Other Modes of Nutrition; Conservation of Food Elements - 1 87 



is reached. Consequently, larger particles that 

 might arise would tend to split into smaller 

 ones when the point of maximum stability 

 was passed. Conceivably such phenomena 

 might lead toward the more complex proc- 

 esses of growth and reproduction such as 

 exist today, but nothing is known about the 

 stages of transition. One cannot say how 

 genes and enzymes first originated, nor how 

 these remarkable entities gradually achieved 

 some primitive form of protoplasmic organi- 

 zation. 



Nature of the First Organisms. It was com- 

 monly believed for many years that the 

 earliest organisms were very primitive green 

 plants, similar perhaps to the blue-green 

 algae (p. 594) of today. Considering what 

 now seems true about the early environment, 

 however, this theory must be abandoned. 

 Carbon dioxide was not present in the early 

 atmosphere and photosynthesis could not 

 occur. It seems more probable, instead, that 

 early primitive "organisms," generating in 

 the rich organic brew that had accumulated 

 in landlocked evaporating basins, derived 

 their energy from fermentation or some other 

 anaerobic kind of metabolism. In other 

 words, the earliest organisms probably were 

 saprophytes, which absorbed organic material 

 from the environment and gained energy 

 from anaerobic processes of metabolism. Such 

 metabolism, to be sure, is relatively ineffi- 

 cient, but it does liberate energy when oxy- 

 gen is lacking, and it does produce significant 

 amounts of C0 2 . Indeed, it seems quite likely 

 that the carbon dioxide of our atmosphere 

 originated completely from the metabolic 

 activities of primitive organisms. This sounds 

 incredible, at first. But when we consider 

 that the green plants of today take up from 

 the atmosphere the total supply of C0 2 once 

 every three hundred years and that the 

 Archeozoic Era endured at least two billion 

 years, the credibility becomes apparent. 



Evolution of Green Plants. Photosynthesis 

 could not develop until an adequate supply 

 of COo became available in the environment, 

 presumably as a result of fermentative proc- 



esses in an ever-increasing population of 

 saprophytic organisms. ATP, undoubtedly, 

 assumed importance as an energy source very 

 early in the evolution of protoplasmic sys- 

 tems; and presumably the cyclic type of 

 photophosphorylation (p. 162), which does 

 not generate free 2 , may be regarded as an 

 early step in the direction of photosynthesis. 

 With the advent of chlorophyll, however, 

 photolysis and consequently the generation 

 of free 2 must have started, so that oxygen 

 began to be an important component of the 

 atmosphere. Accordingly, the greater effi- 

 ciency of oxidative metabolism eventually 

 could be extended from the green plants, 

 where it must have originated, to the sapro- 

 phytes, which previously had lived entirely 

 by anaerobic means, and to new organisms 

 that began to take on the habits of holozoic 

 nutrition. Thus again we are forced to an un- 

 familiar conclusion. Not only have organ- 

 isms been shaped in large measure by the 

 changing environment, but also the environ- 

 ment itself has been fashioned and refash- 

 ioned by organisms. Indeed, even today it is 

 found that the oxygen of our atmosphere is 

 completely replaced every two thousand 

 years as a result of the metabolism of hetero- 

 trophic organisms and the photosynthetic 

 activities of green plants. 



Does Life Exist Elsewhere? Astronomical 

 evidence indicates that planets similar to 

 the earth in size, composition, and tempera- 

 ture occur quite rarely in the universe. But 

 the universe is exceedingly large. In our own 

 galaxy alone, there are thousands of earth- 

 like planets; and at the present time almost 

 a hundred million other galaxies have come 

 into range of the most powerful telescopes. 

 The inescapable conclusion is, therefore, that 

 living things must have arisen and are arising 

 in many places. We do not have to shoulder 

 the whole burden of life's destiny. But we 

 are isolated from other life-bearing planets 

 by stupendous distances, and across such dis- 

 tances, probably, no signals will ever pass. 

 We cannot know how life is faring elsewhere. 

 We can only wish it well. 



