevolution occurs.  In other words, how an organism suddenly develops vastly new structural characteristics that are fine-tuned to the organism which have never been seen before has never been satisfactorily addressed.

Atmospheric Oxygen Levels.  

           Most of the proposed explanations for the vast explosion of new life forms during the Cambrian time epoch concern environmental changes that also seem to be occurring.  One of the most important physical changes that was occurring on the earth was in the levels of oxygen.  It is proposed animal life was encouraged by this increase in oxygen levels throughout the planet.

           The current oxygen rich atmosphere is a late occurrence on this planet.  For much of the Earth’s existence, the level of oxygen would not have supported animal life as we know it – certainly not the fast moving animals with which we are most familiar.  Modern animals use large volumes of oxygen to move their bodies quickly across land; such would be difficult or impossible with an anaerobic (oxygen lacking) respiratory system.  Humans use about 250 cc of oxygen every minute at rest – much more with exercise – without which we would quickly cease to function.

           Bacteria seem to be the primary cause for the earth’s oxygen rich atmosphere with the aid of multiple other factors.  Cyanobacteria are those which produce oxygen as a waste product but only in minute amounts.  Early in the Earth’s history, most of the oxygen they produced was absorbed quickly by the Earth’s crust and mantle as metal oxides were formed.  These reducing minerals acted as a sponge soaking up much of the oxygen produced over countless millions of years.  With tectonic activity, new portions of the Earth’s mantle was repeatedly exposed to the atmosphere sucking up vast quantities of oxygen while oxygen rich exposed portions of the mantle were submerged below the Earth’s surface.  Over time, the quantity of reducing materials in the mantle slowly diminished and less oxygen was absorbed. 

           Models indicate that between 3.9 billion and 2.7 billion years ago, reducing minerals in the mantle and crust efficiently absorbed most of the oxygen produced by the cyanobacteria.  However, between 2.7 and 2.2 billion years ago, gases released from volcanic activity had lost much of their oxygen absorbing ability, and from 2.2 years ago until the emergency of large quantities of animals during the Cambrian explosion, the atmospheric oxygen concentration slowly increased.  One of the most important aspects of this oxygen production was the removal of methane from the Earth’s atmosphere.  Methane is a gas (“swamp gas”) produced by decaying material among other causes.  Even in small quantities, methane is one of the greenhouse gases (such as carbon dioxide) and effectively traps heat into the Earth’s atmosphere.  As the increasing oxygen levels removed more and more methane, the earth cooled and went into several periods of glaciation, the last one occurring just before the Cambrian explosion.

           It is presumed by evolutionists that perhaps one stimulus to advanced animal life was the rising level of oxygen in the Earth’s atmosphere perhaps giving them an evolutionary survival advantage.  However, the precise mechanism underlying the differentiation and production of so much new life merely by increasing oxygen levels has not yet been defined but remains only a theory – a very speculative theory – at best

Biological Changes.  Another mechanism proposed by evolutions to explain the Cambrian explosion is that the biological changes were themselves triggering some of the physical events.  In this scenario, the common use of calcium carbonate shells by newly evolved animals changed the way calcium was distributed in the oceans.  Similarly, organisms may have favored the formation of phosphorus and not the other way around.  

Nutrient Availability.  Abundant evidence suggests that at the end of the last Precambrian era there was a relatively sudden and dramatic increase in the amount of available nutrients.  It has been proposed by some that these nutrients placed a great pressure on the production of more life that also somehow increased evolutionary pressure.  While this may initially seem plausible, upon closer consideration several problems arise.  First, the problem of producing completely new and different animal life is not explained by adding more nutrients to a population!  Indeed, the opposite would seem to be the case for more nutrient production would favor the survival of weaker, less adaptive creatures rather than encouraging the development of new and different creatures.

Cambrian Cessation

The Cambrian explosion has long been explained, as noted above, as an explosion of new life forms brought about by two events;

·        first, the destruction of a large portion of currently existing life to open up niches for new life forms to occupy (as explained by the recent snowball earth), and

·        second, the sudden development of an environment very favorable to life produced an evolutionary pressure toward new and different animals. 

Notice that this hypothesis never addresses the crucial of exactly how extra food might induce development of a new animal phylum, but rather merely assumes that our knowledge is too fragmentary concerning evolution to understand fully the events involved.

           Any explanation that tries to explain the Cambrian explosion using methodological naturalism or evolutionary principles must also explain why there have been no new phyla created since the explosion.  The Cambrian explosion is characterized by the sudden appearance of vast quantities of hugely different and largely unrelated life forms that apparently could not have arisen from each other.  Any explanation that tries to demonstrate how this explosion occurred must also explain why, under apparently favorable life sustaining conditions over the intervening millions of years, no new animal phyla have arisen.  The lack of a realistic explanation becomes even more acute when history subsequent to the Cambrian explosion is considered.  During the intervening 543 million years, there have been several other major mass extinction events when majorities of species living on the Earth were destroyed.  The most catastrophic of these was the Permo-Triassic mass extinction of about 250 million years ago which eliminated an estimated 90% of marine invertebrate species.  Even after this mass extinction, no new phyla appeared even though the number of species approximated levels similar to the very low species diversity found before the Cambrian explosion.  The development events during the Cambrian and Early Triassic period are dramatically different.  Both produced new species, but the Cambrian event resulted in the formation of many new body plans whereas the Triassic event resulted only in the formation of several new species using body plans that were already well established.

           The Cambrian explosion is also characterized by the sudden appearance of complete ecosystems where there are predator and prey relationship, proper food for the various animals, and highly complex interactions among animals.  Thus, it would seem as though the animals which did appear during this brief time interval were those which could survive with each other.  Indeed, an ecosystem implies mutual benefit among the creatures involved so that they helped each other survive and flourish in their particular environment.  The evolutionist then not only has to explain how the Cambrian explosion happened whereby tens of new phyla suddenly appeared (a phenomenon which has not since been duplicated), but also how these brand new animals seemed to be made for each other.  The hypothesis that seems to fit best with the data is that there was a Creator which so fashioned these animals, for the Cambrian explosion is just what one might expect under these circumstances.

Plate Tectonics

[Origin] [Importance to Life] [Global Temperature]
[Earth's Magnetic Center] [Production of Plate Tectonics]

           Naturalists have used the recent discovery of multiple new planets in nearby star systems to imply the ease with which life might spring forth.  Indeed, if Earth-like planets are plentiful then so must life be plentiful throughout the galaxy and Universe.  Most of the planets discovered so far would be very poor candidates indeed for life as they are very large planets with eccentric orbits around their star.  These planets would not be at all similar to Earth and they would generate conditions that could not support carbon based life.  Indeed, it turns out that not only must the planet size be similar to the Earth in order to sustain life, but it must be similar to the earth in several other ways as well.  Perhaps the most surprising requirement for intelligent life is the necessity for plate tectonics.

