THE CONCEPT Of EVOLUTION 711 



A number of reactions by which organic substances can be made 

 from inorganic ones are known. It is believed that originally much of 

 the earth's carbon was in the form of metallic carbides; these could 

 react with water to form acetylene which would subsequently polymerize 

 to form compounds containing long chains of carbon atoms. It has been 

 shown experimentally that high energy radiation, such as that of cosmic 

 rays, can produce organic compounds. This has been demonstrated by 

 M. Calvin, who irradiated solutions of carbon dioxide and water in a 

 cyclotron and obtained formic, oxalic and succinic acids, which contain 

 one, two and four carbons respectively. These are important inter- 

 mediates in the metabolism of living organisms. Irradiation of solutions 

 wath ultraviolet light, or with electric charges to simulate lightning, also 

 produces organic compounds. Harold Urey and Stanley Miller, at the 

 University of Chicago, showed in 1953 that amino acids such as glycine 

 and alanine, and even more complex organic substances, can be formed 

 in vitro by exposing a mixture of water vapor, methane, ammonia and 

 hydrogen gases to electric discharges for a mere week. All of these gases 

 are believed to have been present in adequate amounts in the earth's 

 atmosphere in prebiotic times. 



The spontaneous origin of living things at the present time is be- 

 lieved to be extremely improbable, yet that this same event occurred in 

 the past is quite probable. The difference lies in the conditions existing 

 on the earth: the accumulation of organic molecules was possible before 

 there were living things because there were no molds, no bacteria, no 

 living things of any kind to bring about their decay. Furthermore, there 

 was little or no oxygen in the atmosphere to bring about their spon- 

 taneous oxidation. 



The details of the chemical reactions which could give rise, without 

 the intervention of living things, to carbohydrates, fats and amino acids 

 have been worked out by Oparin and extended by Calvin and others. 

 Most of the reactions by which the more complex organic substances 

 were formed probably occurred in the sea, in which were dissolved and 

 mixed the organic molecules formed. The sea, we may postulate, be- 

 came a sort of dilute broth in which these molecules collided, reacted, 

 and aggregated to form new molecules of increasing size and complexity. 

 The known forces of intermolecular attraction, and the tendency for 

 certain molecules to form liquid crystals, provide us with means by 

 which large, complex, specific molecules can form spontaneously. Oparm 

 suggested that natural selection can operate at the level of these com- 

 plex molecules, before anything recognizable as life is present. As the 

 molecules came together to form colloidal aggregates, these aggregates 

 began to compete with one another for raw materials. Some of the 

 aggregates, which had some particularly favorable internal arrangemeirt 

 would acquire new molecules more rapidly than others and would 

 eventually become the dominant types. 



Once some protein molecules had formed and had achieved the 

 ability to catalyze reactions, the rate of formation of additional molecules 

 would be greatly stepped up. Next, these complex protein molecules ac- 

 quired the ability to catalyze the synthesis of molecules like themselves; 



