186 - The Cell 



perature in the environment and to support 

 the metabolism of green plants and, indi- 

 rectly, of other organisms. 



ORIGIN OF LIVING MATTER 



The origin of living organisms directly 

 from nonliving matter, under present-day 

 conditions in our environment, was a doc- 

 trine widely believed by scientists until about 

 one hundred years ago. First it was thought 

 that worms could come from mud, or mag- 

 gots from meal, and so forth; later it was 

 held that microorganisms, at least, could 

 arise from the nonliving ingredients of de- 

 composing beef broth. Finally, however, the 

 classic experiments of Recli (1650), Spallan- 

 zani (1750), and Pasteur (1860) proved that 

 this doctrine, called spontaneous generation, 

 could not be substantiated. But what alterna- 

 tive was left to science? 



Since the Pasteur experiments, the preva- 

 lent idea has been that some primitive form 

 of protoplasm originated from nonliving mat- 

 ter during the Archeozoic Era (p. 563) — some 

 two billion years ago — when conditions on 

 this planet were quite different. Until re- 

 cently, however, no definitive theory could 

 be formulated, partly because speculations 

 on the nature of the early environment were 

 premature and misleading, ami partly be- 

 cause scientists hesitated to theorize when so 

 very few data were available. Gradually ideas 

 on the nature of the early environment began 

 to clarify, however, and in 1936, the Russian 

 biochemist A. I. Oparin crystalized current 

 thinking in his book, The Origin of Life. 

 Since that time, the contributions of Harold 

 LJrey and Stanley Miller of California, and 

 of George Wald of Harvard University have 

 led to further advances. Finally, therefore, it 

 now seems possible to reapproach the prob- 

 lem in a scientific manner and to ask such 

 questions as: how did organic matter and 

 primitive organisms come into being? and 

 what was the nature of these first forms of 

 life? 



It is necessary to realize that the early at- 



mosphere of the earth did not contain any 

 gaseous oxygen (0 2 ) or nitrogen (N 2 ), but 

 that other gases, particularly hydrogen (H 2 ), 

 ammonia (NH 3 ), methane (CH 4 ), and water 

 vapor (HnO) were abundant. The tempera- 

 ture at the earth's surface was somewhat 

 above the boiling point of water and there 

 was considerable energy in the form of elec- 

 trical discharges, from large cloud masses in 

 the atmosphere, and from ultraviolet radia- 

 tions from the sun, which could penetrate 

 the clouds. 



These conditions — a reducing atmosphere, 

 suitable sources of carbon, hydrogen, and 

 nitrogen, and abundant available energy — 

 seem highly conducive to the formation and 

 stabilization of organic molecules. Conse- 

 quently, some recent experiments in which 

 these conditions were duplicated as closely 

 as possible are of tremendous interest. In 

 1953, Stanley Miller, working from sugges- 

 tions by Harold Urey, arranged to circulate 

 the gases of the early atmosphere through a 

 closed system of vessels into which a con- 

 tinuous series of electric sparks was being 

 discharged. At the end of only eight days, 

 Miller found that significant quantities of at 

 least three (probably five) different amino 

 acids had accumulated in the water trap of 

 the system, and there was evidence that other 

 more complex unidentified organic com- 

 pounds had come into being. In other words, 

 it now seems probable that a variety of or- 

 ganic compounds could have been generated 

 spontaneously in the ancient terrestrial en- 

 vironment and that these could have become 

 well concentrated as the waters of the earth 

 underwent condensation and evaporation. 

 Granting the presence of such organic com- 

 pounds in the early environment, the prob- 

 lem of how life may have arisen is greatly 

 simplified — especially when one thinks of the 

 infinity of time that has been available. 



Certain molecules, especially amino acids 

 and nucleic acids, by virtue of their intrinsic 

 structure can form stable aggregates of con- 

 siderable size. Moreover, such aggregates dis- 

 play a maximum stability when a certain size 



