ORIGIN OF LIFE IN THE OCEANS 121 



theless, their occurrence makes it possible to study the first stages 

 in the origin of life under natural conditions. The original hydro- 

 carbons formed in the earth's crust were only the first step in a 

 long series during abiogenetic organic evolution which took place 

 when the earth was still devoid of life. This took place over a 

 period of many billions of years and led to the emergence of very 

 complicated organic substances with large molecules similar to 

 those in living matter. 



How can we recognize the processes in the chemical evolution 

 for the second stage which took place billions of years ago when 

 the earth was uninhabited by living organisms? At first sight, it 

 appears that methods of geochemistry could be used to study the 

 abiogenetic conversions of carbon compounds taking place today 

 under natural conditions on the earth other than in living matter. 

 These should give the simplest and most reliable answers to the 

 problem. Such a study can certainly give us many valuable data, 

 but it must be remembered that life, having once originated on the 

 earth, radically changed many conditions on the surface of our 

 planet. At present, many of the phenomena which took place in 

 the past do not now occur, but new processes have arisen which 

 did not exist on the surface of the lifeless earth. Therefore, we 

 cannot compare contemporary geochemical data directly with that 

 for the early epochs of our planet. To study the conversions of 

 organic substances, however, we must simulate these early data 

 in laboratory experiments, which artificially reproduce conditions 

 on the primitive earth. Indeed, only a small portion of the hydro- 

 carbons synthesized under abiogenetic conditions could remain in 

 the lithosphere. All volatile carbon compounds gradually escaped 

 from the earth's crust into the atmosphere in the past, and this 

 also occurs now in the case of the natural hydrocarbon gases. 



The atmosphere of the lifeless earth differed from the present 

 atmosphere principally in its reducing character. This is suggested 

 not only by general theoretical considerations, but also by many 

 direct geological observations. Most of the free oxygen of the 

 present atmosphere was undoubtedly formed by the activity ot 

 green plants. If the earth again became devoid of life, free oxygen 

 would very soon disappear from its atmosphere due to adsorption 