           It is interesting when looking at mountains on other planets in our solar system that such mountains are always single and never form in chains.  There is no equivalent to the Rockies, the Andes, the Himalayas, or the other linear mountain ranges that are so common on this planet.   It seems odd to consider that plate tectonics could be a key to the evolution and preservation of life on Earth.  The following factors will illustrate this point,

·        First, plate tectonics promotes high levels of global biodiversity.  The major defense against mass extinction is high biodiversity, and the factor on Earth that is most important toward maintaining biodiversity is plate tectonics. 

·        Second, plate tectonics proves to be the Earth’s global thermostat by recycling chemicals crucial to keeping the volume of carbon dioxide in our atmosphere relatively uniform, and thus it has been important in enabling liquid water to remain on the Earth’s surface for more than 4 billion years,

·        Third, plate tectonics is the dominant force that causes changes in sea level, which are vital to the formation of minerals which keep the level of global carbon dioxide (And global temperature) to remain constant,

·        Fourth, plate tectonics created the continents upon which animal life might thrive.  Without plate tectonics, Earth might look much like it did after the first billion years of its existence; a watery world with only a few isolated volcanic mountains.

·        Finally, plate tectonics makes possible some of our planet’s most potent defense systems; the magnetic field.  Without the magnetic field, the Earth and its cargo of fragile life, would be bombarded by a constant barrage of  potentially lethal cosmic rays, radiation, and solar wind. 

Origin of Plate Tectonics

Geologists of the last century had little difficulty understanding the nature of volcanoes.  Hot magma from deep within the Earth rose to the surface regions and spewed forth lava, ash, and pumice to form a cone.  However, geologists had much more difficulties understanding how non-volcanic mountains formed.  In 1910, an American geologist Frank B. Taylor proposed the continents were drifting upon a soft cushion and when these continents collided with each other, great mountains sprang up at their junction.  This theory was considered heresy and was immediately decried by nearly all other geologists and geophysicists who could envision no means whereby this drift might occur.

Taylor’s hypothesis, however, kindled a spark of interest that would not be suppressed.  Soon, other scientists began to toy with the idea and searching for supporting evidence.  The most successful of these new converts to tectonics was Alfred Wegener who, from 1912 until his death in 1930 was obsessed with this idea.  Wegener was the first to show how the fit of various continents suggested an idea that all continents were once united into a single super-continent.  Furthermore, Wegener used paleontological arguments to strengthen his claim.  He noted that presence of similar fossil species on land masses now widely separated could have come about only if the various continents had once been in contact.  The greatest barrier to the theory of plate tectonics was the lack of any mechanism whereby continents might move.  It was eventually discovered that the answer to this question relies on the different phase states of the Earth’s layers.  These different layers are known as the crust and upper mantle, and the presence of thermal convection to these regions.  Arthur Holmes proposed that the upper mantle might act like boiling water and provide moving cells of material upon which the continents could drift.  The fluid rises as it is heated, and when it eventually gets close to the surface it either spews forth in a volcanic disruption or it begins to flow parallel to the Earth’s surface.  This material then cools and sinks back down into the planet to be reheated once more. 

           Evidence has subsequently come forth from many sources that the theory of continental drift was correct.  We now understand that all continents are masses of relatively low-density rock embedded in a ground mass of more dense material.  The low density rocks have an average composition of granites whereas the higher density rocks which make up the ocean crust are basaltic in composition.  Because granite is less dense than basalt, the granite-rich continents essentially “float” upon a thin bed of basalt.  The Earth has a radioactive interior that constantly generates great quantities of heat as the radioactive elements break down into their various isotopic by-products.  The heat which is produced by this radioactive decay then rises toward the surface and creates great convection cells of hot, liquid rock in the mantle just as Arthur Holmes described.  These gigantic convection currents carry the thing brittle outer layer – known as plates – upon them.

           Many kilometers below the Earth’s surface, the familiar rocks of our crust are exposed to great temperatures and pressures which make them behave in different ways from that with which we are familiar.  The plates are of varying thickness, and their “bottom” is thought by many scientists to coincide with a 1400 degrees Centigrade isotherm where the rock melts into a plastic-like medium.  The difference in viscosity between the overlying plate and the underlying region of lowered viscosity allows the relatively rigid crust to slip as a unit over the mantle.  Plates composed of oceanic crust and mantle are about 50 kilometers thick, whereas the plates with continental crust average about 100 kilometers in thickness.

           We will first discuss the ocean basins to understand the process of tectonics.  The crust lining the bottom of the oceans is largely composed of basalt, the same type of volcanic rock that makes up the Hawaiian Islands.  This material originates within the deeper mangle regions of the Earth where it is heated by radioactivity and ascends toward the surface along rising zones of the convection cells.  As this dense mantle material rises toward the surface, it moves into regions of ever decreasing pressure and the lower density liquid separates from the higher density material rising to the surface as “lava.”  This lava then enters a huge crack in the surface of the planet formed by the pulling apart of two plates when it comes into contact with the ocean water and solidifies into basaltic ocean crust.  The plates then move further apart and more lava then rises to take its place forming a slowly moving conveyor belt of solidified basaltic material. 

           The basalt that forms at the bottom of the oceans in the cracks forming between separating plates has a lower density than the basalt deeper in the mantle because it contains a much higher percentage of silicon.  This basalt has differentiated from its parent compound called peridiotite.  This differentiation from a peridiotite composition to a basaltic composition is the final step of oceanic crust formation.  The composition of continents is different from that of the ocean floors being composed mainly of granite and andesite.  Granite has a characteristic speckled appearance compared to the brown to black color of basalt.  This speckling comes from their containing even more of the white silica compound.  The major step in the formation of continental crust is the differentiation of granite from material of a basaltic composition.  This process takes place in several steps, but the key ingredient is water, and the process is that of subduction.

           Over many millions of years, the oceanic crusts formed from basalt rock gradually moves apart from its birthplace as newer basalt rock gets laid down.  Eventually, however, the spreading ocean flow can spread no further.  As the basalt rock cools it gets denser; also, it acquires some heavy contaminants over time of dense igneous rock called gabbro.  Eventually, the additional weight of the solidifying basalt rock and gabbro causes it to sink, down to as deep as 650 kilometers.  Eventually, the convection cell begins its downward journey back into the deep mantle carrying its veneer of oceanic crust back down with it in regions known as subduction zones.

           Subduction zones are long, linear regions where oceanic crustal material is driven deep into the Earth, apparently not so much as being pushed down but rather by sinking. It is near and parallel to these subduction zones where mountain ranges occur.  These mountain ranges form partly as a byproduct of the collision of two plates which causes the leading edges to crumple and buckle, and partly because of the upwart movement of hot magma.  This magma eventually solidifies into granites and other magmatic rocks parallel to the subduction zones. Most of the world’s volcanoes are located near these subduction zones further bearing testimony to the fundamental link between subduction and mountain building.  Mountain ranges are not found on any other planet in the solar system illustrating that only the Earth has tectonic plates and subduction zones.

