2 The Nature of Biological Diversity 



characteristic must have heen segregated from a community. It seems 

 likely that precellular evolution also involved the segregation of chem- 

 ical constituents from many sources. 



The complexity of living organisms astounds all students of hiology, 

 and the more knowledge the observer has the more astounding do the 

 facts appear to be. Is it not true that the development of a particular 

 minute drop of clear protoplasm, such as a fertilized egg, into a sea 

 urchin and nothing else is one of the greatest wonders of Avhich we 

 have knowledge? Yet the study of the compounds of carbon, nitrogen, 

 oxygen, and hydrogen shows us that a very great variety of these com- 

 pounds is possible. Two illustrations of the varieties of the molecules 

 can be given. The human egg contains strands of total length of about 

 1 meter long of deoxyribonucleic acid containing about 10 9 units, each 

 of which can be chosen in four different ways. The number of possible 

 molecules becomes 4 109 , which is an immense number. To specify a 

 human being at each moment of its existence requires less specific 

 information; much redundancy is possible and surely exists. Again 

 the number of possible proteins consisting of, say, 170 amino acids of 

 20 kinds is very great. If one of each kind were placed in a cubical 

 box, the length of the edge of the box would be 10 raised to nearly the 

 50th power light-years. Not all the possible protein molecules exist 

 now in the observed universe nor could they have existed during the 

 last 4.5 billion years. 



Natural phenomena are very complex in all their various manifesta- 

 tions. We can follow in considerable detail the more simple and iso- 

 lated examples of these phenomena, for example, many chemical reac- 

 tions and structures, the structure of atoms and the nuclei of atoms, 

 and the gross structure of stars and galaxies of stars. The details of 

 natural phenomena exceed in complexity the complexity of structure 

 of the human brain, and we cannot follow these except in an approxi- 

 mate way. But we can say something about the comparative com- 

 plexities of the chemistry of the carbon, nitrogen, oxygen, hydrogen 

 compounds and those of other elements. I shall state my conclusion 

 categorically in this way. There is no known chemistry of other ele- 

 ments which approaches in ordered complexity that of these four ele- 

 ments, and we know enough of the chemistry of all the elements that 

 we can be certain that no other such chemistry exists. Only these ele- 

 ments supply such a complexity of compounds and chemical reactions 

 that we are forced to conclude that only the chemistry of these ele- 

 ments could supply the complexity of structure and behavior which 

 we recognize as those of living things. A system of life based on other 



