110 



from no life, to primitive forms, to bacteria resembling those encoun- 

 tered today, to — after billions of years — single-celled plants and 

 animals with biochemical systems like those found in higher life 

 forms. This progression of microbes has caused events of planetologi- 

 cal significance — the development of an oxygenic atmosphere and 

 the deposition of vast mineral deposits being but two examples. 

 Studies of the origin and early development of life, thus, simply 

 cannot be separated from general investigations of the early history 

 of the Earth as a planet. The geochemical record of the history of 

 the volatile elements on Earth is the record of the history of life on 

 Earth, and an understanding of that record is crucial to an accurate 

 reconstruction of events in the early solar system. 



The record from the early Earth has no parallel at later stages of 

 Earth's history; close analogs may eventually be found on other 

 planets, but no similar environments occur on the modern Earth. In 

 a situation so without precedent, it will be necessary to constantly 

 avoid unjustified extensions of present geochemical models. As it is 

 acknowledged that the early Earth was without multicellular organ- 

 isms or land plants, it must be recalled that this places all the primary 

 productivity in the hands of microorganisms and creates a global eco- 

 system very different from any which has been considered for the 

 past 0.5 b.y. As microbial ecosystems are then recognized as espe- 

 cially important subjects for study by planetary biologists, it must be 

 recalled that the global impact of modern microbial ecosystems is 

 buffered, perhaps powerfully, by the great mass of the biosphere 

 which lies outside them. 



The establishment of a research program at the interface 

 between planetology, geology, and microbial ecology is, if anything, 

 overdue. As it is welcomed and carried forward, the origins of its 

 importance and the uniqueness of the problems it addresses must be 

 kept constantly in mind. The best contributions will result from the 

 arduous confrontation of all the evidence: biological, chemical, geo- 

 chemical, geological, and astrophysical. 



Such a research program must answer a number of questions. 

 For example, can there have been any survival of prebiotic organic 

 matter? How (chemically) might this prebiotic organic material be 

 recognized? When did life first appear? To answer this question, the 

 analysis of rock systems (e.g., Isua, Greenland; Swaziland, S. Africa; 

 and Pilbara, N.W. Australia) is of prime importance. Studies should 



