LECTURES IN BIOLOGICAL SCIENCES 



Rather we shall deal with degrees of probability and ask: 

 Was the origin of lite on earth such an improbable event that 

 it might never occur again in the universe, or was the emergence 

 of life inevitable on a planet such as ours? 



The minimum conditions requisite for life to appear and 

 survive are liquid water, the absence of extremes of acidity and 

 alkalinity, the presence of certain salts, the absence of poisons 

 and high radiation levels, and a long period of time at a rela- 

 tively constant temperature. These factors, however, are not 

 enough. Terrestrial cells are composed of certain alpha amino 

 acids, nucleotides, and carbohydrates and gain their energy by 

 decomposing sugars and organic or fatty acids. These com- 

 pounds are only a few of the many thousand in their size range 

 that can be synthesized in the laboratory. In nature we have 

 known them only as products of living systems, as truly "or- 

 ganic" compounds. Where can we look for a supply of these 

 substances before life appeared? And what are the chances of 

 forming, from these compounds, molecules that duplicate them- 

 selves? Furthermore, if our early organisms were to nourish 

 themselves on C0 2 and sunlight, how could a complex photo- 

 synthetic system, capable of the dozens of synthetic reactions 

 seen in all plants, spring full-blown from barren primeval 

 wastes? Could indeed such complete, organized complexity ap- 

 pear suddenly and synthesize from the very first its own amino 

 acids, nucleotides, and carbohydrates, uniting these to form spe- 

 cific macromolecules that could coalesce in such a fashion as to 

 form a perfect duplicate of the original cell? 



In short, could such complex biochemical mechanisms ap- 

 pear and have the ability to metabolize, grow, and divide— to 

 produce two living cells where only one existed before? Are not 

 these things completely outside the realm of chance or random 

 combination and therefore impossible for certain? 



We do not know the answer. We can only examine such 

 ideas and data as are available and make our own estimates. 

 Let us consider especially two things: first, the building blocks 

 from which the proteins of living cells are made; and second, 

 the problem of growth and division on the simplest level— that 

 of a molecule which can duplicate itself. For unless molecules 

 can duplicate themselves directly or indirectly there is little 

 chance that cells can. 



