LECTURES IN BIOLOGICAL SCIENCES 



acids could be, or indeed were, formed by natural processes, 

 but rather in precisely by what mechanism and in what quan- 

 tity. 



Urey has suggested that the oceans at one time may have 

 contained as much as 10% of organic material. Since the early 

 sea is thought to have had a volume of 150 million cubic kilo- 

 meters (Madison, 1953), this would mean approximately 15 mil- 

 lion cubit kilometers of organic material would have been pro- 

 duced. This amounts to 3.6 million cubic miles of amino acids 

 and other organic compounds. Assuming a sterile period of 

 1.5 X 10 9 years, only 153 pounds of compound per square mile 

 of the earth's surface need be produced per year. This is not 

 an unreasonable rate. 



3. Accumulation and Combination 



No bacteria were present to decompose the amino acids and 

 other substances formed, hence they could accumulate over 

 vast periods of time. Abelson (1957) has recently shown by 

 analyzing shells of the hard-shelled clam Mercinaria mercinaria 

 that some amino acids are stable over such periods of time. This 

 creature is abundant along the east coast of the United States 

 today; in addition, fossil specimens as old as 25 million years 

 have been found there. Abelson compared the amino acids 

 from recent and ancient shells. Those amino acids, such as 

 serine, that are not very heat stable were found only in the re- 

 cent clams. Glutamic acid, glycine, alanine, valine, isoleucine, 

 and leucine, however, were found in both ancient and recent 

 shells. Calculations based on studies of the rate of thermal 

 decomposition of alanine suggest that solutions of alanine might 

 persist for a billion years at room temperature. Actually the 

 age of the earth is about 3.26 billion years, and at least three- 

 fourths of this time had passed before life began. 



Given a plentiful supply of amino acids and nucleotides, 

 would we expect proteins and nucleic acids to be synthesized? 

 The answer that comes first and most easily is no. The proba- 

 bility of forming a decapeptide by chance combination, accord- 

 ing to Blum (1951), is 10"-". It requires energy to put amino 

 acids together to form peptide chains— approximately 3000 

 cal/mole. One way of putting this much energy into a mixture 



