2 LIFE: ITS NATURE AND ORIGIN 



"Although the gene is the simplest vital unit definitely known,* 

 it is possible that in some cases the gene itself is an aggregate of 

 simpler units which have been called 'gene elements,' or 'genels.' 

 The hypothetical lower limit of vital units is in the molecular 

 order, for nothing simpler than a molecule, for example an atom, 

 depends for its increase in numbers upon forces contained only 

 within the unit itself. 



"The question arises: is there in nature any free-living unit of 

 the grade of organization and properties of the gene? It has been 

 suggested that in the bacteriophage and similar ultrafiltrable 

 particles we are dealing with living units (bionts) which either 

 are actually of this degree of simplicity or approach it closely. 



"Considering briefly the bearing of these ideas on the origin of 

 life — a very speculative endeavor — it would seem that no biont 

 as complex in its order of aggregation as a cell, or even a bac- 

 terium, could have been an initial form of life. The structure of 

 a cell is a box-within-box series of units, of successively simpler 

 orders of aggregation, and it seems reasonable to suppose that the 

 simplest of these units was the first to appear in evolution." 6 



To the primal living unit, the cooling earth was in effect a 

 vast sterile, culture medium, and there existed none of the in- 

 numerable and omnipresent mold spores, bacteria, and other 

 contaminants that are now the bane of bacteriologists. On the 

 other hand, while free from competitors and predators, the origi- 

 nal biont had at its disposal only such molecules as chance for- 

 mation gave it. 



But matter itself is pregnant with the possibility of life. Once 

 a self-producing unit was formed, its speedy spread was limited 

 only by the then-existing chemical and physical conditions. From 

 such a unit, its later analogs, and their modified forms and ever- 

 varying progeny, there emerged the enormous numbers and varie- 

 ties of organic molecules now widely available for, and in many 

 cases essential to the life of present-day organisms. Plants and 

 animals surviving under present conditions are adapted to world- 

 wide chemical and physical life with each other, and are in most 

 cases so interdependent that they no longer possess the ability to 

 synthesize all the molecules which they need to live and to propa- 



* According to the criteria of "living" given later in this volume, ultrafiltrable 

 viruses and bacteriophages are vital units, and many of them are resolvable in the 

 electron microscope. 



