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THE LIFE SCIENCES diversity of a single crop is preferable to reliance on a single genetic strain); to indicate the possibilities of utilization of new species of plants and animals as major foodstuffs for man (e.g., the many ungulates of Africa, the Saiga antelope, originally from Alaska, which is much more efficient than sheep and goats at cropping the tundra, or red deer, which are more successful on rocky islands than are sheep); to maximize the food harvest from the oceans and larger bodies of fresh water (the manatee, consumer of water hyacinth, one of the most productive of crops, looks particularly promis- ing); to enable us to predict adequately the consequences of increase in the consumption of fossil fuels as opposed to increase in the utilization of nuclear power; and to enable us to protect naturally or deliberately im- pounded bodies of fresh water-examples without end. Man has claimed this planet as his own. In so doing, he must accept responsibility for the multitudes of species that he has displaced and that he husbands. The planet can never again return to the circumstances that obtained when Homo sapiens was a small wandering clan of hunters. Nor is there any reason to think that desirable. But it can be preserved in beauty with an immense and diverse flora and fauna, while supporting its human population, provided sufficient ecological sophistication is brought to bear. It is regrettable that the need for such understanding has become imperative so early in the life of this young science, which warrants all the support our society can provide. THE ORIGIN OF LIFE The origin of life is the least understood aspect of biological evolution. Significant progress in untangling this puzzle has been made in recent years. This progress, stemming from advances in such diverse fields as cosmology, geochemistry, and molecular genetics, together with the search for extra- terrestrial life, which is one of the prime objectives of the national space program, has now heightened interest in this central problem. In the past, discussions of the origin of life tended to be entirely specu- lative exercises, often tinged with superstition. But this topic has now become a problem for legitimate inquiry, subject to the same intellectual discipline as other attempts to understand evolutionary processes, including the requirement for logical elaboration of hypotheses, avoidance of arbi- trary assumptions, and recourse to observation and experiment. Unfortu- nately, knowledge of the terrestrial environment in the remote past is uncertain and, as the history of this question shows, is liable to drastic
FRONTIERS OF BIOLOGY revision from time to time as new evidence accumulates. In any case, it is impossible to duplicate or approximate the geological time scale, as well as the variety of conditions and the secular changes in these conditions that have occurred during the Earth's history. Because of such constraints, the.most one can hope to claim for conclusions on this subject is a high degree of plausibility. In this respect, however, studies of the origin of life differ in degree but not in kind from other scientific investigations. Life is not one of the fundamental attributes of the universe, like matter, energy, or time, but is a manifestation of certain molecular combinations. These combinations cannot have existed forever, since even the elements of which they are composed have not always existed. Therefore, life must have had a beginning. Current views of the origin of life differ funda- mentally from those of preceding centuries in that they are concerned with the origin of these molecular combinations rather than of organisms en- dowed with mysterious properties. From this standpoint, the origin of life must be viewed as a historical incident in the evolution of our planet, i.e., as an event limited in place and time by prevailing physical and chem- ical conditions. The unique attribute of living matter, from which all its other remarkable features derive, is its capacity for self-duplication and mutation. Living systems reproduce, mutate, and reproduce their mutations. The endless variety and complexity of living organisms and the seeming purposefulness of their structure and behavior are consequences of their mutability. Any system that has the capacity to mutate randomly in many directions and to reproduce those mutations must evolve. On the basis of various geological dating methods, it is estimated that the earth was formed about 4.5 billion years ago. The first hard-shelled animals in the fossil record appear at the beginning of the Cambrian, about 0.7 billion years ago. It is clear that life was present well into the Pre- Cambrian, a period lasting 3.8 billion years, but one cannot yet say how far back. The time when life started is an important parameter because, by difference, it provides the time scale for the organic synthetic reactions leading up to the origin of life. Paleontological evidence comes from ex- amination of various Pre-Cambrian rocks for their fossil remains and organic content. These include the Nonesuch shale of northern Michigan, the Gunflint chert of Ontario, the Soudan shale of northern Minnesota, Bula- wayan limestone of Southern Rhodesia, and the Fig-Tree chert of the Transvaal. All contain isoprenoid hydrocarbons, particularly pristane and phytane, both of which are breakdown products of chlorophyll, and per- haps of other biological molecules. The oldest, the Fig-Tree chert, is three billion years old and contains small amounts of these hydrocarbons as well 123
