124 MISCELLANEOUS GEOLOGICAL CONSIDERATIONS 



has sensibly followed the latter two prescriptions also over the last 

 several billion years. 



4. 'Water, the practical solvent for livdng processes, must be 

 available in liquid form. The kind of life we are talking about and 

 thinking of does not live in uncondensing steam or unmelting ice. 

 The basic requirement, therefore, is that the living planet must be 

 at a proper distance from its star, — in the liquid-water belt — not 

 as close as Mercury is to the Sun, nor as remote as Jupiter.' 



It follows that these four assumptions have held true over the 

 greater part at least of geologic history. Apart from the astronomical 

 position of the planet Earth on an intermediate orbit around its 

 controlling star, this arises mainly from the constancy of solar radia- 

 tion. Any variations in other heat flows, such as variation of terrestrial 

 heat flow, or the hypothetical temporary shielding by stellar dust 

 clouds, appear to have been largely counteracted by the buffering 

 of our atmosphere. The heat retention by the atmosphere, with its 

 feedback mechanisms, seems to be able to temper effectively any 

 possible primary variations in heat flow. It follows that our atmos- 

 phere is a very good glasshouse indeed, a fact which has played a 

 major role in the evolution of life on earth. In view of the fact that 

 this quality of our atmosphere mainly rests with its water vapour, 

 which will have been present in the primeval atmosphere too, we 

 may assume that this glasshouse effect already occurred in the early 

 days of the origin of life on earth. 



THE DANGERS OF COMPARATIVE BIOCHEMISTRY 



As a final remark in this chapter, I should like to add a digression 

 not so much about facts, but about the way in which these facts have 

 been used in some scientific reasoning. In other words, I want to 

 warn against a certain type of deduction drawn from comparative 

 biochemistry that one meets more and more in biological papers on 

 the origin of life on earth. 



The basic assumption of this reasoning is that what is more simple 

 in metabolism, biochemically, is more primitive and consequently 

 older in the history of life. This assumption is entirely unjustified. 

 It has never been tested, and will be very difficult to test. Also, quite 

 possibly, it is false. 



