What really is life? And how do we recognize its formation? These questions seem simple, but the answers are dauntingly complex. In its most common-sense definition, life is able to grow, reproduce, and respond to changes in the environment. By this definition, extremophiles are obviously alive, for example. Yet many crystals can do as much, and they are clearly not life. The great British biologist J.B.S. Haldane pointed out that there are about as many living cells in a human being as there are atoms in a cell. Individual atoms themselves are not alive, though. "The line between living and dead matter is therefore somewhere between a cell and an atom," Haldane concluded.
Somewhere in between atoms and the living cell there is the entity known as a virus. Viruses are smaller than the smallest living cells and do not seem to be alive when isolated (they cannot reproduce), yet they are capable of infecting and then changing the internal chemistry of the cells they invade. Are they alive? In isolation they do not seem to be, but in combination with the host they very well may be. The boundary between living and nonliving is ambiguous at these levels of organization. By the time we reach the level of organization found in bacteria and archaea, however, we are sure that we have unambiguous life. We are also sure that all life on Earth is based on the DNA molecule.
Deoxyribonucleic acid, or DNA, is predominantly composed of two backbones that spiral around one another (the famous "double helix" described by its discoverers, James Watson and Francis Crick). These two spirals are bound together by a series of projections, much like steps on a ladder, made up of the distinctive DNA bases adenine, cytosine, guanine, and thymine. The term base pair comes from the fact that the bases always join up in the same way: Cytosine always pairs with guanine, and thymine always joins with adenine. The order of bases on each strand of DNA supplies the language of life; these are the genes that code for all information about a particular life form.
There might be many kinds of life elsewhere in the Universe, and there is a great deal of speculation among scientists about whether DNA is the only molecule on which life can be based or one of many. It is certainly the only one capable of replication and evolution on Earth, and all life here contains DNA. The fact is that all organisms on Earth share the same genetic code is the strongest evidence that all life here derives from one common ancestor.
Was the rise of life inevitable on this planet? Let us perform a thought experiment: If every environmental condition that ever existed on the Earth during its 4.5-billion-year history were exactly reduplicated in the same order, would life itself again evolve? And if it did, would it evolve with DNA as its crux?
The formation of this complex molecule is thus the starting point for any discussion of life's history on this planet—and perhaps on any other. There may indeed be other ways to produce life; one would be a system where ammonia, rather than water, is the solvent necessary for life. This route may even have been followed, only to be erased later, probably because water is a better solvent than ammonia. (Solvents are a rather humdrum yet essential ingredient in life's recipe. Many of the chemicals necessary for life can be delivered into a cell only in solution, and for that a solvent is needed.) Thus "DNA life" may be either the only type of life that formed or the only survivor.
Life seems to have appeared on this planet somewhere between 4.1 and 3.9 billion years ago, or some 0.5 to 0.7 billion years after Earth originated. However, the fact that no fossils were preserved at this time in Earth's history clouds our understanding of life's earliest incarnation. The oldest fossils that we do find are from rocks about 3.6 billion years of age, and they look identical to bacteria still on Earth today. There may have been earlier types of life that are no longer represented on Earth, but our present knowledge suggests that bacteria-like forms were the first to fossilize.
The Earth formed about 4.5 to 4.6 billion years ago from the accretion of variously sized "planetesimals," or small bodies of rock and frozen gases. For the first several hundred million years of its existence, a heavy bombardment of meteors pelted the planet with lashing violence. Both the lava-like temperatures of Earth's forming surface and the energy released by the barrage of incoming meteors during this heavy bombardment phase would surely have created conditions inhospitable to life. As we recounted in the last chapter, this constant rain of gigantic comets and asteroids would have driven temperatures high enough to melt surface rock. No water would have formed as a liquid on the surface. Clearly, there would have been no chance for life to form or survive on the planet's surface. It was hell on Earth.
As we showed earlier, the new planet began to change rapidly soon after its initial coalescence. About 4.5 billion years ago, Earth began to differentiate into different layers. The innermost region, a core composed largely of iron and nickel, became surrounded by a lower-density region called the mantle. A thin crust of still lesser density rapidly hardened over the mantle, while a thick, roiling atmosphere of steam and carbon dioxide filled the skies. In spite of its being waterless on the surface, great volumes of water would have been locked up in Earth's interior, and water would have been present in the atmosphere as steam. As lighter elements bubbled upward and heavier ones sank, water and other volatile compounds were expelled from the interior and added to the atmosphere.
