QUEST FOR LIFE BEYOND THE EARTH — SAGAN 301 



and bases in the primitive oceans. Under continued ultraviolet 

 irradiation, they combine with phosphorus compounds already in 

 the oceans to form nucleoside phosphates. In turn, the eventual 

 interaction of nucleoside phosphates yields nucleic acids resembling 

 DNA. Because of their characteristic chemical structure, the nucleic 

 acids slowly replicate — that is, they begin forcing the production of 

 other, identical nucleic acids from adjacent building blocks in the 

 primitive oceans. 



Occasionally, an error in replication occurs, yielding different vari- 

 eties of the original nucleic acid molecule. These varieties, however, 

 also reproduce themselves. Some of these new molecules may replicate 

 more rapidly or more efficiently, and they prosper; others do not. 

 Thus, a kind of evolution begins, a natural selection on the molecular 

 level. When, in time, nucleic acid molecules developed which weakly 

 controlled chemical reactions outside themselves, the chain of life 

 from molecule to man began. The critical event has been the produc- 

 tion of the first molecule which could reproduce itself. 



This picture provides a convenient scaffolding for draping our 

 ideas, but there are many problems which remain to be answered. 

 Will enough nucleoside phosphates be produced, and interact, in 

 primitive times, to form many nucleic acids ? How did early nucleic 

 acids control their environment, in the absence of the elaborate con- 

 temporary DNA-ENA-protein transcription apparatus ? What is the 

 elffect of molecular contaminants on the course of prebiological organic 

 chemistry ? What is the origin of the cell ? 



Despite the many uncertainties remaining, certain features of the 

 origin of life are now becoming clear. It is a remarkable fact that 

 the physics and chemistry of the primitive terrestrial environment 

 were such that large numbers of organic molecules were produced — 

 organic molecules which today are intricately entwined in the fabric 

 of life. This has two implications for the possibility of extra- 

 terrestrial life. First, the origin of life may be a highly probable 

 event arising by the operation of very general energy sources on very 

 common primitive planetary environments. Second, fundamental 

 extraterrestrial biochemistry may be of a familiar type, even if extra- 

 terrestrial morphology and physiology are not. 



Although all the planets may have started with similar primitive 

 environments, it is clear that subsequent planetary development has 

 produced a diversity of extraterrestrial environments. The Harvard 

 paleontologist George Gaylord Simpson has emphasized that evolu- 

 tion is an opportunistic, and not a far-sighted, process. Adaptations 

 occur to immediate environmental crises, and not because of any long- 

 term plans. Each evolutionary step must build on the previous ones, 

 and the number of evolutionary "decisions" in the ancestry of any 

 organism is enormous. Thus, we must not expect the inhabitants of 



