THE ORIGIN OF LIFE 



OPARIN AND CHEMICAL EVOLUTION 



The earth may have originated as a fragment from the sun. The probable 

 chemical composition of the earth in the earliest geological periods can be 

 surmised from spectrographic analysis of the sun and of stars in early 

 stages of their physical evolution. All of the elements which enter into the 

 composition of protoplasm were probably present as inorganic com- 

 pounds. Free nitrogen, hydrogen, and oxygen, gases which now form so 

 large a portion of the earth's atmosphere, probably were present at the 

 very beginning, because they are present as free elements in the sun. But 

 they were probably soon lost to outer space, for it is unlikely that the 

 gravitational pull of the earth is great enough to hold such light elements 

 at the high temperatures which prevailed during the earliest ages of the 

 earth's history. So these elements were probably left only in compounds. 

 A large quantity of the hydrogen and oxygen were probably united as 

 water, but this was certainly for long ages present as superheated steam. 

 The hot vapors would rise toward the cold outer layers of the atmosphere, 

 condense and fall as rain, onlv to be acrain converted to steam before 

 striking the earth. Gradually the earth cooled sufficiently to permit the 

 rainfall to strike the earth, then to begin the formation of pools and larger 

 bodies of water. Optimum conditions for solubility and reaction thus ex- 

 isted, and the entire earth was a great crucible for random compound 

 formation and re-formation. 



Origin of Organic Compounds. In such a situation, where the most 

 important elements of organic compounds— carbon, nitrogen, oxygen, and 

 hydrogen— are reacting at random with each other and with many other 

 elements, forming countless compounds which in turn react in such com- 

 binations as may be possible, it is highly probable that, sooner or later, 

 organic compounds or their precursors would appear. It is known that 

 methane (CH4), the simplest of organic compounds, is present in the at- 

 mospheres of some of the cooler stars. And more complex hydrocarbons 

 ( compounds of carbon and hydrogen ) have been found in meteorites, and 

 so it is certain that they can originate without the intervention of living 

 organisms. These may have been preceded by the compounds of carbon 

 with metals, for such metallic carbides crive rise to hvdrocarbons when 

 treated with steam. So it is altogether probable that such compounds were 

 formed in abundance while the cooling earth was still much too hot to 

 permit the existence of life. In a similar way, one would expect the forma- 

 tion of ammonia by the reaction of steam and metallic nitrides. Further, 

 cvanogen (CN) and dicyanogen (NC'CN) were probably also formed. 



Other known pathways for the abiotic origin of organic compounds 

 include ultraviolet irradiation of formic acid, which yields large organic 

 molecules, including amino acids; the action of cosmic rays upon carbon 

 dioxide and water vapor, which yields organic acids; and the action of 

 lightning upon an atmosphere of methane, hydrogen, ammonia, and 

 water vapor, which yields a mixture of organic compounds, including 

 amino acids. All of these reactions have been proven in the laboratory. 

 Using the last-mentioned method, with electrical sparks discharged into 



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