34 J. D. BERNAL 



which would diffuse out of the atmosphere. Nitrogen would steadily be built 

 up in the atmosphere and elementary sulphur or iron sulphide would be deposited. 

 The other main oxidizing reaction is strongly endergonic 



2H2O -> O2 + 2H2 or O2 + 4H etc. 



but the energy to effect it can be provided by short-wave radiation at the top of 

 the atmosphere. The concentration of molecular oxygen would tend to rise 

 until it sufficed to oxidize the hydrogen emitted by the dehydrogenation of NH3 

 and SH2 and also the ferrous iron derived from weathering of rocks. Free energy 

 available for biochemical reactions may also have been available from this source. 

 What this equilibrium concentration of oxygen would be is still a moot point 

 but it would seem on biochemical evidence alone that oxidation reactions, 

 though now furnishing most of metaboUc free energy, are not primitive. In 

 any case the high concentration of atmospheric oxygen now observed in the 

 atmosphere is now derived from photosynthesis, the mechanism of which, at 

 least using visible sunlight, seems to have a fairly elaborate biochemical history. 

 The type of endergonic reaction which could lead to organic compounds is, 

 however, now part of the photosynthetic cycle of reactions. 



CO2 + 2H ^ HCOOH formic acid 

 or CO2 + 4H ^ H2CO formaldehyde + H2O 



None of the actual reactions which occurred in the primitive ocean are Ukely 

 to be as simple as those indicated above. More probable variants and inter- 

 mediates will certainly be discussed at this Symposium. I only cite them to show 

 firstly that the major features of current metaboUsm were already present in the 

 primitive hydrosphere and secondly that the actual basic compounds which make 

 up hving organisms can be most easily envisaged as constructed mainly of carbon 

 dioxide and ammonia by not implausible processes which do not involve unsup- 

 ported hypotheses about the constitution of the primitive atmosphere. They are 

 moreover thermodynamically sound even without the assumption of the utiHza- 

 tion of free energy derived from the sun. The free energy suppHed on this hypo- 

 thesis would be simply due to the greater reduction potential of the surface 

 hthosphere with respect to an atmosphere from which hydrogen can escape. 



Life, geologically speaking, consists of the interference with secondary htho- 

 sphere-atmosphere reactions so as to produce a small but ever-renewed stock 

 of organic molecules. 



One of the major tasks of biopoesis is to establish the nature and the ordey- of 

 formation of the simpler molecular species out of which the more complex com- 

 pounds and polymers now found in living organisms were derived. I have no 

 competence in this field, but to complete the picture, I am indicating on the 

 diagram. Fig. i, the kind of molecular genealogical table that needs to be drawn 

 up. Both the hues of derivation and the order need to be examined. Indeed this 

 table is less a statement of probabihties than a setting of problems which may be 

 solved ultimately by a critical use of present-day biosynthetic changes (for it 

 does not follow that the present derivations were the same at the outset) and a 



