Chemical Diversity and the Origins of Life 77 



On the conventional theory that Earth was originally incandescent with approxi- 

 mately its present mass, the oceans can only have formed after there was a 

 soUd crust and its components come from those parts of the total mass that 

 later came near enough to the surface to be washed. Similarly the atmosphere 

 consists of those original volatile components that were heavy enough to be 

 retained by gravitation, together with subsequent exhalations. On the accre- 

 tional theory, on the other hand, it is probable that a much larger proportion 

 of the whole mass was exposed to hquid water during the process of condensa- 

 tion. It is already difficult, on the conventional theory, to account for the low 

 salt content of the ocean; present-day rivers would bring down that amount in 

 a fraction of the time for which the ocean is thought to have existed. On the 

 accretional theory, the problem of what materials were initially present in solution 

 is raised in an even more acute form and a recognition of this may keep us 

 from being dogmatic about the composition of the pools that first formed on 

 Earth. 



It is unlikely, on either theory, that the probiotic atmosphere contained O2 

 because of the immense preponderance of Fe++ over Fe+++ in rock. O2 is now 

 produced by photosynthesis at such a rate that each molecule of it in the atmos- 

 phere has a 'half-hfe' of only a few thousand years before returning to CO2 or 

 H2O. It is tempting to assume that O2 had this origin initially but that assumption 

 cannot accoimt for the immense deposits of sedimentary Fe+++ and sulphate. If, 

 as seems certain, these are made by atmospheric oxidation of Fe++ and S or 

 H2S, it is hard to see where all the C or other reducing material that is simul- 

 taneously produced during photosynthesis has gone to. As Herbert Spencer 

 first pointed out, the C in shale, coal and oil is approximately equivalent to the 

 O2 in the atmosphere now. A probiotic origin for O2 must be sought and one is 

 readily available in photolysis of H2O in the upper atmosphere followed by loss 

 of H2 into space. The atmosphere is a system through which the CO2 now present 

 in limestone and dolomite, and the O2 now present in sulphates or instrumental 

 in making Fe+++, has flowed. The quantities present in, or represented by, these 

 'fossil' forms of CO2 and O2 are very much greater than the quantities in the 

 atmosphere or ocean now. This, taken in conjunction with the fact that rates of 

 production and sequestration are uncertain by factors of at least 10, makes it 

 legitimate to conclude that dogmatism about the composition of the atmosphere 

 at any stage in Earth history before perhaps the Jurassic is out of place. Very 

 large amounts of O2 and CO2 have passed through it and the concentration of 

 either at any phase of history is still a matter of assumption. If a certain type 

 of atmosphere is necessary for a certain type of biopoesis (see p. 78, footnote) it 

 is reasonable to postulate that atmosphere for argument but it is not reasonable, 

 on existing information, to try to exclude some other type of atmosphere or 

 mode of biopoesis. The fact that amino acids and related substances are made 

 when electrical discharges pass through a mixture of CH4, NH3, H2O and H2 [4] 

 is sometimes quoted as evidence that that was the composition of the 

 probiotic atmosphere. This would be logical only if it were demonstrated that 

 similar substances are not formed in other gas mixtures and if we know that these 

 substances were the substrate from which the first Ufe grew. There are geo- 



