30 Life: Its Nature and Origin 



plicated almost beyond the limits of our imagination. The search 

 for those properties of life which will throw light on its origin has 

 been an arduous undertaking complicated by many factors. The 

 total evidence of phylogeny shows clearly that life began as a 

 simple cell suggesting that a study of the simplest unicellular 

 organisms may be the chief avenue for deducing the properties of 

 the primeval cells of first life; but here difficulties arise. In the first 

 place, it is highly unlikely that any living cell is as primitive as the 

 primeval cell because each persisting kind of cell has been evolving 

 to some extent during the two billion or more years that life has 

 existed. In the second place, it is difficult to study life. The bacteria 

 and viruses are excellent objects of study for many experiments on 

 chemical and physical properties, but they are too minute for 

 observation and manipulation beyond a gross degree. Experiment- 

 ing with many of the unicellular nucleate organisms circumvents 

 a number of these difficulties but presents the same problem in that 

 the various functions of the organisms are crowded together in a 

 small space. From this standpoint, the multicellular organisms are 

 better subjects for study because certain cells such as muscle tissue 

 or nerves may become so highly specialized for the performance 

 of one biological function that in them the processes of particular 

 functions may be studied to special advantage. Yet these specialized 

 cells are so different from the primeval cell that great care must 

 be exercised in extrapolating from one to the other. 



By bringing together information from these various sources, it 

 is possible to reconstruct a sort of idealized cell combining proper- 

 ties basic to all life and in its essential features probably approach- 

 ing closely the primeval cells which were the beginning of all the 

 existing life on this planet. Life progresses and continues from 

 generation to generation by the growth and division of cells and in 

 no other way. Our problem, therefore, is resolved into two parts: 

 How do cells grow, and how do they di\'ide? The answers to these 

 questions are essential not only to deductions concerning the origin 

 of life but also to an understanding of its evolution. 



The cell has three principal structural components: the cell wall, 

 essentially a membrane which separates the contents of the cell 

 from its external environment; the nucleus, the central dense por- 

 tion containing the highly complex gcnic material; and the cyto- 

 plasm, usually a viscous aqueous colloid containing a tremendous 

 assortment of simple and complex molecules, including proteins, 

 free amino acids, carbohydrates, fats, and metallic compounds. 