THE LIFE SCIENCES as microfossils of bacterial and algal size and form. If these are genuine fossils and residues of biological activity and are not the consequence of subsequent contamination, then life started between 3 and 4 billion years ago. The conditions on the earth's surface at that time are not entirely certain. But all the indications are that the atmosphere was unlike that of the mo- ment, that in place of oxygen, nitrogen, water, and carbon dioxide, there were methane, ammonia, carbon monoxide, hydrogen, and lesser quan- tities of hydrogen cyanide and formaldehyde. This is a reducing atmos- phere, in contrast to the oxidizing atmosphere of the moment, and was probably generated largely by outgassing of the initial solid matter of the earth. On this assumption, a variety of experiments have been conducted to ascertain what circumstances might have led to the kinds of organic compounds characteristic of living material. In the first such successful experiment, an electric discharge was passed through a mixture of ammonia (or nitrogen), methane, and water vapor above boiling water. Aldehydes and hydrogen cyanide were formed under these conditions, and these in turn reacted to form detectable amounts of the amino acids, glycine, alanine, serine, and aspartic and glutamic acids. Remarkably, these are the very amino acids that, to the present day, are the most abundant amino acids of proteins. By increasing the amount of hydrogen cyanide in such preparations, and by varying the energy source to simple application of heat or ultraviolet radiation, the number of products formed has been extended significantly. Among these products are hydrogen cyanide and its polymers such as dicyandiamide, which in turn have led to the forma- tion of the major purines, guanine and adenine, and, under the correct conditions, to guanylic and adenylic acids. It was more difficult to find conditions that would lead to the formation of pyrimidines, but both cytosine and uracil have been obtained in impressive yields. In yet other experiments, at 100° C clays such as kaolin catalyze formation of such sugars as glucose and ribose in good yields from dilute solutions of form- aldehyde. Moreover, cyanide and its polymers have been found to catalyze synthesis of random polypeptides from relatively concentrated solutions of amino acids, and of random polynucleotides from mononucleo- tides. In sum, therefore, conditions that may or may not mimic those of the prebiotic era on this earth, but that are as close as current theory can suggest, result in the formation of a large number of the primary building blocks of biological macromolecules as well as primitive macromolecules, thereby setting the stage for the origin of life. Beyond those facts, all is speculation. The problem is to ascertain how these noninformational proteins and polynucleotides combined to form a
FRONTIERS OF BIOLOGY self-duplicating system. No point is served by repetition here of current highly speculative hypotheses. It must suffice to indicate that the necessary raw material would have accumulated somewhere on earth as a consequence of the physical and chemical conditions on the earth's surface. If, over the passage of 1 or 2 billion years, molecules with a very low order of accidental catalytic activity served to catalyze further syntheses either on their own surfaces or on the surfaces of other molecules, the earliest beginnings would have been made. According to this concept, life is not necessarily a highly unlikely event, but rather the almost obligatory consequence of the zero-time conditions on the earth's surface. One further set of concepts warrants recital. From the notion described above, once primitive life began, and the crudest membrane surrounded such macromolecular packages, the materials available for further trans- formation, like those that contributed to the primitive genetic apparatus, must have been those that were present in the original "primordial soup." It is in that context that one finds explanation of the fact that the nucleo- tides not only serve as the building blocks of all forms of nucleic acids, but also participate in intermediary metabolism as the coenzymes for hundreds of metabolic processes presumably because "they were there." ATP be- came the energy source for cellular reactions because it was there. In time, as the original supply of organic materials began to dwindle, selective ad- vantage would come to those primitive cells that "learned" to synthesize what they required from other organic materials still present in the medium. In a metabolic pathway such as those we have considered, there is invari- ably a set of intermediates that serve no purpose but as substrate for the transformations that lead to the desired end product. Patently, a cell that had "learned" to convert what we currently recognize as a starting com- pound e.g., glucose to one of the first stages in the current biosynthetic pathway would not have benefited at all by such an event. Synthetic path- ways as we now know them must have evolved backward in the sense that the next capability when guanine and adenine began to disappear should have been an enzyme that could make use of the hypoxanthine also formed in the primordial reactions. When that was gone, utilization of some other substrate to make hypoxanthine would again confer high sur- vival value. It is of interest, therefore, that aminoimidazole carboxamide, even today an intermediate in purine biosynthesis, was also formed under the same rudimentary circumstances that have been shown to lead to the formation of adenine, guanine, and hypoxanthine. Patently, the information at hand is a far cry from genuine understanding of the origin of life, but it may well be that an important beginning has been made.