102 Inside the Living Cell 



cesses which might be thought to represent at least a step or two in 

 the elaboration of living forms. 



It is a very curious thing that, with one or two exceptions, practi- 

 cally no intermediate stages between simple inorganic substances 

 and living things or their products are known to exist. Practically all 

 the organic substances on the earth are products of life. It is possible 

 that the hydrocarbon oils which exist very plentifully in some strata 

 are an exception, but this is uncertain and it is quite likely that they 

 are to a considerable extent themselves the products of highly de- 

 veloped life, like the coal deposits. This is demonstrated by the fact 

 that, in some cases, they contain 'optically active' compounds, i.e. 

 when two possible forms of a compound exist, which are mirror 

 images of each other, only one occurs. This is an indication of living 

 origin, because natural processes (except crystallization) do not 

 usually distinguish between the two optically active forms of one 

 molecule. 



We do not find organic compounds being elaborated by any pro- 

 cess which does not involve life anywhere in the world. 



We are forced to adopt one of three alternatives: 



(1) Life did not originate in this world. As Arrhenius suggested, 

 when he was confronted with the difficulty of finding sufficient time 

 for the evolution of life, it might have arrived from other worlds in 

 the form of spores. However, this only shifts the difficulty of account- 

 ing for the origin of life to other places, and in any case the evidence 

 of cosmology suggests that the earth has existed for an appreciable 

 fraction of the period of existence of the whole universe. 



(2) Life did originate on this world, but the conditions at the surface 

 of the earth have altered so greatly, that reactions leading to life no 

 longer occur. This view has been put forward and supported by the 

 Russian scientist, A. I. Oparin, in a book on The Origin of Life, which 

 was first published in an English translation in 1938. Oparin argues 

 that in the early stages of the earth's history, the surface rocks con- 

 tained considerable quantities of metallic carbides, which, by react- 

 ing with water, would form first methane or acetylene and later more 

 complex hydrocarbon compounds. This would provide an 'organic' 

 environment, which would be much more favourable to the forma- 

 tion of complex organic compounds than that which exists now. 

 There is evidence for hydrocarbons like methane in the atmospheres 

 of other planets. Methane, as well as ammonia, have been detected 

 in the atmosphere of Jupiter and methane alone in Uranus and Nep- 

 tune. 



Consider what would happen if such an atmosphere were exposed 

 to intense solar radiation. The sun produces not only visible light, but 