           Volcanoes occur along subduction zones due to the different composition of magma and basalt.  Basalt when first formed at the ocean bottom is unhydrated.  However, water gradually seeps into the rock and is added to the crystalline structures of the rock hydrating the basalt.  Water poor minerals actually can incorporate rather large amounts of water into their structure.  The newly hydrated minerals have a lower melting point when compared to the unhydrated variety, so as the hydrated basalt descends into the subduction zone, it is readily melted and rises back toward the surface.  The magma from a volcano eventually cools into andesite (named after the Andes mountains), and granite.  The crucial fact is that the granite that makes up the continents is of lower density due to its higher silicone content than the basalt upon which it rests, and which is on the bottom of the oceans.  The less dense continents float on a sea of basalt and can never be sunk into a subduction zone.  Continents once formed cannot be destroyed (although they can be eroded).  The continents can be split and fragmented to drift from one place to another, but their basic volume cannot be significantly reduced.  Through time, in fact, the number and volume of continents on the Earth has gradually increased.  Geologist Davie Howell, in his book Principles of Terrane Analysis, estimates that eh volume of continents increases by about 650 to 1300 cubic kilometers of rock each year.

           Tectonic plates can intersect with each other in three different ways:

·        The bottom of oceans where spreading two plates spread apart; hot magma then arises between the two plates producing new ocean floor,

·        Areas where the plates grind side by side such as at the San Andreas fault,

·        Regions where plates collide at the subduction zones, which are associated with linear chains of active volcanoes such as the Cascades and the Aleutian Islands <![endif]>

Plate Tectonics are Important to Life

The majority of Earth’s biodiversity today is based on continents and there is no reason to believe that this relationship has changed over the past several hundred million years.  As the continents have grown through time, they have affected the planet’s overall reflectivity (albedo), he occurrence of glaciation events, oceanic circulation patterns, and the amount of nutrients reaching the oceans.  All of these factors have biological consequences and affect global biodiversity.  Plate tectonics promotes biodiversity by increasing the variety of habitats in which animals can life.  A world with mountainous continents, oceans, and myriad islands such as those produced by plate tectonics produce a far more complex world than would totally land or ocean dominated planets.  Furthermore, changes in continental position would affect ocean currents, temperature, seasonal rainfall patterns, the distribution of nutrients, and patterns of biological productivity.  The deep sea is the area on Earth that has the least biodiversity; over two-thirds of all animal species life on land, and the majority of marine species live in the shallow-water regions that would be more affected by plate tectonic movements.

As continental positions gradually change through time, the relative abundance of north-south and east-west coastlines can change.  Also, the larger the continents the lower the environmental heterogeneity and the fewer species and less biodiversity that would be formed.  If many or all of the continents were welded together into a super-continent, biodiversity with would be expected to be lower than if the land masses were broken up into many smaller areas.  

Plate Tectonics and Global Temperature

One of the most interesting facets about plate tectonics and why we have dwelt upon this arcane topic is that it is probably essential for any form of advanced life to exist on a planet.  For animal life based upon DNA to exist, water is necessary and must be abundant on a planet’s surface.  In desert environments, there is little biodiversity, but in rainforests in the same latitude, biodiversity is considerable.  For complex life to be attained and then maintained, a planet’s water supply must,       

·        Be large enough to sustain a sizable ocean on the planet's floor,

·        Have migrated to the surface from the planet’s interior,

·        Not be lost to space, and

·        Exist largely in liquid form.

Plate tectonics play a role in all four of these criteria.

            Earth is about 0.5% water by weight, and must of this water arrived among the planetesimals that took part in the planet’s early development and accretion.  Incoming comets dumped other volumes of water here after the Earth accreted.  The relative importance of these two major processes is largely unknown at this time.  Once water has arrived on the surface of a planet, its maintenance becomes the primary requirement for attaining and sustaining animal life.  The maintenance of liquid water is controlled largely by planetary temperatures, which are by-product of the greenhouse gas volumes of a planet’s atmosphere.  The temperature of Earth (and other planets) is determined by several factors.  The first is related to the energy input form the sun.  The second is a function of how much that that energy input from the sun is reflected back into space.  The third is related to the volume of “greenhouse gases” maintained in a planet’s atmosphere.  The volume of greenhouse gases in the atmosphere is not constant but is are eventually broken down or undergo a change in phase; therefore, if supplies are not constantly replenished, the planet will grow colder until the freezing point of water is reached.  Once water freezes, then the planet will grow colder rapidly as ice will reflect heat back into space more efficiency than does liquid water.  The most important sources of planetary greenhouse gases are volcanic eruptions which occur on planets with or without tectonic plate systems.  Even so called “dormant” volcanoes are venting carbon dioxide into the atmosphere in great quantities.  On any planet with volcanism there is usually an abundance of greenhouse gases – too much in some cases (perhaps becoming so in the case of the Earth!), and this is where plate tectonics becomes crucial.

            Greenhouse gas composition are byproducts of complex interactions among the planet’s interior, surface, and atmospheric chemistry.  One of the most important by-products of plate tectonics is the recycling of mineral and chemical compounds bound up in any planet’s sedimentary rock layer.  On non-plate tectonic worlds, vast quantities of sedimentary material are formed by erosion and then form layer upon layer of rock, permanently taken out of planetary circulation.  The only means for these sedimentary rocks to be exhumed is through some process leading to mountain building.  However, mountain building on non-plate tectonic worlds is limited to the formation of large volcanoes over hot spots. However, with plate tectonics, the interaction of plates, formation of mountain chains, and the process of subduction all lead to a recycling of many materials.  One of the most important of these materials is putting carbon dioxide back into the atmosphere.  As limestone is subducted deep into the mantle, it changes in the process returning large amounts of carbon dioxide into the atmosphere.  Since carbon dioxide is a greenhouse gas, this leads to global warming keeping liquid water in the liquid state.

            The burning of large amounts of hydrocarbon fuels has largely produced the recent onset of global warming on our planet.  We have become more interested in the reverse process; that is global cooling.  The most important element in reducing atmospheric carbon dioxide is the weathering of minerals known as silicates such as mica and feldspar (contained within granite).  The basic chemical reaction is CaSiO3 + CO2 = CaCO3 + SiO2.  When the first two chemicals in this equation combine, limestone (calcium carbonate) is produced and carbon dioxide removed from the atmosphere.  The basic process responsible for getting granite to undergo this chemical reaction is weathering.  The rate of chemical weathering increases as a planet warms as a warmer planet produces more rain and severe weather that in turn produces more weathering of granite surfaces.  As the rate of weathering increases more silicate material is made available for reaction with the atmosphere and more carbon dioxide is removed thus causing cooling.  Similarly, as the planet cools, weathering will reduce and the carbon dioxide content of the atmosphere will begin to rise causing warming to occur.  The carbon-silicate weathering cycle cannot work efficiently on planets without plate tectonics.

