The Biochemical Evolution of Genetic Material 



441 



A study of the comparative biochemistry 

 of present higher plants and animals, bac- 

 teria, and many viruses, shows that they all 

 form or require the same 20 or so amino 

 acids. Accordingly, nucleic acid and pro- 

 tein are, perhaps, the most durable geo- 

 chemical features of the earth, having per- 

 sisted more than a billion years. Because 

 of the intimate relationship between amino 

 acids and genetic material, it would seem 

 highly desirable to learn about the evolution 

 of all kinds of organic (carbon-containing) 

 compounds, including amino acids and poly- 

 peptides, energy-rich compounds (like ATP), 

 catalysts (like iron-containing compounds), 

 and energy-capturing compounds (like 

 chlorophyll). 



Our present understanding is that at an 

 early stage in the history of the earth there 

 was a reducing atmosphere, which was rich 

 in water, hydrogen, methane, and ammonia, 

 but poor in free oxygen and CO2. Using 

 mixtures of gases predicated as being present 

 in such a reducing atmosphere, and exposing 

 such mixtures to heat, ultraviolet light, or 

 electrical discharges (man-made lightning), 

 it has been possible to produce large amounts 

 and a large variety of amino acids. Other 

 experiments have, by heat treatment, con- 

 verted amino acids into polypeptides, and 

 have produced purines like adenine from 

 simpler components. Such experiments are 

 expected to lead us to understand better the 

 organic evolution which took place on earth 

 prior to the formation of the first genes or 

 organisms. 



We are not restricted to this planet, how- 

 ever, in our search for information regarding 

 either pregenic, preorganismal evolution, or 

 postgenic, postorganismal evolution. The 

 present universe is about ten billion years 

 old, the earth being about half this age. 

 Because the universe contains vast numbers 

 of stars (suns) with planets, there must be 

 numerous suns the size of our own that have 

 planets which are about the same size as 



the earth and which are about the same dis- 

 tance from their suns. Some of these planets 

 are surely younger and others older than our 

 own. What is the possibility that a chemical 

 and biological evolution similar to that 

 which occurred on earth would take place 

 on these or other planets? The answer to 

 this question will depend, of course, upon 

 the chemical composition of these other 

 planets. 



Most of the universe is composed of hydro- 

 gen and helium (most of the earth's hydrogen 

 having escaped from its atmosphere in 

 the distant past). Of the remaining atoms, 

 however, the universe has abundant oxygen 

 and nitrogen and, in fact, is relatively richer 

 than the earth in carbon, the atom essential 

 for the organic compounds which have 

 played so important a role in chemical and 

 biological evolution on earth. It is, there- 

 fore, likely that there are numerous places 

 in the universe where the initiation of an or- 

 ganic chemistry of biological interest would 

 be possible. Since the earth is a relatively 

 poor place for such an evolution (which 

 nevertheless occurred), there are almost 

 surely numerous planets in the universe like 

 our own, which contain earher stages in 

 chemical evolution, similar stages in biologi- 

 cal evolution, as well as those which are 

 older, and very probably have more ad- 

 vanced types of organisms. 



We already have evidence for the occur- 

 rence of organic radicals like CH, CN, CC, 

 and CO in comets, and for organic molecules 

 of an asymmetric type on Mars. Astrono- 

 mers have also reported variations in ap- 

 parent color and texture of Mars with changes 

 in season. These evidences are very strong 

 that Mars contains appreciable quantities 

 of organic matter, although one cannot yet 

 decide whether these have a preorganismal 

 or an organismal origin. 



Further information about the chemistry 

 of the sun and planets will doubtless be pro- 

 vided by telescopes of various kinds placed 



