29 



their construction. They are traces, rather than fossil organisms 

 themselves. Research in this phase is vigorous and ongoing; over the 

 past two decades it has resulted in substantial increases in our knowl- 

 edge about Proterozoic evolution. 



3. The ancient phase: The later portion of the Archean Eon, 

 extending from 3.5 to 2.5 b.y. ago, is very poorly known. Here, the 

 whole fossil record consists only of some seven or eight stromato- 

 litic deposits, and of a few units known to contain microfossils or 

 suggestive micro fossil-like objects. Indeed, only one diverse, cellu- 

 larly well-preserved microbiota has as yet been detected in rocks of 

 this age. The oldest assured, bona-fide records of life now known are 

 those contained in the sediments of the Warrawoona Group of 

 Western Australia (fig. III-3). Those rocks are approximately 3.5 b.y. 



Figure IH-3.- Filamentous, prokaryotic (i.e., bacterium-like) microfossils in 

 petrographic thin sections of carbonaceous chert from the Early Precambrian 

 (ca. 3.5 b.y. old) Warrawoona Group in the North Pole Dome region of the 

 Pilbara Block, northwestern Western Australia. Photo courtesy of J. W. Schopf 



