26 



4.43 b.y. The complete accumulation of the Earth is thought to have 

 taken somewhat longer than that of the meteorite parent bodies, and 

 it is therefore unlikely that the Earth is older than 4.55 b.y. Most 

 likely the "corrected" lead isotope age of the Earth (4.53 b.y.), as 

 discussed above, is within 50 m.y. of the time at which the Earth was 

 formed and differentiated into silicate mantle and iron core. The real 

 uncertainty in this figure could easily be 40 m.y., but is unlikely to 

 be as large as 100 m.y. A value and range of 4.50 ±0.1 b.y. would 

 seem conservative. This implies at least 0.5 b.y. — indeed, something 

 like 0.6 or 0.7 b.y. — between (1) the formation of the Earth as a 

 solid differentiated planet and (2) the oldest known rocks. 



VISIBLE TRACES IN THE ROCKS 



What then is known about the history of life on Earth? What 

 were the trends that shaped the course of evolution, and over what 

 time periods did they occur? 



Two generalizations are clear. First, the history of life on Earth 

 is on the whole the history of microscopic, rather than of visible, 

 organisms. Because of our naturally anthropocentric myopia, as well 

 as the relative ease with which fossils of the larger plants and animals 

 can be discovered and studied, the history of organisms large enough 

 to be seen with the unaided eye has received a disproportionately 

 large share of scientific attention. The real situation is vastly differ- 

 ent. Indeed, it is now known that the Earth's biota was composed 

 solely of microscopic forms of life and their colonies for nearly 85% 

 of the total history of life on this planet. All larger organisms are by 

 comparison recent additions, interesting and significant, but they 

 have been preceded by a long and well-developed evolution of 

 microscopic forms. 



Second, the type of life inhabiting the globe and the nature of 

 evolutionary trends through time depend upon the peculiar geology 

 of our planet. Earth, unlike all other known bodies of the solar sys- 

 tem, is an aqueous planet, some 71% of its surface being covered by 

 a thin, watery veneer. It is thus not surprising that liquid water 

 (H 2 0) is the major component of every known form of life. Water is 

 the "universal solvent," the fundamental medium without which life 



