Most scientists are confident that life had already arisen 3.8 to 3.9 billion years ago, at about the time when the heavy bombardment was coming to an end. The evidence indicative of life's appearance is not the presence of fossils but the isotopic signatures of life extracted from rocks of that age in Greenland.
The oldest rocks on Earth that have been successfully dated via radiometric dating techniques are mineral grains of zircon about 4.2 billion years old. The Greenland rocks (from a locality called Isua) are thus only slightly younger. The Isua rock assemblages, which include sedimentary (or layered) rocks as well as volcanic rocks, have yielded a most striking discovery. They contain ratios of the light and heavy isotopes of carbon, indicating formation in the presence of life. The isotopic residue in the Isua rocks reveals an excess of the isotope carbon-12 compared to carbon-13. A surplus of carbon-12 is found today in the presence of photosynthesizing plants, because all living organisms show an enzymatic preference for "light" carbon. The inference is that if early life existed at Isua, it may have used photosynthesis for its energy sources. But there is no fossil evidence that life existed so long ago—only this enigmatic and provocative surplus of a carbon isotope that in our day is a sign of life's presence. If the excess of light-carbon isotope is indeed a reliable indication that ancient life existed at Isua, and perhaps elsewhere on Earth, as early as 3.8 billion years ago, it leads to a striking conclusion: Life seems to have appeared simultaneously with the cessation of the heavy bombardment. As soon as the rain of asteroids ceased and surface temperatures on Earth permanently fell below the boiling point of water, life seems to have appeared. But how?
There are still more questions than answers about life's origin on Earth. Yet the sophistication of the questions now being addressed by legions of scientists tells us that we are well along in the investigation. Among the most pressing of these questions: Did life originate in only a single setting or in several? Did the key chemical components—the building blocks—come from different environments to be assembled in one place? Was life's origin "deterministic"? That is, could different environmental conditions produce the same molecule of life, the familiar DNA? Were the individual stages in the origin of life (such as the formation of amino acids, then of nucleic acids, and then of cells) dependent on long-term changes in the Earth environment? Did the origin of life change the environment such that life could never originate again? At what stage did evolution take over to influence the development of life? And, perhaps most interesting of all, can we infer the nature of the settings of life's origin from the study of extant organisms—creatures living on Earth today?
Determining how the first DNA molecules appeared on Earth has been a very difficult scientific problem, and it is still far from solved. No one has yet discovered how to combine various chemicals in a test tube and arrive at a DNA molecule. Added to this is the fact that conditions on the early Earth would have been in many ways horrific for natural "chemistry" experiments involving reactions that now routinely take place in what we humans call "room temperature." Temperatures on the early Earth even 3.8 billion years ago, about the time that the first life on Earth may have appeared, may have been far higher than those of today (although some astrobiologists think that Earth was colder then than it is now because the sun was fainter). Many other aspects of early environmental conditions would clearly have been deleterious to much of the life now on our planet. For example, with an oxygen-free atmosphere the amount of ultraviolet radiation reaching Earth's surface would have been far higher than it is today, making delicate chemical reactions on the planet's surface very difficult. But we know that life did arise and that the most important step in the process was the formation of DNA, life's basic information center.
To build DNA—and, ultimately, life—requires the following ingredients and conditions: energy, amino acids, factors that make chemical concentration possible, catalysts, and protection from strong radiation or excess heat. The chemical evolution of life entails four steps:
1. The synthesis and accumulation of small organic molecules, such as amino acids and molecules called nucleotides. The accumulation of chemicals called phosphates (one of the common ingredients in plant fertilizer) would have been an important requirement, because these are the "backbone" of DNA and RNA.
2. The joining of these small molecules into larger molecules such as proteins and nucleic acids.
3. The aggregation of the proteins and nucleic acids into droplets that took on chemical characteristics different from their surrounding environment.
4. The replicating of the larger complex molecules and the establishment of heredity. The DNA molecule can accomplish both, but it needs help from other molecules, such as RNA.
RNA molecules are similar to DNA in having a helix and bases. But they differ in having but a single strand, or helix, rather than the double helix of DNA. They also differ in the makeup of their base composition: Instead of thymine, they contain a base called uracil. Most RNA is used as a messenger, sent from DNA to the site of protein formation within a cell, where the specific RNA provides the information necessary to synthesize a particular protein. To do this, a DNA strand partially unwinds, and an RNA strand forms and keys into the base-pair sequence on the now-exposed DNA molecule. This new RNA strand matches with the base pairs of the DNA and, in so doing, encodes information about the protein to be built. This process is called translation.
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