30 



old; evidence indicates that diverse types of microorganisms, prob- 

 ably including photosynthesizers, were already extant at that time. 



4. The most ancient phase: The Early Archean (Hadean), 

 extending from the time of formation of the Earth, 4.5 b.y. ago, to 

 about 3.5 b.y. ago, is all but unknown; only one rock unit has as yet 

 been detected in this oldest portion of the geologic column, and that 

 unit (the Isua Supracrustal deposits of western Greenland) has been 

 severely altered by postdepositional motion, heat, and pressure. 

 Indeed, the sequence has undergone at least five stages of alteration, 

 including severe metamorphism (technically of amphibolite-grade, 

 viz., to 500°-600° C, and 2-8 kilobars), a sequence of events so 

 severe as to make meaningful interpretation of the Isua "fossil 

 record" virtually impossible (if, in fact, organisms that could give rise 

 to a fossil record actually existed in Isua time). Isua has been studied 

 mainly over the last 5 years. 



Thus, the fossil record as now known — rich and varied as it is 

 in younger rocks, and scant to nearly nonexistent in older rocks — 

 provides only the most limited insight into the timing and nature of 

 those events that led to the origin of life. The evidence establishes 

 that communities of complex, diverse microorganisms were extant at 

 least as early as 3.5 b.y. ago, and that these organisms resembled 

 modern bacteria in morphology, in ecology, and perhaps in biochem- 

 istry. Certainly, the origin of living systems must have occurred some 

 time earlier, perhaps long before 3.5 b.y. ago. When it occurred, and 

 how long the origin-of-life event took, is certainly not known. 



THE CARBON CHEMISTRY OF ANCIENT ROCKS: 



AN OPPORTUNITY 



As just discussed, rocks more than a few billion years old are 

 scarce, not because only few of them were made but because their 

 survival has been difficult. The examples that we have are, quite 

 literally, battered veterans. Even the layered pattern of stromatolites 

 rarely remains. If the rocks themselves have been mostly destroyed 

 or significantly altered, it follows that their organic molecular con- 

 stituents, which are much more fragile, must be in relatively poorer 

 shape. Delicate information-rich biological macromolecules have not 

 a chance of survival intact. On the other hand, not all organic matter 



