22 FROM FISH TO PHILOSOPHER 



ized in such a manner as to preserve and to reproduce 

 itself, and even this process presents considerably less 

 difficulty theoretically than it did ten years ago. 



The first forms of life— and they may have been highly 

 diversified— were certainly below the cellular level, be- 

 cause even the unicellular animals or plants, or Protista, 

 represent an advanced stage in evolution in which the 

 organism has acquired elaborate mechanisms for nutri- 

 tion and reproduction. Since most of these unicellxilar 

 organisms are soft-bodied, it is not to be expected that 

 anything more than traces of their existence will be 

 found in the fossil record. Indeed, few fossil vestiges of 

 life of any kind are discoverable before the Paleozoic era; 

 the most abundant are deposits of calciimi-containing 

 algae, with only rare filmlike fossils which might be the 

 remains of bits of seaweed. That other plants, probably 

 of microscopic size, were also fairly abundant is indi- 

 cated by the presence of carbonaceous matter, in the 

 form of coal or particles of carbon in sedimentary de- 

 posits, but what forms they may have had remains a 

 mystery. A few trails or burrows, presumably made by 

 wormUke creatures living in the mud, present the only 

 evidence of animals. 



Then, with the opening of the Cambrian, the first 

 period of the Paleozoic era— and the first period shown 

 in the geologic sequence as illustrated in Figure 2— 

 hving organisms suddenly appear throughout the seas in 

 great variety and already diversified into all the major 

 branches of the marine invertebrates: the Foraminifera, 

 sponges, coelenterates, worms, brachiopods, gastropods, 

 echinoderms, and arthropods. Whether these inverte- 

 brates were evolved from a single primordial form or 

 from several forms that from the beginning had de- 

 veloped independently cannot be said. Given the com- 

 position of the sea and air, the nature of available atoms 

 and molecules, they would in any case have pursued a 

 closely parallel course. 



