THE EVIDENCE FOR EVOLUTION 721 



are probably the transformed remains of plants and animal bodies. Al- 

 though graphite can originate inorganically, its distribution in the rocks 

 suggests that it was formed organically. If the amount of graphite in these 

 rocks can be taken as a measure of the amount of living things in the 

 Archeozoic, and there are reasons for believing that this is justified, then 

 life must have been abundant in the Archeozoic seas, for there is more 

 carbon in these rocks than in the coal beds of the Appalachians. 



Proferozoic Era. The second geologic era, which lasted about one 

 billion years, was characterized by the deposition of large quantities of 

 sediment, and by at least one great period of glaciation during which ice 

 sheets stretched to within 20 degrees of the equator. There was less vol- 

 canic activity in this than in the preceding era and the rocks are better 

 preserved. Only a few fossils have been found in Proterozoic rocks but 

 they show not only that life was present but that evolution had pro- 

 ceeded quite far before the end of the era. Plants and animals were 

 differentiated, multicellular forms had evolved from unicellular ones 

 and some of the major groups of plants and animals had appeared. 

 Sponge spicules, jellyfish, and the remains of fungi, algae, brachiopods 

 and annelid worm tubes have been found in Proterozoic rocks. 



Paleozoic Era. A second great revolution ended the Proterozoic 

 era. During the ensuing 360,000,000 years of the Paleozoic every 

 phylum and class of animals except birds and mammals appeared. Some 

 of these animals appeared and became extinct in a short time (geo- 

 logically speaking) and their fossils provide convenient markers by 

 which rocks of the same era in different localities can be correlated. 



The fossil deposits of the first three periods of the Paleozoic era, 

 the Cambrian, Ordovician and Silurian, were mostly laid down in the 

 seas. Large shallow seas covered most of the continents during these three 

 periods and they teemed with life. Many of these forms had hard skele- 

 tons or armor coverings which left a good fossil record. The organisms 

 living in the Cambrian were so varied and complex that they must have 

 evolved from ancestors dating back to the Proterozoic era. Appar- 

 ently both plants and animals lived in the sea, and the land was a curi- 

 ous lifeless waste until the Ordovician, when plants became established 

 on land. The Cambrian seas contained small, floating plants and animals 

 that were eaten by primitive, shrimplike crustaceans and swimming 

 annelid worms. The sea floor was covered with simple sponges, corals, 

 echinoderms growing on stalks, snails, pelecypods and primitive cephalo- 

 pods. An exceptionally well preserved collection of Cambrian fossils 

 was found in the mountains of British Columbia; it included annelids, 

 crustaceans, and a connecting link similar to peripatus. The most numer- 

 ous animals were brachiopods and trilobites. Brachiopods, sessile, bi- 

 valved plankton feeders, flourished in the Cambrian and the rest of 

 the Paleozoic. One of the present day brachiopods, Lingula, is the 

 oldest known genus of animals and is almost identical with its Cam- 

 brian ancestors. The trilobites (Fig. 16.2) were primitive arthropods, 

 with flattened, elongated bodies covered dorsally by a hard shell. The 

 shell had two longitudinal grooves that divided the body into three 

 lobes. On the ventral side of the body was a pair of legs on each somite 

 but the last, and each leg was biramous, had an outer gill branch and 



