MYSTERIES OF LIFE AND EXISTENCE — SNODGRASS 521 



was not until plants acquired chlorophyll and developed the process 

 of photosynthesis that oxygen was liberated from carbon dioxide and 

 energy became available by oxidation. Thereafter organic substances 

 were produced in abundance by living things themselves, and the life 

 processes were speeded up by tlie acquisition of enzymes. Since all 

 the earliest known animals were air breathers, the plants must have 

 had the water well oxygenated by the time of the Pre-Cambrian, and 

 made the land habitable for all the air-breathing creatures that came 

 later to populate the earth. There is no doubt that plants are now 

 the ultimate source of all animal food; it is difficult to believe, how- 

 ever, that we are indebted to them also for all the oxygen we breathe 

 today. Another theory contends that oxygen and other gases have 

 seeped through the earth's crust from the interior. 



The biochemical theory of life's origin here reviewed should be 

 as convincing as any visionary theory on the subject could be. It is 

 the hope of the biochemists that it may sometime be demonstrated by 

 the artificial synthesis of life in the laboratory. In this event, the 

 mystery of life would not be its origin, but the nature of the chemical 

 forces in organic compounds by which they give life to lifeless 

 matter. 



There have been other theories concerning the origin of life on the 

 earth. A fanciful idea that once appealed to the credulous held that 

 germs of life prevaded the universe and that drifting through space 

 they have developed on any planet that offered favorable conditions. 

 This theory neatly avoids the question of life's origin. It may read- 

 ily be admitted that if the same chemicals and the same conditions are 

 present on some other planet as on ours, life may have appeared 

 there as on the earth. But it is highly improbable that evolution fol- 

 lowed the same course and produced higher forms of life comparable 

 to those that have evolved here. The "man from Mars" will probably 

 be found to be just as fictitious as the "man in the moon." 



Down to almost modern times a belief has persisted in the spon- 

 taneous generation of living creatures, such as the production of mag- 

 gots from the flesh of dead animals, or the origin of microorganisms 

 from the scum of foul water. Even in the writer's early days we 

 never doubted that threadworms {Gordius) found in the old horse 

 watering-troughs were animated horse hairs. Modern biology has 

 callously put an end to all such romance. 



From the theoretical primitive forms of life it is still a long way 

 to the organized cell with its nucleus, chromosomes and genes, centri- 

 oles, and mitochondria. Unfortunately, however, as Oparin (1938) 

 says, "the origin of the cell is perhaps the most obscure point in the 

 whole study of the evolution of organisms." No intermediate forms 

 are known, and theories have not yet bridged the gap. Viruses are 

 not a primitive form of life because they can reproduce only in other 



