CHAP. 32] THE ORIGIN OF LIFE 847 



of H2CO per ion pair) by the use of 40 x 10^ eV helium ions from a 60-in. 

 cyclotron (Garrison et al., 1951). While the simplest organic compounds were 

 indeed synthesized, the yields were so small that this experiment can best be 

 interpreted to mean that it would not have been possible to synthesize organic 

 compounds non-biologically as long as the Earth had oxidizing conditions. This 

 experiment is important in that it induced a re-examination of Oparin's 

 hypothesis of the reducing atmosphere (Urey, 1952, 1952a). 



2. The Primitive Atmosphere 



The geological record gives us little (and contradictory) evidence as to the 

 nature of the atmosphere when the Earth was formed. Opinions as to the nature 

 of the early atmosphere range from regarding it as strongly reducing to strongly 

 oxidizing and include all types of atmospheres of intermediate degrees of 

 oxidation. 



The discussion here is based on the assumption that the conditions on the 

 primitive Earth were favorable for the production of the organic compounds 

 which make up life as we know it. There are many sets of conditions under 

 which organic compounds could have been produced. All these conditions are 

 more or less reducing. However, before accepting a set of conditions for the 

 primitive Earth, one must show that reactions known to take place will not 

 rapidly change the atmosphere to another type. The jiroposed set of conditions 

 must also be consistent with the known laws for the escape of hydrogen. 



Cosmic dust clouds, from which the Earth is believed to have been formed, 

 contain a great excess of hydrogen. The planets Jupiter, Saturn, Uranus and 

 Neptune are known to have atmospheres of methane and ammonia. There has 

 not been sufficient time for hydrogen to have escaped from these planets owing 

 to their lower temperatures and higher gravitational fields. It is reasonable to 

 expect that the Earth and the other minor planets also started out with reducing 

 atmospheres and that these atmospheres became oxidizing due to the escape of 

 hydrogen. 



The meteorites may be the closest approximation we have to the solid 

 material from which the Earth was formed. They are observed to be highly 

 reducing, with the iron mostly as metallic iron with some ferrous sulfide, the 

 carbon as elemental carbon or iron carbide, and the phosphorus as phosphides. 

 The carbonaceous chondrites contain several per cent of organic carbon. 



The overall chemical change has been the oxidation of the reducing atmo- 

 sphere to the present oxidizing atmosphere. This is caused by the loss of hydro- 

 gen which results in the production of nitrogen, nitrate, sulfate, free oxygen 

 and ferric iron. As we shall discuss, many complex organic compounds would 

 have been formed during this overall change, thereby presenting a favorable 

 environment for the formation of life. It is extremely doubtful that life could 

 begin unless large quantities of organic compounds were present on the Earth. 

 Therefore, the first living organism must have been formed by the time sig- 

 nificant quantities of oxygen appeared in the atmosphere, for otherwise the 



