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Molecular fossils are ambiguous. They tell us practically nothing about the size and shape of what producedthem. They can, however, clearly indicate the biochemical capabilities and functions of what produced them. There are, as we know, a variety of organic molecules synthesized by living systems, but they are stable to very different degrees. Proteins and their constituent amino acids would be fascinating fossils, but proteins are essentially non-existent as fossil molecules and extensive searches for amino acids have led to the following conclusions. Amino acids are present in ancient rocks and the majority are in the L-form, which is characteristic of life. However, in rocks over 60 million years old, the L- and D-forms occur in about equal amounts. Hence, it appears that L-amino acids, when associated with ancient rocks (Precambrian, over 600 million years old), are probably of relatively recent origin.

The most useful, it now seems, of the fossil molecules are the straight-chain hydrocarbons. These are very stable compounds and are easily recovered from ancient rocks by washing in benzene and methane. The occurrence of pristane and phytane, products of the chlorophyl molecule, can be used to infer the presence of organisms capable of photosynthesis.

However, there is still lively debate regarding the interpretation of molecular fossils. The work on the generation of organic compounds in the absence of life has shown, as we discussed earlier, a quite extraordinary array of spontaneously formed substances. Amino acids were among them. Someorganic geochemists suggest that the long-chain hydrocarbons should be added to this list. And so the search is still underway for unambiguous fossil molecules. The field is, nonetheless, considered to be a promising one by many researchers.

Finally, under problems of direct observation of fossils, we must consider the species problem. Unavoidably, fossil specimens are named and classified and this places them beside extant or neontological forms. The problem now is this: Are the criteria for species identification the same for paleontological and for neontological material? We raise this question because both are used in the study of evolution. And when evolutionary sequences are constructed from extinct forms leading to modern extant forms, we might wonder if we are using comparable terms for both forms. In brief, the answer is that we are not. The problem is not a serious one if we keep in mind certain precautions.

To describe apparently related fossil forms as members of different species we apply the kinds of differences apparent between comparable extant species. Some workers prefer to work only with differences that justify generic names. This degree of caution is not always necessary, but it is more desirable than the reverse case in which every little difference between two fossils is used as an excuse for describing new species. The necessary precautions come down to an awareness of the natural variation that occurs in any species, a sense of the genetic basis for such variation, and then a cautious usage of species designations. In this way we can quite confidently combine our data on extinct and extant forms and expand the data base used for evolutionary studies.

The relative positions of fossils: Chronology. The most informative aspect of the fossil record is that it allows us to perceive evolutionary history directly. This can happen, however, only when we have determined the relative ages of fossils correctly. The basic rule is that the oldest fossils are toward the bottom of the pile. The pile we refer to is the bedded sequence of fossils. The oldest fossil beds are laid down first, the next oldest on top of them, and so on up to the surface where today's forms are living. Ideally, then, the evolutionary sequence is simply read from bottom to top. But things are rarely ideal. Two phenomena, at least, can confuse the ideal state of affairs. One of these is that fossil beds can be moved. Geological folding and uplift, which can obscure an otherwise quite readable story, occur. Folding often reorients layers or strata of fossil beds relative to each other and uplift in one areacan raise a bed that was at the bottom, and quite old, in another area. Another troublesome phenomenon is the loss of strata or beds. Erosion can remove millions of years of sedimented history and thus produce gaps in the fossil record. Such gaps, depending on the amount of discontinuity they cause, can pose genuine problems in our understanding of how one form evolves into another.

The answer to the foregoing largely depends on common sense as well as the competence of the geologist and the paleontologist. Common sense tells us to go slow in interpreting sequences from fossil beds until the geologist can determine the degree of folding, uplift, and erosion. The paleontologist can then compare the results from one stratum with those from a comparable stratum elsewhere and thus carefully reconstruct historical sequences. Ultimately, when we deal with history, we look for absolute measures of time, not just relative ones. We want to know precisely how long ago a certain event occurred and not just that it occurred earlier or later than other events. The most informative method in this regard is an analysis that depends on the amount of specific radioisotopes present in a fossil bed or a fossil.

There are two aspects to learning what fossils can teach us. The first comes from the direct observation of fossil evidence. The second is less direct because it is concerned with the relative position of fossils in the rocks where they are found.

Direct observation of fossils. Except for rare cases in which organisms are immediately preserved, e.g., the frozen mammoth or, another case, insects trapped in amber, fossil remains present us only with a part of some organism. So, obviously, one problem in interpretation is the reconstruction of the whole organism. Perhaps the most famous practitioner in this area was Cuvier, the father of paleontology. A fossil tooth was a fascinating clue to Cuvier. From it he could make educated guesses as to the size of the original organism and its age and its food. The latter then allowed cautious speculation as to the nature of such internal organs as the digestive system. This led to more information on body size and shape. And so on. Comparable work is being done today and the reconstruction of whole humans from parts of their skull is a case in point. Such reconstructions can be extremely useful in casting light on the creatures of bygone times. In some cases specially preserved fossils, such as the impression of dinosaur skins, allow us to reconstruct details that we would otherwise have had no knowledge of.

