The Problem of Stages in Biopoesis 51 



are characterized by the presence of lipoprotein membranes giving rise to other 

 types of organelles — mitochondria, plastids etc. The sixth stage envisages the 

 formation of a membrane-enclosed cell, the first organism of the type of the 

 present day bacteria. The seventh stage is that of the appearance of the nucleated 

 cell of the protozoa, which is the basis of all the later evolution of species both 

 plant and animal. 



In the other columns of the Table i have put down hypothetical sequences of 

 biochemical evolution running in parallel with the structural evolution outlined 

 above which is shown in more detail in Fig. i. Here there is, on account of 

 the greater complexity, no such clear sequence. The evolution of the main meta- 

 bolites — amino acids, sugars, fats, enzyme systems, proteins and co-enzyme 

 systems, nucleotides — must have proceeded in close interaction with each other 

 but the pattern is still difficult to unravel. Nor are the correlations with the stages 

 in the evolution of structure by any means as yet verifiable. Indeed some of the 

 correlations suggested have been inserted only to provide the elements of a 

 plausible detailed account. For instance, the origin of pentose sugars has been 

 referred to the third or eobiont stage while that of the nucleic acids only occurs 

 in the fourth stage. Good arguments could be adduced for referring them indeed 

 to the second and third stages but that of the sugars must in all cases precede 

 the nucleoside of which it is part. A similar doubt affects the position in the 

 sequence of photosynthesis, which by providing abundant new energy must 

 have led to a great break-out of vital evolution. It may, however, as I have earlier 

 maintained, have occurred in a more primitive form from the very beginning, 

 using ultraviolet light in the first place. I have abandoned this view because I 

 now think that the essential anabolic step — the reduction of carbon dioxide to 

 carbohydrate — can have been effected in the dark with hydrogen and free energy 

 supplied by the oxidation of ammonia to nitrogen. 



I realize that this is still a most debatable point, but the picture of an enlarged 

 table in which every alternative suggestion finds some place is one I could not 

 contemplate at this stage, though it might be well worth while to appoint a com- 

 mittee to draw one up as a provisional working hypothesis after this Symposium. 

 The major problems of biopoesis, as I have seen them, are difficult enough to 

 solve and no doubt others have found or will find many more before a generally 

 accepted scheme can be drawn up. However, I list nine of them here in what I 

 think may have been chronological order, and it is to be hoped that some of them 

 will receive elucidation in the course of our present discussions. 



1 . What was the constitution of the original atmosphere ? Was it oxidative 

 or reductive in character ? Did the carbon dioxide come from the lithosphère or 

 from oxidation of hydrocarbons ? 



2. What was the nature of the first simple organic compounds ? Had they a 

 carbohydrate or amino character? Or were both formed simultaneously? 



3. At what stages were triose and cyclic (pentose) sugars formed ? At what 

 stages were pyrimidines formed ? When were purines formed from them ? 



4. What was the precursor of porphyrins as the molecule trap for photo- 

 synthesis ? 



