846 MILLER [OHAi*. 32 



not subject to experimental investigation. S. A. Arrhenius offered a second 

 theory that hfe developed on the Earth as a result of a spore or other stable form 

 of life coming to the Earth in a meteorite from outer space or by the pressure of 

 sunlight driving the spores to the Earth. One form of this theory assumes that 

 life had no origin, but like matter, has always existed. The presence of certain 

 lonof-lived radioactive elements shows that the elements were formed about 

 5 X 10^ years ago. If the elements have not always existed, it is difficult to 

 understand how life could have always existed. Another form of this theory 

 assumes that life was formed on another planet and traveled to the Earth. This 

 hypothesis does not answer the question of how life arose on the other planet. 

 In addition, it is doubtful that any known form of life could survive very long 

 in outer space and fall through the atmosphere without being destroyed. 

 Therefore, while this theory has not been disproved, it is held to be highly 

 improbable. If micro-organisms are carried on interi^lanetary rockets, space is 

 likely to become contaminated, and this would make it impossible to disprove 

 this theory conclusively. For this reason, space vehicles are being sterilized. 



A third hypothesis holds that the first living organism arose from inorganic 

 matter by a very improbable event. This organism, in order to grow in an in- 

 organic environment, would have to synthesize all of its cellular components 

 from carbon dioxide, water and other inorganic nutrients. Our present know- 

 ledge of biochemistry shows that even the simplest bacteria are extremely 

 complex and that the probability of the spontaneous generation of a cell from 

 inorganic compounds in a single event is much too small to have occurred in the 

 approximately 5 x 10^ years since the Earth's formation. 



The fourth proposal is that life arose spontaneously in the oceans of the 

 primitive Earth, which contained large quantities of organic compounds similar 

 to those which occur in living organisms. This theory was outlined mainly by 

 Oparin (1938) and it forms the basis of most of the present ideas on the origin 

 of life. Oparin argued that if large amounts of organic compounds were in the 

 oceans of the primitive Earth, these organic compounds would react to form 

 structures of greater and greater complexity, until a structure would form 

 which we would call "living". In other words, the synthesis of the first living 

 organism would involve many non-biological steps, and none of these steps 

 would be highly improbable. Oparin further proposed that the atmosphere 

 was reducing in character and that organic compounds might be synthesized 

 under these conditions. This hypothesis implied that the first organisms would 

 be heterotrophic, meaning that they would obtain their basic constituents from 

 the environment instead of synthesizing them from carbon dioxide and 

 water. 



In spite of the argument by Oparin, numerous attempts were made to 

 synthesize organic compounds under the oxidizing conditions now present on 

 the Earth (Rabinowitch, 1945). Various sources of energy acting on carbon 

 dioxide and water failed to give reduced carbon compounds except when con- 

 taminating reducing agents were present. The one exception to this was the 

 synthesis of formic acid and formaldehyde in very small yield (10"'^ molecule 



