ORIGIN OF ORGANIC COMPOUNDS 143 



that the first carbonates were formed. At the same time 

 direct photochemical changes of methane and ammonia were 

 going on, for both of them absorb ultraviolet light, methane 

 at a wavelength below 1,450 A and ammonia at a wavelength 

 below 2,250 A. Under these conditions methane forms hydro- 

 gen, higher saturated hydrocarbons and unsaturated hydro- 

 carbons, particularly ethylene. The ethylene thus formed 

 can be converted photochemically into acetylene and a whole 

 series of liquid hydrocarbons. Ammonia is decomposed 

 photochemically into NH2 -f h with the formation of hydrazine 

 NH2NH2 and other nitrogenous substances. The radicals which 

 were thus formed in the primitive atmosphere of the Earth 

 such as — CH3, ^CH2, =CH, — NH2, ^NH, and — oh reacted 

 with one another, giving rise to a large number of different 

 sorts of organic compounds, the simplest oxygen- and 

 nitrogen-containing derivatives of hydrocarbons.^^* 



The oxygen which was produced by the photolysis of water 

 must have reacted not only Avith ammonia and hydrocarbons 

 but also with other reduced substances, for example by 

 oxidising hydrogen sulphide and metals, particularly iron. 

 Thus, in spite of the continued photolysis of water and escape 

 of hydrogen, free oxygen did not appear in the atmosphere 

 of the Earth in significant amounts for a long time. 



On the basis of a study of the distribution of the isotopes 

 of sulphur in its oxidised and reduced compounds H. G. 

 Thode and colleagues^^^ reached the conclusion that the 

 original transition of the atmosphere of the Earth from the 

 reduced to the oxidised state occurred only 700 or 800 million 

 years ago, that is to say, at a time when, according to all the 

 evidence, life already existed on the Earth and photosynthesis 

 may even have begun. 



On the basis of a study of the abundances of isotopes of 

 lead and other elements various authors have given estimates 

 of the age of the Earth ranging from 3-4 to 5.3 x 10^ 

 years. ^^®'^^'' It follows that for at least 2-3 x 10^ years the 

 atmosphere of the Earth was reduced, or undergoing gradual 

 transition to the oxidised state, and that under these condi- 

 tions there occurred on the surface of the Earth the abiogenic 

 formation first of the simplest and later of more complicated 

 organic compounds. 



