LIFE IN THE UNIVERSE — HUANG 



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significant role in shaping a star, yet chemical action is responsible for 

 the emergence and evolution of living organisms. And although we 

 can predict that in about 10 billion years or less our sun will become a 

 white dwarf, there is no way of telling how man will evolve in even 

 10 million years. 



Wliat is the reason for this? Gravitational interaction is very 

 simple and is described by Newton's law of gravitation. The number 

 of possible nuclear interactions is very large indeed, since there are 

 hundreds of different atomic nuclei ; nevertheless, we could still list all 

 conceivable reactions. Hence we can compute them and even predict 

 the evolution of stars by the law of gravitation and our knowledge of 

 nuclear physics. 



But how big is the total number of chemical reactions — both in- 

 organic and organic — that one may conceive ? Unable to estimate such 

 a number, I am probably safe in saying that it is larger than any astro- 

 nomical figure we can find in our textbooks. It is this wealth of chemi- 

 cal activity that makes a prediction of the emergence and evolution of 

 living organisms difficult, if not permanently elusive. 



If we cannot compute the time scale of biological evolution, we must 

 find it out empirically. Here on earth it took about 3 billion years for 

 humans to evolve from atoms. I have suggested earlier that since 

 biological evolution occurs through the random processes of muta- 

 tion and selection, its average time scale is probably of the same order 

 of magnitude — a few billion years. On this basis, for successful bio- 

 logical evolution on a planet, the luminosity of its parent star must 



Table 1 