Plate Tectonics and the Earth’s Magnetic Field

            Cosmic rays are high-energy particles that constantly bombard the earth, traveling at velocities approaching the speed of light.  They come from many sources, including the sun and sources from deep space including supernovae, the explosions of stars.  These catastrophic events send great numbers of high-energy particles into space some of which bombard the Earth.

            In The Search for Life in the Universe, by D. Goldsmith and T. Owen, the authors speculate that without some form of protection from these high-energy particles, life on earth would be extinguished within several generations.  Fortunately, the vast majority of these particles are deflected by the Earth’s magnetic field.  The innermost layer of our planet – the core – is made up of mainly iron in the liquid state.  As the planet spins, it creates convective movements in this liquid that produces a giant magnetic field surrounding the entire planet.  The convections cells within the liquid core is enabled by heat loss; heat must be exported out of the core in order for the whole system to work.  Joseph Kirschvink of Cal Tech has suggested that without plate tectonics, there would not be sufficient heat loss across the core region to produce the convective cells necessary to generate Earth’s magnetic field; thus, there would be no convective currents and no magnetic field.  Furthermore, the magnetic field around the earth protects our atmosphere from being blown away by solar wind – particles periodically aimed at the earth from the sun in great volumes produced by solar storms.  There is good reason to believe that without plate tectonics, there would be no magnetic field and perhaps no animal life.

            Plate tectonics plays at least three crucial roles in maintaining animal life; it promotes biological productivity; it promotes diversity; and it helps maintain equable temperatures necessary for animal life.  It is difficult to determine at this time the rarity of plate tectonics.  However, there is evidence that the emergence of plate tectonics on Earth depended upon the presence of a large companion moon.

Production of Plate Tectonics

The adequate abundance of silicates for carbon dioxide removal requires an active plate tectonics system.  The Earth needs three things for plate tectonics to occur: a stable dynamo (electromagnetic generator) at its core, a powerful interior source of radioactive decay, and an abundant supply of liquid surface water. The presence of any one of these processes would be "unexpected" by natural processes but for the three of them to be present simultaneously is mind boggling.

The Earth's dynamo works with enduring efficiency because several independent factors fall within certain narrow ranges.  These factors include,

·        Solar and lunar gravitational torques,

·        The frequency or period of the core’s gyrations (its “precession”),

·        The ratio of the inner core radius to the outer core radius,

·        The relative abundances of silicon, iron, and sulfur in the solid inner core,

·        The outer core’s magnetic Reynolds number (a measure of viscous flow behavior in the magnetic medium),

·        The ratio of inner core magnetic diffusivity (a measure of how well a magnetic field diffuses throughout a conducting medium) to outer core magnetic diffusivity,

·        And the viscosity of the material at the boundaries between the solid inner core and the liquid outer core, also between the liquid outer core and the mantle

The Earth's radioactive elements are also necessary to provide the long-term heat needed for volcanism to occur - also important in plate tectonics.  Two very unlikely events brought the necessary radioactive elements to the earth for this to occur.  First, the gas cloud that condensed into the Sun and its planets formed adjacent to both the fresh remnant of a Type 1 supernova and the fresh remnant of a Type II supernova.  Each contributed radioactive and life essential heavy elements to the emerging solar system.  Second, between 4.4 and 4.5 billion years ago, a Mars sized object crashed into the Earth hitting at the proper speed, angle, and location to transfer its radioactive and other heavy elements to the Earth's interior.  The lighter material of both the collider and the Earth formed a debris cloud that eventually condensed into the Moon.

A problem arises, however, because radioactivity naturally declines with time.  Therefore, the energy released from radioactive decay contributed less and less toward maintenance of plate tectonic activity.  Thus, if erosion were to continue at the same rate, eventually all the continents would be eroded into the ocean.  However, the Moon acts as a tidal brake with its gravitational tug gradually slowing the Earth's rotation rate.  Strategically, this reduced rotation rate results in a just-right reduction of erosion.

Life itself has also provided a critical piece in the Earth temperature stabilization picture.  The essential species and the entire matrix of life forms supporting their existence - in other words, entire ecosystems - existed at the right population levels in the right locations at the right times to assist in controlling the quantity of greenhouse gases, that in turn has kept the Earth's temperature in life's safe range for nearly four billion years.  Furthermore, this regulation of the Earth's surface temperature in the context of a steadily brightening Sun mandates a carefully timed progression of new life forms.  For example, the most advanced plants on Earth have vascular bundles which are more efficient than other more primitive plant species in accelerating erosion.  Thus, as plate tectonics and erosion gradually decline, Earth needs more and more of these advanced plants to sustain adequate carbon dioxide levels to maintain proper temperature.  This increase in advanced plant forms has coincided with a commensurate reduction in more primitive forms which are less efficient in producing erosion.

Unique Earth - Sun Relationship

Perhaps one of the most wonderful and unique relationships in natural science has been played out over the past 4 billion years since our current solar system came into being.  This relationship is between the Earth and the Sun, and concerns how the Sun has been gradually brightening over that time period, and how the Earth has been simultaneously and independently changing so as to assume a relatively stable temperature to support the emergence of life.  Without this delicate balance, the Earth would have become either too hot or too cold to allow for the emergence of intelligent life.  While we worry today about global warming and the influence this warming might have upon the environment, a much greater threat to life on Earth has existed over time with the gradual brightening of the Sun. 

The Sun was about 30% less luminous about 3.8 billion years ago when life first emerged on this planet than it is now.  Knowing that a drop of only 1-2% of the Sun's brightness would plunge the planet into an overwhelming ice-age (assuming current atmospheric conditions), or an increase of only 1-2% would boil away the oceans and cook all life, the question emerges as to how life survived during an increase in the Sun's luminosity of 30%.  This has been called the "faint Sun paradox" and emerges as one of the most elaborate and wonderful interplay of forces to permit a stable Earth environment.

The Sun was born about 4.5 billion years ago with the gravitational collapse of a huge gas cloud, and with this collapse became progressively more hot until nuclear fusion could occur.  Initially, these nuclear reactions were unstable causing the luminosity of the Sun to fluctuate widely.  Furthermore, during the initial 500 million years of the Sun's existence, the emission of ionizing radiation - in particular X-rays - was fifty times greater than current levels.  Thus, during the first half-billion years of the Sun's existence, the Earth would have been a very inhospitable place for the emergence of life.

