23 



III. THE TWO RECORDS: 

 IN THE ROCKS AND IN THE CELLS 



From the perspective of the long sweep of human history, over 

 the past two centuries or so, less than a dozen human generations, 

 scientists have begun to study, and to comprehend, the history of life 

 on this planet. Progress has been impressive — nearly 200,000 spe- 

 cies of fossil organisms have been discovered and described; the 

 evolutionary continuum that links life of the modern world to that 

 of earlier biotas has been extended far into the geologic past; and 

 great strides have been taken toward deciphering the timing and 

 nature of the major events in the development of life on Earth. 



Early investigators interested in the history of life concentrated 

 on those problems most readily amenable to investigation, and 

 already by the mid- 1800s when Darwin's On the Origin of Species 

 first appeared, the broad outlines of the history of animals and 

 plants were rapidly coming into focus. Indeed, it is this fossil 

 record - that of the "Phanerozoic Eon" of Earth history, the ages of 

 trilobites, coal swamp flora, dinosaurs, and the like - that normally 

 comes to mind when one thinks of the history of life on Earth. Yet 

 the Phanerozoic extends only a scant 600 m.y. into the geologic past 

 (fig. III-l). But this is only 15% of geologic time. What came before, 

 during the first 85%? How long before the advent of visibly large life 

 did the earliest organisms first appear? And what does the geologic 

 record reveal about the origins of life itself? These questions and 

 others like them have long been pondered — indeed, Darwin regarded 

 their solution as a necessary prerequisite to ultimate acceptance of 

 his theory of evolution — but it has only been within the past quarter 



Finely layered stromatolitic structures, of bacterial and/or 

 blue-green algal origin, from Early Precambrian rocks near 

 Bulawayo, Rhodesia, about 2600 million years in age. These 

 structures are among the oldest stromatolites - and thus 

 among the oldest fossil evidences of life - now known. 



Photo courtesy of J. W. Schopf. 



