GENERAL ZOOLOGY 



The Origin of Life 



Tt has been calculated that the earth was detached from the sun between 

 5 and 10 billion years ago. For a few billion years after its formation, the 

 earth was presumably a glowing mass, far too hot to furnish an environment 

 for any form of life. The historic record of the existence of organisms begins, 

 in terms of our present knowledge, possibly as long ago as 3 billion years. 

 Sedimentary rocks dating from this age contain spores of simple plant-like 

 organisms. The level of organization of these fossilized traces of early life 

 reveals, however, that they were the product of immense evolutionary ad- 

 vance during long antecedent periods of time. We must suppose that as the 

 earth cooled and the watery hydrosphere condensed, living systems arose at 

 least some hundreds of millions of years before the appearance of the organ- 

 isms represented by our earliest fossils. There is no direct evidence about 

 the nature of the environment on earth during these vast reaches of time. 

 It may be speculated that in the primordial seas conditions of temperature, 

 radiation, and chemical constitution favored the formation and persistence of 

 an enormous number and variety of carbon compounds. Further, through 

 random combinations among these substances, physicochemical systems of a 

 relatively stable nature, with the fundamental characteristics of life, must 

 have arisen. It has been suggested that these "proto-organisms" may at one 

 stage have appeared in the form of "gene strings" — as aggregations of proteins 

 and nucleic acids with the capacity of self-duplication, able to select materials 

 from the surrounding medium and organize them into replicas of their own 

 structure. As previously indicated (p. 262), we may interpret the existing 

 viruses as modern representatives of this level of organization. It is possible 

 that the viruses originated as free-living forms, in an environment which sup- 

 plied all their requirements, although now they find only within the cells of 

 other organisms the materials necessary for self-duplication. 



It has been emphasized that at the present time living systems are not 

 known to originate through any process of spontaneous generation. They 

 occur only as organisms, and organisms arise only from pre-existing organisms. 

 The fact that this is demonstrably true under present conditions does not 

 eliminate the probability that living systems once arose, under very different 

 conditions, from precursors that were non-living. We may simply reason as 

 follows: life was not always present upon earth, and since life is now present, 

 it must have originated at some time from non-living materials. 



With the appearance of primordial living systems, the stage was set for the 

 great drama of organic evolution. As the earthly environment continued to 

 change, variant forms of organisms were always available with the capacity of 

 survival under new conditions, and more conservative or less adaptable forms 

 became extinct. The history of life on earth, from its inception to the present 

 day, is an account of the evolutionary changes of living things. It is one of 

 the most fascinating stories known to man. 



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