394 CELL HEREDITY 



atmosphere, screening the short-wave ultraviolet radiation from the 

 earth. The experimental results are in accord with a second hypothesis: 

 that most of the molecular oxygen in the atmosphere is of biological 

 origin, more specifically, of photosynthetic origin. 



In this anaerobic oceanic broth, given the millions of years needed for 

 improbable events to occur, molecules may have come together spontane- 

 ously to form aggregates of different types with varying degrees of order. 

 Perhaps natural selection played a role at this early time, so that those 

 aggregates better able to take up substances from their environment and 

 capable of overcoming the problems of size by duplicating would win out 

 in the competition for materials. Furthermore, some complexities could 

 be virtues. When the supply of certain substances became low, those 

 aggregates which assumed the capacity of synthesizing the limiting factor 

 from other substances would have a selective advantage. The ability to 

 undergo anaerobic fermentation and secure energy and substrates by 

 breaking down organic molecules would give the aggregate a degree of 

 freedom from its environment and confer a selective advantage. Thus 

 primitive "organisms" may have been established, with gradually in- 

 creasing organization, structure and complexity, from which we inherit 

 those features of cells so widely distributed as the common denominators 

 of living things. 



As long as no organisms were about, chemical evolution could 

 continue. But with the appearance of the first living things, the organic 

 chemical environment would be consumed, metabolized, and changed. 

 Organisms evolved, making their own organic substances from those in- 

 organic materials in essentially limitless supply, such as carbon dioxide, 

 which is constantly being formed as a product of fermentation. Photosyn- 

 thesis is one way in which organisms learned to make sugar from carbon 

 dioxide and water. The catalyst which traps the energy in sunlight is 

 the pigment, chlorophyll. Some of the precursors from which it is 

 synthesized today are able to absorb light and may have been catalysts 

 in the past. They would have been improved upon step by step as the 

 ability to make the complex chlorophyll molecule evolved. 



The earth's atmosphere gradually became pervaded with molecular 

 oxygen, a by-product of photosynthesis. Now aerobic respiratory metab- 

 olism could be evolved as an efficient improvement over anaerobic 

 fermentation. It may be envisaged that the supply of compounds re- 

 quired by these primitive systems underwent localized depletion. If so, 

 any form capable of carrying out a terminal step in biosynthesis would 

 have enjoyed a great selective advantage, since presumably the penulti- 

 mate compound would still have been present in excess. Horowitz de- 

 veloped a scheme of this sort, by which biosynthetic pathways may 



