862 MiLLKB [chap. 32 



When the methane and ammonia of the primitive atmosphere had been 

 converted to carbon dioxide and nitrogen by photochemical decomposition in 

 the upper atmosphere, water would be decomposed to oxygen and hydrogen. 

 The hydrogen would escape, leaving the oxygen in the atmosphere and thereby 

 resulting in oxidizing conditions on the Earth. It is likely, however, that most 

 of the oxygen in the atmosphere was produced by photosynthesis from plants 

 instead of by the photochemical splitting of water in the upper atmosphere. 



The evolution of multicellular organisms probably occurred after the de- 

 velopment of photosynthesis. The evolution of primitive multicellular organisms 

 to more complex types and the development of sexual reproduction can be 

 understood on the basis of the theory of evolution. 



6. Life on Other Planets 



Life as we know it requires the presence of water for its chemical processes. 

 We know enough about the chemistry of other systems, such as those of silicon, 

 ammonia and hydrogen fluoride, to realize that no highly complex system of 

 chemical reactions similar to that which we call "living" would be possible in 

 such media. Also, much living matter exists and grows actively on the Earth 

 in the absence of oxygen, so that oxygen is 7wt necessary for life, as is stated so 

 often. Moreover, the protecting layer of ozone in the Earth's atmosphere is not 

 necessary for life since ultraviolet light does not penetrate deeply into natural 

 waters, and also because many carbon compounds capable of absorbing the 

 ultraviolet would be present in a reducing atmosphere. 



It is possible for life to exist on the Earth and to grow actively at temperatures 

 ranging from 0°C, or perhaps a little lower, to about 70°C. It seems likely that 

 if hot springs were not so temporary, many plants and possibly animals would 

 evolve which could live in such temperatures. Plants are able to produce and 

 accumulate substances which lower the freezing point of water and hence can 

 live at temperatures below 0°C. At much lower temperatures the rates of 

 reaction would probably be too slow to proceed in reasonable periods of time. 

 At temperatures much above 120°C reaction velocities would probably be so 

 great that the nicely balanced reactions characteristic of living things would 

 be impossible. In addition, it is doubtful if the organic polymers necessary for 

 living organisms would be stable much above 120°C, even allowing for the 

 amazing stability of the enzymes of thermophilic bacteria and algae. 



Only Mars, Earth and Venus conform to the general requirements so far as 

 temperatures are concerned. Mars is known to be very cold and Venus to be too 

 hot. Recent observations of the black-body emission of radio waves from 

 Venus indicate surface temperatures of 290 to 350°C (Mayer et ah, 1958). 

 Infrared bands of water have been detected recently by observing Venus from 

 a balloon at high altitude. The presence of water on Venus would make life 

 possible, but if the radio-wave temperature is in fact the temperature on the 

 surface, then hfe could only exist on dust particles in the atmosphere. 



Mars is known to be very cold with surface temperatures of -f 30 to — 60°C 



