ON THE ORIGIN OF LIFE 



THE BUILDING BLOCKS 



1. Earth and Atmosphere 



There is now overwhelming evidence that the entire visible 

 universe is composed of the same set of 92 elements familiar 

 to the student of terrestrial chemistry. However, the relative 

 abundance of the elements here is very different from the aver- 

 age of the universe, because the universe is made principally 

 of hydrogen. The interstellar medium, which is actually very 

 rarified, is about 90% H, 9% He, with O, C, N, and Ne making 

 up approximately 1%, and Fe, Si, Mg, S, etc., making up 0.2 

 to 0.3% (Sokolov, 1957). These exist not only as elements but 

 also as compounds. In addition to CH, and CN found in space, 

 enormous amounts of methane as well as ammonia are found 

 in the atmospheres of Jupiter, Saturn, Uranus, and Neptune. 

 Comet heads contain N 2 and C.,, C 3 , CN, CH, OH, and CO as 

 frozen unsaturated radicals. There is sufficient C0 2 in the 

 atmosphere of Venus to form a 0.6-mile-thick layer at STP, 

 while the atmosphere of Mars contains about twice the C0 2 

 found in our atmosphere. Of greater interest is the observation 

 that certain meteorites, upon heating, give off carbon monoxide 

 and methane as well as other gases. In addition, certain hydro- 

 carbons have been observed. If we judge from the present 

 make-up of the universe, therefore, we assume it is likely that 

 many simple carbon compounds existed in space before the 

 earth was formed. 



It is now generally agreed that the earth and other planets 

 condensed from a protoplanetary cloud surrounding the sun. 

 Now, since the earth is not composed principally of H, it is 

 evident that the condensation involved loss of the greater part 

 of the mass of the original cloud. Two general views, w r ith 

 many variations, have been put forth concerning the details of 

 this condensation. 



Kuiper, Fesenkov, and others (reviewed by Fesenkov, 1957) 

 suggest that the earth condensed as one large mass and that 

 considerable heating occurred with two results. First, the origi- 

 nal atmosphere was completely lost, and second, any carbon 

 compounds formed in space were decomposed. 



Urey, Levin, and others (Urey, 1952; Levin, 1959) have sug- 



