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atmosphere at that time was certainly not highly reducing; in fact, 

 the bulk composition of the Isua rocks is surprisingly similar to that 

 of younger rocks in equivalent settings. In this sense they give elo- 

 quent testimony to the notion that 3.8 b.y. ago the Earth had 

 already settled down to a regime that is quite similar to that of the 

 present day. 



Elemental carbon is present in these sediments. The Isua sedi- 

 ments have been heated to such high temperatures since deposition 

 that virtually no extractable organic compounds remain. At present, 

 a biological origin for carbon in the Isua rocks seems unproven. Life 

 could have started before the time when Isua was formed, perhaps 

 much closer to the birth of our planet. A search for sedimentary 

 rocks older than 3.8 b.y. is obviously needed. 



Several continents are known to contain 3.5-b.y.-old sedimen- 

 tary rocks, and these ancient areas may contain enclaves of even 

 older rock units. Areas in Australia and southern Africa, and an area 

 in central Greenland that is currently covered by ice may turn out to 

 be particularly promising targets in the search for more ancient 

 rocks. 



We believe that our Earth is about 4.6 b.y. old. At present we 

 are forced to look to other bodies in the solar system for hints as to 

 what the early history of the Earth was like. Studies of our Moon, 

 Mercury, Mars, and the large satellites of Jupiter and Saturn have 

 provided ample evidence that all of these objects were bombarded by 

 bodies with a wide variety of sizes shortly after they had formed. 

 This same bombardment must have affected the Earth as well. The 

 lunar record indicates that the rate of impacts decreased to its pres- 

 ent low level about 4 b.y. ago. On the Earth, subsequent erosion and 

 crustal motions have obliterated the craters that must have formed 

 during this epoch. Since it is generally believed that life on Earth 

 began during this period, the bombardment must have been part of 

 the environment within which this event occurred. 



Perhaps the most significant aspect for our consideration is the 

 realization that some of the impacting objects were large enough to 

 punch through the crust of Earth and formed large basins that could 

 be flooded by lava. These would be the terrestrial analogues of the 

 large circular maria on the Moon, the basins on Mars, and the ringed 

 structures on Jupiter's moons — Callisto and Ganymede. This is a 



