48 THE BIOLOGICAL APPROACH 



the presence of free oxygen in our atmosphere. Therefore, in the 

 absence of free atmospheric oxygen, even the anaerobic Hfe we now 

 know would have difficuhy in surviving, not because its metaboHc 

 processes would not be adapted to suit that atmosphere, but because 

 it would be killed off by the ultraviolet sunrays directly reaching the 

 surface of the earth. Only if shielded by water or soil, in lakes and 

 oceans, or in pores in the soil, would there be a chance for early 

 life similar to our present anaerobic microbes to survice in such an 

 environment. 



ANOXYGENIC PRIMEVAL ATMOSPHERE 



All present-day theories about a natural origin of life on earth, 

 theories which all go back to the original ideas of Oparin, all postu- 

 late an early or primeval atmosphere of reducing character, in which 

 there is no free oxygen. 



Oxygen will have been present at that time only in chemical 

 compounds, of which water will have been the most important. Apart 

 from water, this early atmosphere, and also, of course, the early 

 hydrosphere, the contemporaneous rivers, lakes and oceans, will also 

 have carried carbon and nitrogen, mostly in compounds also, and a 

 host of other elements. Amongst these sulphur and phosphorus may 

 have been the most important as catalysts in early energy cycles. 

 The difference with our present atmosphere lies in the fact that there 

 could not have been present, in that primeval atmosphere, an ap- 

 preciable amount of free oxygen. 



Views on the relative abundance of the elements and their com- 

 pounds in that early atmosphere and hydrosphere vary with different 

 authors. The mixture of water with these compounds in the early 

 hydrosphere is often referred to as the 'thin soup', which obviates 

 the necessity of making too strict an estimate of its composition. 



INORGANIC SYNTHESIS OF 'oRGANIc' COMPOUNDS IN PRIMEVAL 



ATMOSPHERE 



The important thing is not the exact composition of the primeval 

 atmosphere and hydrosphere. The important thing is the absence of 

 free oxygen and the presence, consequently, of relatively simple com- 

 pounds of the elements C, O, H, N, S and P, which nowadays occur 

 exclusively as natural organic compounds, accompanying compounds 