Temperature at the core of the Sun reached 17 million degrees Centigrade igniting the fusion of hydrogen into helium gradually increasing the amount of helium.  A greater percentage of helium causes the nuclear furnace to burn more efficiently gradually increasing core temperature.  With greater efficiency of nuclear fusion the Sun's surface temperature also gradually increased as well and will further increase until all of the hydrogen at the Sun's core has been converted into helium.  This whole process takes about 9 billion years for a star the size of the Sun, so we are now at about exactly the half-way point.

The paradox that exists, however, is even though the Sun's luminosity was about 30% less than current levels 3.8 billion years ago when life first emerged, the surface temperature of the Earth was only marginally different than today.  Both liquid water and life began to abound at this point, both of which would be impossible were the Earth's surface temperature different.  This balancing act continued for the next nearly 4 billion years whereby the Earth compensated for different solar luminosity through a complicated series of environmental changes.

Initially, the reduced luminosity of the Sun was compensated for by an increase in greenhouse gases, especially carbon dioxide.  These gases served to insulate the earth conserving heat from the less luminous sun producing a climate which approximates our own.  As the ancient Sun slowly brightened, there was a continuous supply of silicates exposed to the atmosphere (containing silicon, oxygen, and metals that comprise more than 90% of the Earth's crust), and a continuous burial of carbon-rich organic matter the decomposition of which would produce additional carbon dioxide.

In the presence of liquid water, silicates chemically react with carbon dioxide from the atmosphere forming carbonates and sand in the process.  As exposed silicates react with carbon dioxide, new silicate deposits need to be brought up to the surface.  This requires the plate tectonic system to push silicates above the ocean floor producing islands and continental land masses.  Then, erosion must "plough" the crust so that more silicates are constantly brought into contact with the atmosphere.  Erosion is a complicated process.  Multiple processes determine its efficiency including the Earth's rotation rate, average rainfall, average temperature, average slope of the land masses, and the types and quantities of plant species on the land masses.  If erosion proceeds too slowly, silicates cannot maintain an adequate pace of carbon dioxide removal - too much erosion removes too much, too quickly.

Organisms also are involved in the planetary heat stabilization process.  Photocynthetic plants, plus bacteria and methanogens (which remove methane from the atmosphere - another greenhouse gas), also work to take water, methane and carbon dioxide from the atmosphere chemically transforming them into fats, sugars, starches, proteins, and carbonates.  If these compounds get buried before they can decay or be eaten by other organisms, they help in the task of reducing greenhouse gases.  (Humans have benefited greatly as they form a wealth of biodeposits such as limestone, marble, fossil fuels, and concentrated metal ores).  

Thus, fine tuning removal of the greenhouse gases requires participation by factors that govern silicate erosion, tectonic activity, volcanism, and extent of plant life.

The Moon, Jupiter, and Life on Earth
[Moon] [Jupiter]

There is strong likelihood that without the Moon and Jupiter, animal life would not have developed on Earth.  Both are key elements for different reasons.

The Moon.  The likelihood that an Earth sized planet would have such a large moon is small as the conditions suitable for moon formation were common for the outer planets but rare for the inner ones.  Of the many moons in the solar system, nearly all orbit the giant planets in the outer solar system. The only moons of the terrestrial inner planets are Phobos and Diemos – two tine (10 kilometers in diameter) moons of Mars.  Some of the moons orbiting the larger planets are huge; Ganymede orbiting Jupiter is nearly as large as Mars, and Saturn’s Titan is nearly that large but also has an atmosphere more dense than our own (although much colder).  The Moon is nearly a third the size of the Earth, and in many respect is more like a twin than a moon.  The only other case in the solar system with a moon comparable in size to its planet is in the case of Pluto and its moon Charon.

The Moon plays three roles that affect the development and survival of life on Earth,

·        It controls tides,

·        It stabilizes the tile of Earth’s spin axis,

·        It slows the Earth’s rate of rotation

Of these three factors, the most important is its effect on the tilt of the Earth’s spin axis relative to the plane of its orbit that is called its “obliquity.”  Obliquity is the cause of seasonal changes, and for most of the Earth’s history, its obliquity has not varied by more than a degree or two from its present value of 23 degrees.  Although the direction of the tilt varies over periods of tens of thousands of years as the planet wobbles – much like a spinning top – the angle of the tilt relative to the orbit plane remains almost fixed.  This angle is nearly constant for hundreds of millions of years because of the gravitational pull of the Moon.  Without the Moon, the tilt angle would wander in response ot the gravitational pulls of the sun and Jupiter.  The montly motion of our large Moon damps any tendencies for the tilt axis to change.  If the Moon were smaller or more distant or if Jupiter were larger or closer, or if the Earth were closer to or father away from the Sun, the Moon’s stabilizing influence would be less effective.  Without the stabilizing effect of the Moon, the Earth’s spin axis might vary by as much as 90 degrees.  Mars, a planet with the same spin rate and axis tilt but without a large moon, is believed to have exhibited changes to its tilt axis of 45 degrees or more.

            The tilt of a planets spin axis determines the relative amount of sunlight that fall on the polar and equatorial regions during the seasons and strongly affects a planet’s climate.  On planets with a moderate spin axis, the majority of solar energy is absorbed in the equatorial regions where the moon is always high in the sky.  The poles of such a planet are in total darkness for half a year and constantly illuminated for half a year.  The highest altitude that the sun reaches in the sky at the pole is exactly equal to the number of degrees tilt of the spin axis.  The planet Mercury provides an example of what can happen on a planet whose spin axis is nearly perfectly perpendicular to the plane of its orbit.  Mercury is the closest planet to the sun and most of its surface is very hot, although the poles are covered with ice.  The planet is very close to the sun, but as viewed from the poles, the sun is always on the horizon.  In contrast to Mercury’s lack of tilt, the planet Uranus has a 90-degree tilt, and one pole is exposed to sunlight for half a year while the other experiences extreme cold and darkness.

            Constancy of the tile angle is the factor that provides long-term stability of the Earth’s surface temperature.  If the polar tilt axis had undergone wide deviations from its present values, Earth’s climate would have been much less hospitable for the development of higher life forms.  One of the worst possibilities is that the excessive tilt axis could have led to the total freezing over of the oceans which might be difficult to recover from.  Astronomer Jacques Laskar noted,

“These results show that the situation of the Earth is veruy peculiar.  The common status for all the terrestrial planets is to have experienced very large scale chaotic behavior for their obliquity, which in the case of the Earth and in the absence of the Moon, may have prevented the appearance of evoluted forms of life. … We owe our present climate stability to an exceptional event: the presence of the Moon.”

In the distant future, the Moon will lose its controlling influence upon the Earth’s spin axis.  The Moon is slowly moving outward from the Earth at the rate of about 4 centimeters a year), and within 2 billion years it will be too far away to have enough influence to stabilize Earth’s obliquity.  The Earth’s tilt angle will begin to change as a result, and the planet’s climate will follow.  Furthermore, the sun’s increase in brightness will continue over the years, so that at the time when our planet’s spin axis begins to wander, the sun will be hotter, and both effects will decrease the habitability of the Earth.