The heavy bombardment by comets and asteroids lasted more than half a billion years and finally began to diminish around 3.8 billion years ago as the majority of debris was incorporated into the planets and moons of our solar system. During the period of heaviest impact, the steady bombardment would have scarred our planet by craters in the same manner as the moon. Yet the comets and asteroids raining in from space delivered an important cargo with each blow. Some astronomers believe that much, or even most, of the water now on our planet's surface arrived with the incoming comets; others think that only a minority of Earth's water arrived in this fashion.
Comets are made up of dust and volatiles, such as water and frozen carbon monoxide, and there is no doubt that a good many of them hit the early Earth. These cargoes of water slamming into Earth would have turned instantly to steam. The dense early atmosphere remained hot for hundreds of millions of years. Perhaps 4.4 billion years ago, its surface temperatures might have dropped sufficiently, and for the first time liquid water would have condensed from steam onto our planet's surface, successively forming ponds, lakes, seas, and finally a planet-girdling ocean. The study of ancient sedimentation suggests that by slightly less than 3.9 billion years ago, the amount of oceanic water on Earth may have approached or attained its present-day value. But these were not tranquil oceans or oceans even remotely similar to those of today.
We have only to look at the Moon to be reminded of how peppered Earth and its oceans were during the period of heavy impact, between 4.4 and 3.9 billion years ago. Each successive, large-impact event (caused by comets larger than 100 kilometers in diameter) would have partially or even completely vaporized the oceans. Imagine the scene if viewed from outer space: the fall of the large comet or asteroid, the flash of energy, and the evaporation of Earth's planet-covering ocean, to be replaced by a planet-smothering cloud of steam and vaporized rock heated (at least for some decades or centuries) far above the boiling point of liquid water. It is difficult to conceive of life—whatever its form—surviving anywhere on the planet during such times, unless that survival occurred deep underground.
Scientists have made mathematical models of such ocean-evaporating impact events. The collision with Earth of a body 500 kilometers in diameter results in an almost unimaginable cataclysm. Huge regions of Earth's rocky surface are vaporized, creating a cloud of "rock-gas" several thousand degrees in temperature. It is this superheated vapor, in the atmosphere, that causes the entire ocean to evaporate into steam. Cooling by radiation into space would take place, but a new ocean produced by condensing rain would not fully form for thousands of years after the event. Much of the revolutionary detective work behind these conclusions was described in 1989 by Stanford University scientist Norman Sleep, who realized that the impact of such a large asteroid or comet could evaporate an ocean 10,000 feet deep, sterilizing Earth's surface in the process.
How ironic that the comets may have brought some of Earth's life-giving, liquid water—a prerequisite for life—and then snatched that gift away for a time with each successive large-impact event. Yet it is not only water that these comets may have brought. They could have played a role in determining the chemical evolution of Earth's crust. And they may have contributed another ingredient to the recipe for what we call life: They may have brought organic molecules—or even life itself—onto our planet's surface for the first time.
If some time machine made it possible to visit the Earth of about 3.8 billion years ago, at the end of the period of heavy bombardment, our world would surely still appear alien to us. Even though the worst barrage of meteor impacts would have passed, there still would have been a much higher frequency of these violent collisions than in more recent times. The length of the day was shorter, because Earth was rotating far faster than it does now. The sun was much dimmer, perhaps a red orb supplying little heat, for it not only was burning with less energy than today but also had to penetrate a poisonous, turbulent atmosphere composed of carbon dioxide, hydrogen sulfide, steam, and methane. In such an environment, we would have had to wear spacesuits of some sort, for only traces of oxygen were present. The sky itself might have been orange to brick red in color, and the seas, which surely covered all of the planet's surface except for a few scattered, low islands, would have been muddy brown and clogged with sediment. Yet perhaps the greatest surprise to us would be the utter absence of life. No trees, no shrubs, no seaweed or floating plankton in the sea; it would have seemed a sterile world. Somehow, the fact that we have not yet detected life on Mars seems consistent with its satellite images. A waterless world fits our picture of a lifeless world. Even when the young Earth was covered with water, however, it was still devoid of life. But not for long.
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