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.