More difficult to interpret are the traces left by organisms, such as worm tracks left in the mud of ancient seas or the molecular fossils, mentioned earlier. In these instances we may never know the exact nature of the organism that left such evidence of its existence. Nonetheless the information we do have is useful. It helps complete the picture of what type of organism was alive at a certain time in the past history of this earth. Worm tracks, for example, document, at the least, that worms were present. Even though we may not be able to say which species or class or maybe even which phylum, we do know that crawling things of a certain approximate size were part of the ecosystem under study. That enhances, though admittedly in a limited way, our understanding of past life.

Fossils appear when erosion or digging uncovers them. Erosion goes on all the time, in total disregard of the presence or absence of fossils. Hence many fossils are uncovered and may be casually destroyed. The ideal fossil is one whose position in its surroundings is still identifiable; and it represents as complete a record as possible of the original organism. Isolated bones and teeth are important--they have told us much about human evolution--but complete skeletons are, of course, much more informative, though much less likely to be found.

Sedimentary formations are naturally the richest source of fossils for the paleontologists who seek them. Here fossils were formed and preserved through deposition of muds and silt. When such a bed of fossils is found great care is taken in the removal of specimens. Often, chunks of the sedimentary material are removed and worked on with special tools in a laboratory so as to assure intact specimens. Depending on the size and nature of the fossil, the paleontologist's tools are appropriately varied. In looking at microscopic fossils, the techniques of a gem cutter and polisher are utilized. This assures thin, transparent preparations for the microscope. Larger fossils may require accurate splitting of sedimentary layers followed by the addition of preservatives, e.g., plastics, to protect the fossil remains. And large dinosaur bones are often covered with plaster at the site of their excavation to protect them from rain and other wear and tear. Most fossils are destroyed before they catch the eyes of fossil-hunters. Sedimentary rocks are changed through movements of the earth's crust or through heating, and this damages or destroys fossils. When we put this fact together with the realization that relatively few organisms are successfully fossilized, we can conclude that the fossils we have recovered are a very limited sample of the fife of the past. But that conclusion protects against the extremes of wild speculation orpessimistic conservatism in interpreting the fossil record. Neither the extreme of assuming we have a complete record of the past nor the extreme of assuming we can say nothing of the past is justified.

Any trace of life from the past can be called a fossil. This includes the molecular products of life as well as worm tracks in the mud of some bygone time. It also includes various kinds of hard parts, such as shells, bones, or teeth. It can include the impressions left by soft parts, which may be flattened and distorted by the pressures of the overlying mud that turned to rock or replaced by chemicals, which have no direct connec-tion to living systems, but nonetheless are a record of fife in the past. Perhaps the most extraordinary fossils are the living things that were frozen during an Ice Age. A mammoth, which is an extinct relative of the elephant, has been found which was trapped in the frozen Alaskan tundra some milllions of years ago and preserved there. Discovered and recovered some years ago, it provides us with an extraordinary fossil.

Fossils are obviously formed in a variety of ways, and therefore, there is no one answer to our first question, How are fossils formed? Fossil formation is a chance event. It depends on living things, or any of their many products, being in a place where they are relatively undisturbed. Or, if they are disturbed, where the resulting changes still retain the record of the past. The examples of petrified wood show this dramatically. Here the original wood has been completely replaced by various chemicals from the environment. These latter were selectively deposited in various parts, even down to cellular details of the wood. Thus, despite replacement of the original living parts, a marvelously detailed record of that life is retained.

More typically, the fossilization of an animal, such as one of the ancestors of present-day horses, occurs along the following lines. The organism in question, say, for example, the grazing form called Merychippus that lived about 20 million years ago, died on the grassland where it lived. In a relatively short time the scavengers of that time--giant hyenas and vultures, and a whole array of other forms including beetles and flies and their larvae--would have consumed and dismembered the carcass of Merychippus. Such a fate would have precluded any fossil formation and was by all odds the most common situation. Another, less likely possibility was that a Merychippus was drowned while crossing a river. If the body sank and was covered by silt, the possibility of a fossil resulting is more likely. However, if the river dried up--perhaps our grazer was simply caught in a flash-flood--before the drowned animal was well buried, scavengers could have destroyed the carcass.

Oxygen penetrates surface waters and enters the soil to some extent. These areas have also evolved oxygen-dependent life forms, but also important are the accumulated organiccompounds that have dissolved in water and that continually enrich the soil. Such compounds transform a barren dust or sand to the rich topsoil that supports plant life and forms the basis for all food chains on land. Similarly, organic compounds are important to oceanic plankton and zooplankton. Plankton form the basis for life in the surface waters of the ocean. That life supplies not only organic compounds at the surface, but also contributes to the quiet rain of detritus that reaches the ocean floor and sustains life even at the greatest ocean depth. Such fife lives in permanent darkness, with minimal oxygen and almost always low temperatures. But the fact that it exists has changed the ocean from an originally lifeless dilute soup of
1. The methanogens have at least two enzymes, related to methane metabolism, and not known to occur in other organisms.
2. The methanogens contain no cytochromes, the proteins widely used by other organisms for electron transport.
3. Most prokaryotes have peptidoglycan in their cell walls; Wthanogens have none.
4. The transfer RNA (tRNA) of other organisms carries a distinctive squence TψCG in one part of each tRNA molecule. In its place, the methanogens have either ψψCG or UψCG. (ψ is a modified uridine.)
5. Changes occur in rRNA after it has been transcribed. (The change of uridine to "pseudo-uridine" or ψ, in 4 above, is an example.) Such changes in the methanogen 16S rRNA are very different from those found in other prokaryotes.

