the process? Just what is the detailed history of molecular oxygen as a compo- 

 nent of the Earth's atmosphere? For our models of the prebiological era on 

 Earth, what are we to assume to be the history of the flux of solar (and other?) 

 radiations reaching the surface? What is the possible significance of large-body 

 impacts with the Earth during some or all of this phase of evolution? 



From the foregoing discussion, it should be clear that the intellectual content 

 in the field of exobiology goes far beyond attempts to detect life on another 

 planet. Thus, while exobiology has historically been narrowly viewed as the 

 search for extraterrestrial life, in point of fact, the field today is better described 

 as an interdisciplinary science devoted to the study of evolutionary biology. As 

 such, it encompasses the origins and history of the major elements required for 

 life; their processing in the interstellar medium and in protostellar systems; 

 their incorporation into organic compounds on the primitive Earth and on other 

 celestial objects; the interactions of an evolving planet with the evolution of 

 complex organic compounds; the conditions under which chemical evolution 

 resulted in replicating molecules; and the subsequent interactions between an 

 evolving biota and further planetary evolution. To implement the objectives of 

 this discipline, investigators in the field are studying different aspects of the 

 evolutionary process in order to synthesize from these studies a plausible "road 

 map" that leads from the origin of the universe to the establishment of a sus- 

 tained biota on Earth. It is reasonable to expect that biologists will acquire new 

 and important information in the future from ground-based studies of terrestrial 

 and extraterrestrial materials, as well as from laboratory demonstrations of 

 critical chemical and biochemical pathways involved in chemical evolution. 

 Moreover, one can readily assume that telescopic probing of the solar system 

 and beyond by physical scientists will provide fresh insights into many of these 

 issues. However, successful implementation of the broad program of inquiry that 

 constitutes modern exobiology requires that biologists interact directly with 

 astronomers, astrophysicists, atmospheric chemists, geochemists, and other 

 physical scientists in order to resolve many of the open questions in this field. In 

 this regard, the opportunities provided by space technology are especially 

 intriguing. Direct measurements of the compositions of the atmospheres and/or 

 surfaces of objects such as comets, Jupiter, Titan, Saturn, Neptune, and 

 Uranus-all of which are known to contain at least simple organic molecules— 

 and investigations of the chemistry of carbonaceous asteroids, can provide 

 valuable insights into the nature of organic chemical evolution within the solar 

 system. Detailed analysis of solar system objects, particularly the Moon and 

 Mars, including careful assessment of their cratering histories, could advance our 

 understanding not only of the early history of the solar system during the 

 prebiological era on Earth, but also of later epochs throughout biological 

 evolution. 



