Life: Its Nature and Origiii 55 



which the cell is left alive and still in possession of enough of its 

 fermentation system to provide energy for growth. Although not 

 necessarily evidence for it, this demonstration at least is in harmony 

 with the idea that primeval cells evolved by adding molecular 

 machines (or perhaps occasionally groups of machines) one after 

 another to the earlier and less inclusive organism. 



These are only a few highlights of the events which must have 

 occurred in the evolution of primeval life. Other aspects, details, 

 and speculations on this subject may be found in papers by many 

 investigators, including Blum (1951, 1955), Calvin (1959), Fox 

 (1960), and Wald (1954) 



Evolution of Sex 



We may presume that changes leading both to elaboration ot 

 chemical compounds and to incorporation of new molecular ma- 

 chines into the structure of the organism arose by genetic mutation 

 and that they were associated with enzymatic compounds con- 

 trolhng inheritance. This means that individuals possessing these 

 new physiological characters would have, for example, better 

 enzymatic action, better buffering, or new sources of food avail- 

 able. Natural selection for such improved traits would be strong. 



An even stronger selection pressure would have been exerted if 

 different but compatible improvements, arising as unit characters 

 in different individuals, could have been combined in a single cell. 

 It is therefore plausible to assume that some sort of recombination 

 mechanism, that is, sex, arose fairly early in the evolution of life. 



How a sexual mechanism started is unknown. Perhaps it originated 

 in the form of small, freed genetic particles similar to the "combin- 

 ing principles' of bacteria. Dougherty (1955) suggested that the 

 boundaries of early cells broke down at maturity, freeing into the 

 aqueous medium bundles of genetic material which later recom- 

 bined to form new cells and that during this process different 

 combinations of characters resulted. It is also possible that tend- 

 encies for occasional diploidy arose in some line and that the first 

 sexual types arose from this beginning. 



Implicit in a regular sexual process are two mechanisms: one 

 insuring that a diploid cell divides into haploid cells, another in- 

 s:; ^ig that haploid units combine to form diploid units. The exact 

 icchanism triggering the chromosomal halving has not yet been 

 discovered, but a mechanism bringing about the fusion of haploid 

 units has been partially demonstrated in certain yeasts. It has been 



