LECTURES IN BIOLOGICAL SCIENCES 



well supplied with enzymes to use the products of photosynthesis. 



Many suggestions as to when and how photosynthetic and 

 autotrophic organisms evolved have been presented (Gaffron, 

 1960). The difficulty of oxidizing various organic compounds 

 in a reducing environment has led to the proposal that some 

 form of photosynthesis occurred at a very early stage. 



ft is not possible to begin to do justice here to the many 

 proposals, based on comparative biochemical studies, that have 

 been made. 



DISCUSSION 



Thus we pass almost imperceptibly from a gaseous cloud 

 about the sun to cells whose survival depends on adaptation to 

 changing conditions. Only those cells that can adapt, survive. 

 The stage is now set for the gradual unfolding of the bio- 

 chemical possibilities inherent in the elements— the results of 

 which we now observe in the entire biosphere and reflect in 

 ourselves. The evolution of cell division from simple mechani- 

 cal breakage of droplets to the complex mitotic mechanism has 

 been discussed in detail elsewhere (Anderson, 1956). The con- 

 clusion that life spontaneously appeared on earth would not, 

 I think, have greatly surprised St. Thomas Aquinas or other 

 early scholars who saw nothing unusual in the idea that living 

 things could arise from inanimate substances. 



How probable were these events, taken in their entirety? 

 Certainly many of the individual steps are more probable than 

 was once thought. The very first requirement, for example, is 

 a planet with a suitable atmosphere and a stable temperature 

 for long periods of time. When the near-collision hypothesis 

 of the origin of the planetary system was in vogue, planetary 

 systems were thought to be very rare— as rare as a special near- 

 collision between two suns. With the emergence of the dust- 

 cloud hypothesis, matters completely reversed. The rotation of 

 a condensing cloud of cosmic dust may, in many instances, be 

 sufficient to cause an excessive centrifugal strain on the con- 

 densing star if all the rotational energy ends up in the star. 

 Present evidence, however, suggests that more than half the 

 stars in our galaxy are double or multiple systems. The angu- 



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