Altogether, then, significant differences are seen between the methanogens and typical prokaryotes. Whether this will justify a new kingdom of organisms, as hinted at by Carl Woese and his colleagues, remains to be seen. For the present it is convenient to refer to the methanogens as a very special group of prokaryotes. spontaneously formed compounds to a collection of complex ecosystems with interdependent inhabitants. This awareness of co-evolution, the concept that life evolves along with the changes it produces in its environment, is emerging as a key concept in our search for life on other planets. This search is called exobiology--life beyond the limits of this earth, or outer life--and one of its leading practitioners in the United States, Cyril Ponnamperuma, of the University of Maryland, uses it as a criterion for predicting the probabilities of extraterrestrial life. In his laboratory Ponnamperuma mimics extraterrestrial conditions on the moon or on Mars, for example, so that he can study the spontaneous formation of molecules needed for life. When the first satellite descriptions were received on earth from Mars, Ponnamperuma set up his Mars atmosphere on earth and correctly showed that life on Mars was highly unlikely, a result confirmed by subsequent tests performed by the spacecraft that landed on the surface of Mars.

One of the most interesting evolutionary finds in recent years is the prokaryotic form, the methanogen. Their special evolutionary significance has been described by the American biophysicist and microbiologist Carl Woese and his colleagues. The methanogens are best classified as chemoautotrophs. They live in a hot, reducing environment. Their cell walls, their enzymes, and certain of their nucleic acids differ markedly from those of other prokaryotes, although their overall cellular organization is clearly prokarvotic. But especially because the nucleic acids of their ribosomes (essential in protein synthesis) are so different from any others known, Carl Woese and his colleagues have suggested that the methanogens might merit a kingdom of their own. That suggestion has not been acted upon, but it is worth mentioning to highlight the special significance of the methanogens. For our purposes, they represent a group of extraordinary chemoautotrophs that depend on a reducing atmosphere. They could wellbe living representatives of early chemoautotrophs--of those forms that first emerged when the ecosystems were evolving and that retain a need for a reducing atmosphere.

The Gaia hypothesis. This brings us to consider somewhat more carefully the changes that occurred in the atmosphere, hydrosphere, and the surface soils of the earth as life originated and evolved. The idea that life and its environment have co-evolved has been called the Gaia hypothesis by the British scientist James E. Lovelock. As the by-products of liv-ing systems accumulated on the surface waters and in the air and the soil of the earth, living things had to adapt to ever new surroundings. We have already said that the appearance of oxygen as a by-product of photosynthesis changed the atmosphere from a reducing one to an oxidizing one. This made possible chemical respiration as we know it today. Most living things are aerobic and animals, in particular, depend on oxygen for fife. Hence the change to an oxidizing atmosphere was an essential precursor to animal life and evolution.

Horowitz's proposal goes as follows. Suppose an early organism needs organic substance A as a precursor for a vital substance. Substance A eventually comes into short supply because many similar organisms are competing for it. Suppose, further, that A is being produced spontaneously from the following reaction, occurring outside the cell, B + C → A. Any cell that can take up B and C from the environment and produce A now has a selective advantage over all the other cells that depend on an outside source of A.

The uptake of B and C may require, first, a changed membrane protein. This can occur through mutation. Proteins already in the cell might catalyze the reaction B + C → A. Further, mutations could yield a more efficient catalyst. The cell with such new capabilities would now predominate in those parts of the early seas in which survival was possible. And then, in time, B or C or both would come into short supply. What then? A situation like that regarding the formation of A would occur. The cell or cells able to form B from its precursors and C from its precursors would be at a selective advantage. And this situation could repeat itself many times throughout the early history of the earth, which is now reliably estimated as being 4.6 × 109 years old.

The overall result of this early evolution would be the emergence of biosynthetic pathways. Such pathways would be like those known today, in which reactions are catalyzed by highly specific enzymes formed under the control of genes. Also, of necessity, there would be a general direction in this evolution. The biosynthetic pathways would belengthening in the direction of simpler and simpler precursors. Eventually, organisms would probably become so biosynthetically sophisticated as to require only such inorganic precursors as carbon dioxide, water, ammonia, sulfur, and phosphorus or their compounds or both. They might, also, by this time have evolved means for capturing energy from sunlight for biosynthesis. In short, nutritionally, they would have evolved from being chemoheterotrophs to being photoautotrophs.