Lunar Tides.  The Earth also experiences tides which are due to the gravitational effects of both the sun and the Moon.  These two bodies produce bulges in the ocean pointing both toward and away form the Moon and the sun.  The complexity of the Earth’s present tidal effects is well known by those making their living upon the ocean.  The effect is complicated as both the Moon and the sun cause ocean pulges on their respective near and far sides of the planet.  When the Moon lines up with the sun every two weeks, the tidal changes are at a maximum, and when they are 90 degrees apart in the sky, the range of tidal change is minimized.  With a smaller or more distant moon, the lunar tides would be lesser and would have a different annual variation.

            Soon after the Moon formed, it was perhaps 15,000 miles from the earth.  Instead of tides being a few meters high like they are today, they might have been hundreds of meters high with the moon was first created.  The extreme effects of such a close moon could have strongly heated the Earth’s surface.  There would have been tremendous flexing and torsion of the Earth’s crust, along with frictional heating which may have actually melted the rocky surface.  The retreat of the Moon is a natural consequence of gravitational pull between the Moon and the tidal bulges.  The Earth’s lunar tidal bulges don’t actually correspond with a line drawn from the Earth to the Moon, but rather lead ahead as the Moon orbits the planet.  This offset produces a torque that causes the Earth’s spin rate to decrease slowly and the distance between the Earth and the Moon to slowly increase.  As we have previously noted, laser measurements indicate that the Moon is receding from the earth at the rate of about 4 cm/year.  As the Moon gets further away, the Earth spins slower; coral records show that about 400 million years ago there were about 400 days in a year; the Moon was closer to the Earth and the Earth was spinning faster.  The coupling of these two effects is due to conservation of angular momentum, the same physical property that allows ice skaters to spin faster by pulling their arms closer to their bodies.  Interestingly, Triton, the large moon of Neptune, is in a retrograde orbit causing it to gradually move closer to its planet.  It will collide with Neptune in a few hundred million years.

The Moon’s Origin.  We have seen how the Moon stabilizes the Earth’s rotational axis and the seasons unlike the other planets.  And, we have also seen that the Moon itself is somewhat unusual with respect to its size; it is very large relative to the planet about which it rotates.  This leads us to the possibility that the origin of the Moon was also unusual – and this indeed appears to be the case.

            Before the Apollo Moon rocks were brought back home, the most popular theories regarding origin of the Moon held that it had formed cold and consequently that the rocks would retain records of the earliest history of the solar system. Theories on the lunar origin usually fit into three categories; it formed in place, it formed elsewhere and then was captured by the Earth, or it was somehow ejected from the Earth.  The newest model that seems to best fir the data indicates that a Mars-sized projectile hit the Earth.  Debris from this collision was ejected into space, and some if it remained in orbit where self-collisions would cause it to form a thin, orbiting ring of rocks similar to the rings of Saturn.  The Moon would form by collision and sticking – the processes of accretion that built most of the bodies in the solar system.

            There are several observations which are consistent with this collision model of lunar formation.  First, the moon is depleted of elements such as zinc, cadmium, and tin.  These are relatively volatile elements which would have been vaporized in the impact, and the resulting vapor would not have condense well in the heat of the moon’s surface and would then have been swept away into space and lost to the Earth-moon system.  Other elements and compounds that would have been lost in this collision include nitrogen, carbon, and water.  One of the surprising findings from the Apollo mission was the lunar samples were exceedingly dry.  Unlike rocks from the Earth, they contained no detectable water – not even in crystalline form.  Another remarkable property of the lunar samples was they are highly depleted in the iron-binding elements that tend to concentrate in the metallic iron cores of planets.  When planetary cores form from molten iron sinking into the planet’s center, the iron-binding elements such as platinum, gold, and iridium are incorporated into the falling iron and are highly depleted from the crustal and mantle rocks.  That the lunar rocks was similarly depleted of these iron-binding elements was unexpected because the moon does not have a significant iron core.  The mean density of the Moon is 3.4 times water, very similar to that of rocks on the lunar surface, and much lower than that of the Earth which is 5.5 times water.  If the moon had a significant iron core, then its mean density would be much higher than that observed.  Additionally, seismic and magnetic data also indicate no evidence of a significant iron core.  Finally, the moon and Earth are very similar with respect to similarities in trace element concentration as well as the composition of various isotopes.

            The Moon apparently formed as the result of a collision of a that was about a fourth of the final mass of the Earth, with an Earth that was about half its current size.  The collision between these two masses briefly resulted in a single mass; however, inertial effects literally ripped the mass apart resulting in material being shed into space.  Some of this material then fell back to Earth, while others accreted to form the Moon over a few tens of thousands of years.  When the Moon did form, it was about 15,000 miles away from the Earth’s surface.  With the Moon so close, the Earth would have been spinning at a rate that a day would only be five hours long.  The height of the tides would have been fantastic – hundreds of feet – resulting in an inertial drag upon the Earth that would rapidly have slowed its rotation.  It is possible that the Earth’s violent history was responsible for its plate tectonic structure.


            Jupiter is a giant ball of gas consisting of mostly hydrogen and helium.  In the interior of the planet, the gas acts more like a metal as the electrons are not restricted to one atom but rather move freely from atom to atom.  At pressures of a million atmospheres in the center of Jupiter, hydrogen is in a metallic state.

            Jupiter is over 300 times the mass of the hearty and is by far the most massive of the planets orbiting the sun.  Jupiter formed from accretion of gas from the solar nebula in addition to a few solids.  Jupiter formed very quickly, and its rapid growth had major effects upon other planets, especially those which were trying to accumulate mass just inside its orbit.  A terrestrial planet was in the process of forming midway between Mars and Jupiter, but because Jupiter formed first, its development was aborted.  This failed planet is now the asteroid belt, a region where original planetesimals and fragments still survive but were never assembled into a planet.  The largest of these rocks is the asteroid Ceres – a rounded object 1000 kilometers in diameter.

            The asteroids are the source of meteorites, and detailed examinations of these ancient rocks gives insight into the nature of planet formation.  These rocks from heaven are the oldest radiometrically dated rocks, and suggest that while initially there was a period of growth when colliding materials led to accretion but that later, during most of the solar system’s history, collision of rocks have occurred with such high energy that they led to erosion and disruption – not growth.  It has been asserted that without the influence of the giant planet Jupiter, both Mars and the asteroid planet would have grown to the size of the Earth.

