Life: Its Nature and Origin 51 



FIRST LIFE 



Different authors have considered hfe to begin at various steps in 

 the chain of chemical evokition. It seems most reasonable to con- 

 sider that life began when a cellular protein-containing system arose 

 which, at some definite point in its growth, automatically and 

 inexorably divided to form daughter cells which just as inexorably 

 grew into forms exactly like their parent. 



This concept does not require the premise that life was mono- 

 phyletic. Many living lines could have arisen independently (Keo- 

 sian, 1960). The first might have perished from a variety of non- 

 predatory causes, or it might have been destroyed by other more 

 virile types. Certain lines may have fused. However, a polyphyletic 

 origin is not at all a necessary prerequisite of this theory. If a 

 single organism arose answering our description of life, this could 

 have been sufficient to begin the whole subsequent sequence of 

 living forms. Whether or not the first life was monophyletic or 

 polyphyletic, uninterrupted generations of some lines must have 

 succeeded each other from near the dawn of life, some billion or 

 more years ago, down to the present. Until photosynthesis evolved, 

 the biochemical staples formed in the air and in the escaping gases 

 of the earth must have provided the energy and food needed for 

 growth and reproduction. 



Whether or not the facts and theories presented above indicate 

 the path along which life evolved, they indicate that in the pre- 

 biological world a complex organic chemical and physical evolu- 

 tion occurred that could have led ultimately to the evolution of 

 life. It would seem also that, before life evolved, complex chem- 

 icals underwent a natural selection based on longevity, ability to 

 absorb other chemicals, and ability to withstand changing ecological 

 stresses of the environment. The ultimate surviving macromolecules, 

 those which made the transition to living matter, unquestionably 

 imposed on life certain restrictions concerning its future evolution. 

 Any subsequent single change would have to be compatible with 

 the already existing chemistry. 



EVOLUTION OF THE MODERN CELL 



Because the form of the first living organisms is not known, it is 

 difficult to do more than speculate on the evolution of primeval 

 living things into the complex structure represented by even the 

 simplest known complete living entity, such as a bacterium. The 



