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presented with a comparative approach in assessing the early evolu- 

 tion of the solar system and the origin of life. We are adjusting our 

 sights to the understanding of the planetary stage on which life 

 enters as a player. Was the ancient Earth the only stage on which life 

 could play? This is the deepest question that future research must 

 answer. From comets to planets, NASA is the prime agency gathering 

 the data necessary for providing an answer to this question. 



THE EARTH 



Studies of the origin of life require an accurate reconstruction 

 of conditions and events on the early Earth. Investigations of those 

 matters, once rare, are becoming more common as new techniques 

 become available and as other developments in Earth science allow 

 problems to be more clearly defined and profitably attacked. As this 

 work proceeds, it must constantly be borne in mind that the early 

 Earth was, quite literally, "a different planet." The contrasts 

 between surface conditions on the early Earth and those on the 

 modern Earth are nearly as large as the contrasts between the surface 

 conditions presently found on the Earth and on Mars. It is not just a 

 play on words, therefore, to speak of a "mission to the early Earth" 

 just as we might speak of a "mission to Mars." While the latter 

 involves the use of spacecraft and a journey of millions of miles, the 

 former presents equal challenges (and promises equal scientific 

 returns) in a journey across billions of years. 



The space sciences thus have a special contribution to - and 

 should have a special interest in - the studies of the earliest phases of 

 Earth's history. The space scientist's avoidance of "geocentric bias," 

 the unwarranted attribution of Earth-like characteristics to other 

 planets, is perfectly appropriate to studies of the early Earth. 

 Equally, the development of an accurate view of the earliest stages of 

 Earth's history can provide crucial information regarding the origin 

 and formation of planets generally, and can significantly constrain 

 theories regarding the origin and development of the solar system. 



Contrasts between the ancient and the modern Earth are 

 nowhere greater than in biology, which has embodied a progression 