            Mars might have developed into a planet very similar to the Earth; however, because of the gravitational effects of Jupiter, fewer mass accretions occurred retarding Mars’ growth.  Because Mars is a much smaller planet, it did not develop a significant metallic core; therefore, it has only a small magnetic field.  The insignificant magnetic field around Mars could not protect the planet’s atmosphere from depletion due to solar wind.  The depleted atmosphere was then unable to protect Mars from solar and cosmic radiation which over time would sterilize the planet’s surface making it ever more difficult for life to appear.  Similarly, if the Earth had been a little closer to Jupiter, or if Jupiter had a somewhat larger mass, then the “Jupiter effect” that aborted the formation of the asteroid planet and reduced the size of Mars would also have affected Earth rendering it a smaller planet.  If the Earth had been smaller, then it too might not have been able to keep its atmosphere and oceans rendering its long-term suitability for life much less ideal than present conditions.

            Jupiter also played a role in reducing the number of large asteroids in the inner solar system, objects which might have produced sterilizing impacts with the Earth.  Jupiter is 318 times more massive than the Earth, and its enormous gravitational influence even impacts the inner reaches of the solar system.  The early history of the solar system produced enormous numbers of asteroids which failed to coalesce into a planet and which might impact the Earth with adverse results.  However, over billions of years, most of these asteroids have fallen victim to the gravitational effect of Jupiter falling into the planet never to be seen again.  Alternatively, some of these asteroids were ejected out of the solar system, into the Sun, or into the Oort cloud of comets.  Jupiter was the major cause of this purging of the middle region of the solar system.  The objects that still survive today to impact the Earth are survivors of the gravitational pull of Jupiter, and survive in three special ecological regions:

·        The Oort cloud beyond Pluto,

·        The Kuiper belt of comets just beyond the outer planets,

·        The asteroid belt largely confined between the orbits of Jupiter and Mars.

The impact rate into the Earth of a 10-kilometer object occurs every 100 million years – the last one occurring about 65 million years ago which is thought to have killed the dinosaurs and the time of the K/T extinction.  George Wetherill of the Carnegie Institute of Washington has estimated that the rate of impact of these 10 kilometer objects might be 10,000 times as great if Jupiter had not come into being and purged many of the leftover bodies out of our region of the solar system.  If the Earth had been subject to impacts with 10-kilometer asteroids every 10,000 years rather than every 100 million years, it seems very unlikely that life – let along animal life – could have survived.

            Special Formation of Jupiter.  It is unclear whether planets similar to Jupiter and Saturn form in solar systems with planets similar to the Earth.  Jupiter is unique in several respects, however.  First, Jupiter grew very quickly – it grew to a mass of 15 Earths before Mars grew to 10% of an Earth mass.  The reason for this very rapid early growth is because Jupiter formed in a region of the solar system where water exists in the form of ice, whereas Earth and the inner planets formed in a region of the solar system where water exists in the form of liquid water.  As ice, water helps the accretion process accelerating the conglomeration of rocks into a larger body.  Jupiter’s growth to a giant planet first started with a giant core of rock, metal, and ice, forming a 15 Earth mass body.  At this size, the gravity of the core could attack and hold hydrogen and helium, the gases which account for 99% of the mass of Jupiter. 

            Not only are large planets like Jupiter and Saturn beneficial for life on Earth as their gravitational pull clears asteroids from the Earth’s vicinity, but also we are fortunate that the orbits of these two planets is roughly circular.  An irregular, very elliptical orbit might bring these planets dangerously close to the inner planets destabilizing their orbits with disastrous consequences for any life that might be on them.  Indeed, an elliptical orbit for large planets may be the norm rather than the exception.  Many of the large planets that are being discovered rotating about other stars have very irregular orbits.

What Are the Odds?

We have so far examined several factors (among others) which make the Earth an ideal habitat for life

·        its temperature,

·        atmosphere,

·        tectonics,

·        percentage of land,

·        position from the sun,

·        sun that is not too hot, nor too large, does not burn too quickly, but is increasing gradually in brightness over time,

·        earth axis tilt stabilization by the Moon, and

·        protection by Jupiter are some of these positive influences. 

What are the odds that these same occurrences happened elsewhere in the Universe to allow intelligent life as on Earth (at least we suppose intelligence!)?  In other words, how rare is the Earth?

In order to try to get an estimation of the odds relating to the creation of a planet like the Earth, it is necessary to create a solar system similar to ours.  The first major object that comes into existence in a solar system is its sun; therefore, we start with the odds relating to the creation of a sun smilar to ours.

The Sun.  We should probably start with similar ingredients to that which formed our solar system; that is, the same mass and elemental composition.  According to most theorists, this might create a star identical to ours – but then again, it might not.  The spin rate of the new star might be different from that of our sun with unknown consequences.  We need to develop a star that has similar mass than ours; one that will burn long enough to let live development occur, one that does not pulse or rapidly change its energy output, one that does not produce too much ultraviolet radiation, and one that is large enough – but not too large.  Of 100 stars, perhaps only two to five will yield a star as large as our sun; the vast majority of stars in the Universe are smaller than our sun.  While smaller stars could have planets with life, most would be so dim that Earth-like planets would have to orbit very close in order to receive sufficient energy to melt water.  However, being close enough to get adequate energy from a small star would lead to tidal lock whereby the same side of the planet always faces the sun.  A totally locked planet is probably unsuitable for advanced life.

Planetary System.  If we were to examine planetary systems about a star that is similar to our sun, there little chance that we would find one similar to ours.  Namely, it would be unlikely to locate a system with a Jupiter-like planet and three other gas giants orbiting outside four terrestrial planets with a halo of comets surrounding the entire mix.  If we were to examine 1000 planetary systems, there would likely be none that would be identical to our solar system today.  James Kasting from Penn State University believes that planetary spacing is not accidental but regulated by physics, and that if the solar system were to reform many times, we would get the same number of planets each time.  But, the extra-solar planets that have been discovered seem to exhibit an enormous diversity of spacing and orbits with their positions not being nearly so orderly as Kasting’s theory might suggest.  Ross Taylor states, “Clearly, the conditions that existed to make our system of planets are not easily reproduced.  Although the processes of forming planets around stars are probably broadly similar, the devil is in the details.”  Indeed, no one knows whether a planet the size of Jupiter would always form or whether there would be a couple of planets like Mars.  A planet would probably form in a position similar to that of the Earth, but nobody knows whether that planet might be smaller, larger, or in a slightly different orbit or orbital eccentricity. 

Even if all these events occurred more or less the way they have with our solar system, would life form?  And given life, would animal life appear once more?  Can there be animal life without the utterly chance events that occurred in Earth’s history such as a Snowball Earth, or an inertial interchange event, for instance. 

Thus, in summary, the set of questions are as follows:

·        How many of all planets in the Universe are terrestrial planets (as opposed to giant gas planets such as Jupiter and Saturn),

·        What is the percentage for all planets of the Universe?

