8 The Nature of Biological Diversity 



plies no effective energy for these metabolic processes. The radio- 

 activity of the rocks is unavailable because it is much too violent in 

 character and because it is largely dissipated within the rocks. Cosmic 

 rays are unimportant, as can be seen from the table. 



Electrical energy of lightning is too violent to be of direct use, but 

 it does produce some compounds of nitrogen which are useful. 



Volcanic heat is of no value today as a source of energy for meta- 

 bolic process and was probably of minor or in fact negligible impor- 

 tance in the past because of its violent and destructive character and 

 its very sporadic appearance. What is needed for the evolution of life 

 as well as its maintenance is a steady source of energy which will be 

 maintained over billions of years. 



Living things require compounds of certain kinds which contain 

 greater quantities of energy than do the degradation products. The 

 particular thermodynamic function that measures the spontaneous 

 character of chemical reactions at constant temperature and pressure 

 is the Gibbs free energy. In any spontaneous reaction this function 

 decreases. Hence, living organisms of the animal or saprophytic plant 

 type must be supplied with chemical compounds of higher free-energy 

 content than the compounds which they discard. In some way free 

 energy must be supplied to these waste products in order that they 

 can be reconverted into the food chemicals again. Such free energy 

 can be supplied by high-energy sources such as sunlight, radioactivity, 

 electric discharges, etc. It could be supplied from heat sources, pro- 

 viding a suitable heat engine operating between high- and low-tem- 

 perature heat reservoirs is available in accordance with the second 

 law of thermodynamics. Merely high-temperature regions such as 

 water pools alone, supplied by volcanoes or hot springs, are not 

 sufficient to accomplish this. In considering this problem of the 

 energy source for chemical evolution of life, these requirements of 

 thermodynamics should be kept in mind. Though warm water may 

 have accelerated reactions between compounds with suitable free- 

 energy content, warm water alone cannot provide the compounds of 

 high free-energy content. 



At present the source of energy is sunlight in the red end of the 

 spectrum, which is absorbed by chlorophyll. However, in prebiological 

 times before the appearance of chlorophyll, ultraviolet light, absorbed 

 in the atmosphere to produce high-energy compounds which in turn 

 were transferred to the oceans, must have furnished most of the free 

 energy required for prebiological evolution. As Table 3 shows, there 

 is only a small fraction of the energy in the solar spectrum below 



