PRE-CAMBRIAN AND PALEOZOIC ERAS 145 



most one-celled plants and animals, and most coelenterates (e.g., jelly- 

 fishes) are notably lacking in structures likely to be preserved as fossils. 

 Some or all of these are the forms we should expect to have been present in 

 the early days of life on the planet. In the second place, most of the 

 Archeozoic rocks were later subjected to metamorphism; hence most fos- 

 sils present were destroyed (p. 135). As noted previously, however, fossils 

 of algae have been found in rocks approximately 2600 million years old 

 (p. 138 ). Since these simple plants doubtless possessed the power of photo- 

 synthesis (p. 84), this method of capturing energy from the sun and utiliz- 

 ing it in synthesis of organic compounds has existed on the earth for over 

 two billion years (Briggs, 1959). 



In addition to these fossils, indirect evidence that life was present in 

 Archeozoic oceans is furnished by deposits of graphite in rocks of this era. 

 Graphite, the "lead" of our pencils, is composed of carbon, as is coal. In 

 later periods of earth history deposits of carbon in the form of graphite and 

 coal represent the remains of vegetation. Accordingly we may reasonably 

 conclude that Archeozoic graphite was also derived from simple plant life, 

 probably in the main from algae. 



The presence of iron ore in Archeozoic rocks is also sometimes considered 

 evidence of the existence of life, since iron ore frequently represents the 

 result of bacterial action. Since, however, iron ore may be deposited by 

 processes that do not involve the action of living things, the evidence here 

 is not so conclusive as it is in the case of graphite. 



Limestone in deposits of this era may also have been derived from living 

 organisms. The fossils of algae, mentioned above, are of this nature. Some 

 of the limestone deposits from later periods represent the massed shells 

 of such protozoans as foraminiferans and the skeletons of such coelenter- 

 ates as corals. Yet some limestone is of inorganic origin and hence the 

 mere presence of limestone does not prove that life existed at the time the 

 deposit was formed. 



PROTEROZOIC ERA 



Like rocks of the preceding era, many Proterozoic rocks 

 were subjected to metamorphism, with consequent destruction of any fos- 

 sils they may have contained. Since there are, however, large deposits of 

 unmetamorphosed Proterozoic rocks, the scarcity of fossils in the latter is 

 somewhat surprising. The most abundant fossils from this era consist of 

 globular masses of limestone representing the remains of colonies of 

 algae. Many of these deposits are of large size, analogous in numerous 

 ways to the coral reefs in our present oceans. 