·        Of the terrestrial planets in the Universe, how many of them have sufficient water to form an ocean,

·        Of all the planets with an ocean, how many have any land,

·        Of those with land, how many have continents (rather than scattered islands),

Similar questions abound for each step of the developmental process.

The Odds of Life

           Frank Drake developed a thought-provoking formula which sought to predict how many civilizations might exist in our galaxy.  The point of this formula was to estimate the likelihood of detecting radio waves sent out from other technologically advanced civilizations.  How called the Drake equation, it has had enormous influence to development of the Search for Extraterrestrial Intelligence (SETI) project.  The equation is a string of independent factors that when multiplied together give an estimate of the number of intelligent civilizations in our galaxies.

           As originally developed, the Drake equation is,

Nx  X fs X fp X ne X fi X fc X fl = N


·        Ns = Number of stars in the Milky Way galaxy,

·        fs = fraction of sun-like stars,

·        fp = fraction of stars with planets,

·        ne = planets in a star’s habitable zone,

·        fi = fraction of habitable planets where life does arise,

·        fc = fraction of planets inhabited by intelligent beings

·        fl = percentage of a lifetime of a planet that is marked by the presence of a communicative civilization.

When Drake first published his famous equation, there were great uncertainties in most of the factors.  The number of stars in our galaxy is between 200 and 300 million.  The number of star systems with planets, however, was very poorly known in Drake’s time (about 1950).  Although many astronomers believed that planets were common, that was only theory.  Carl Sagan estimated that an average of ten planets would be found around each star.  Now that numerous stars have been examined, it appears that only about 5% of stars have detectable planets.  Because in general, only large gas-giant planets are detectable, this figure really shows that Jupiter clones close to stars or in elliptical orbits are rare.  Spectroscopic examination indicates that stars that appear to have planets also appear to be rich in metals similar to our own sun.  Thus, according to some astronomers conducting these studies, there seems to be a causal link between high-metal content in a star and the presence of planets.  Our own star is metal-rich; in a study of 174 stars, astronomer G. Gonzales discovered that the sun was among the highest in metal content.  It appears as though we orbit a rare star after all!

Other studies indicate that planetary systems such as our own might also be rare.  At a large meeting of astronomers in Texas in 1999, it was announced that 17 nearby stars had been observed to have planets the size of Jupiter orbiting them.  What seemed obvious, however, was that none of the planetary systems being discovered seemed to be like our own.  Most of the Jupiter-like objects which orbit these stars travel in elliptical orbits, not circular orbits – which is the rule in our solar system.  In such planetary systems, the possibility of an Earth-like planet existing in a stable orbit is low indeed.  A Jupiter coming in close to the sun would have destroyed the inner rocky planets by causing the inward planets either to spiral into their sun or to be ejected out of their solar system into the cold, dark space between stars.

Furthermore, it seems as though the frequency of planets orbiting stars is probably much lower than originally anticipated.  As previously noted, Carl Sagan in 1974 estimated that the average number of planets orbiting each star is ten.  This was echoed by Goldsmith and Owen in their 1992 book, The Search for Life in the Universe

The most common stars in the Universe are M stars – fainted than the sun and nearly 100 times more numerous than solar-mass stars.  These stars can generally be ruled out as having life near them because their “habitable zones” where surface temperatures could be conducive to life are uninhabitable for other reasons.  To be warmed by these cool stars, the planets would have to be so close to the star that tidal effects from the star would force them into synchronous orbits with one side of the planet always facing the star while the other side would be permanently dark.  On the other hand, stars which are much more massive than the Sun would have stable lifetimes of only a few billion years which would probably be too short for the development of advanced life.  Each planetary system around a star similar to our own sun might not have an Earth-sized terrestrial planet orbiting the star in its habitable space.  When we take into consideration factors such as the abundance of planets and the location and lifetime of the habitable zone, the Drake Equation suggests that only between 1% and 0.001% of all stars might have planets with habitats similar to those on the Earth.  Furthermore, of those stars similar to the sun with planets similar to the Earth, how many of them have an Earth with a large Moon and a Jupiter-sized planet in just the right position and circular orbit to protect that planet. 

           We can now arrive at an estimation of the odds of finding intelligent life by examining these factors,

·        Stars in the Milky Way,

·        Fraction of stars with planets,

·        Fraction of metal-rich planets,

·        Planets in a star’s habitable zone

·        Stars in a galactic habitable zone,

·        Fraction of habitable planets where life does form,

·        Fraction of planets with life where complex animal life arises,

·        Percentage of a lifetime of a planet that is marked by the presence of complex animal life

·        Fraction of planets with a large moon

·        Fraction of solar systems with Jupiter-sized planets,

·        Fraction of planets with a critically low number of mass extinction events

With these added elements, the number of planets with animal life gets smaller and smaller.  Clearly, many of these terms are known in only the roughest details; but it does seem that the Earth may indeed be extraordinarily rare.

The Final Piece - Man

Sadly, life on Earth is coming to an end.  The Sun is getting progressively hotter and the greenhouse gases need to be continuously reduced in order to reduce surface temperature.  However, there is a limit under which carbon dioxide - the most important greenhouse gas - cannot fall as photosynthesis must continue or all green plant life would die.  The current carbon dioxide level is 375 parts per million in Earth's atmosphere; when the atmospheric carbon dioxide level falls below about 225 parts per million, all photosynthetic life will die - followed shortly by all animal life.  Alternatively, if we continue with our polluting ways dumping carbon dioxide into the atmosphere, the increased greenhouse gases combined with a brightening Sun will kill all animal life long before we have to worry about loss of photosynthesis.

The timing of man's arrival on Earth toward the end of its life sustaining capacity seems tragic at first.  However, upon closer inspection, we seem to have been provided a great gift.  Scanning the Earth's surface, we see great evidences of man's presence; farms, ranches, towns, cities, transportation and communication facilities, a plethora of building materials which are all derived from nearly four billion years of life - and death.  From dead life we get gems, sand, steel, asphalt, concrete, copper, limestone, marble, plastics, etc.  Most of the energy that drives civilization comes from the biosphere in the form of petrochemicals, wood, kerogen, and so forth.  Must of the fertilizers that support agricultural production also come from biodeposits - phosphates, nitrates, and other chemicals.

Such a bountiful provision speaks of a great Provider who carefully planned and prepared the planet through the ages for human life.  Furthermore, this Provider has given man the ability to use the bio-deposits around him for his own benefit.  But perhaps even more important, we have also been able to use this intelligence to understand the "big picture" - how the cosmos seems to have been put together with fine detail and "just right" physical laws to ensure man's survival.  The more we understand about the complexities of life, the fine tunedness of physical laws to ensure this life, how this planet is engineered to keep surface temperature at a life-supporting level over billions of years despite a tremendous increase in solar energy output, and just the wonder of the cosmos that is being revealed by our largest telescopes, we come to realize the transcendent power and kindness of our Creator.