Vlll 



Just such a scene — we can infer the details rather well from the 

 complex fabric of the rock samples — would present itself at the 

 spot, now the western coast of Australia, where the oldest trace of 

 life in all the Earth is found. The time is long ago indeed, a time we 

 can estimate to within a few percent from a secure, mutually con- 

 firming set of radioactive decay measurements. The signs of copious 

 algal life, which bears a remarkable resemblance to the same forms 

 found throughout the record of the rocks up to the present day, 

 occur almost as early as the first dated rocks. One must emphasize 

 that this teeming life, single-celled, though colonial in nature, was 

 about all that lived on Earth, not only for the first pages of the 

 record but for four-fifths of our whole past. Not until a time only 

 0.7 billion years (b.y.) back can we surely see any relic of life more 

 mobile than the algal and bacterial mats. Indeed, they themselves 

 become more complex in microstructure and more powerful in their 

 chemistry over the 3 b.y. of their evolution and change. No life is 

 mobile (beyond the drift of plankton) until about that time, 0.6 or 

 0.8 b.y. ago. And it requires another couple of hundred million years 

 before anything alive, either plant or animal, can break close contact 

 with the waters, to stand well above the coast, the marsh, or the 

 damp soil. All the forms of life we see in the familiar fossil record, 

 everything so graphically drawn by the paleontological artists, all 

 those feathery sea lilies, bulge-eyed trilobites, all sharks and dino- 

 saurs, all ancient birds and beasts, all dawn palms or big kelp, all that 

 crawls and flies and swims and stalks, all that branches or flutters in 

 the wind, belong to the last 10% or 15% of life's long history on 

 Earth. 



The direct rock record supports one major plausible inference: 

 All life we know has evolved from single, small-celled beginnings, 

 forms like those still to be found as copious and vigorous participants 

 in the great geochemical cycles, the blue-green algae and their kin, 

 the bacteria. These had filled the shallow-water world in full vigor 

 even by the time of our earliest evidence. Theirs, of course, is the 

 triumph of lowly biochemistry — not motion, not sensory response, 

 not even structure on the scale of naked-eye visibility. It is rather the 

 microstructure, the complexities at the level of molecular helixes, 

 sheets, tubes and rods, and the complex biochemical pathways they 

 enable, which evolved over the entire first half or so of life's biog- 

 raphy. Thus, it is biochemistry we must search out back to the time 



