clarke: early forms of life 603 



BIOLOGY. — Geochemical evidence as to early forms of life.- 1 F. 

 W. Clarke, Geological Survey. 



When life began on earth the conditions favorable to its devel- 

 opment were, generally speaking, somewhat different from what 

 they are today. Rocks derived from the remains of living or- 

 ganisms did not, of course, exist; and the only sediments were 

 those due to erosion, increased, doubtless, by volcanic dust and 

 other ejectamenta. The surface of the land was made up of 

 primitive rocks, and among them the specifically lighter varieties 

 probably predominated. By erosion these rocks were gradu- 

 ally decomposed, and their more soluble constituents were taken 

 up by the primeval waters, whose character gradually changed 

 as the process of erosion went on. At first, silica and alkalies 

 passed into solution, with lime and magnesia in much smaller 

 proportions. A large part of the lime and magnesia in the waters 

 of today is derived from the solution of limestones of organic 

 origin, which came into existence later. Possibly algal and 

 foraminiferal limestones were among the earliest to form large 

 masses, but of that it is not well to be too positive. We can 

 only assume that the simplest forms of life came first, even 

 though their remains have since been obliterated. In geologic 

 time the complex forms are relatively modern. 



So far as we are able to judge from anything like positive evi- 

 dence, the earliest living organisms were aquatic, and their 

 physical constitution was determined by the character of their 

 environment. Whether the waters were warm or cold we do 

 not know, and speculation upon that subject is hardly profitable. 

 Whether the primeval ocean was fresh or saline is also uncertain, 

 but we can assert that the composition and concentration of its 

 dissolved salts have undergone great changes and are still chang- 

 ing. The enormous load of saline matter annually poured into 

 the ocean by rivers is evidence that can not be ignored. Part 

 of that load remains in solution, part is precipitated, either di- 

 rectly or through the agency of plants and animals, and so the 

 changes are brought about. These changes in the environment 



1 Published with the permission of the Director of the U. S. Geological 

 Survey. 



