32 



The Nature of Biological Diversity 



Chemical Evolution 



I wish to discuss briefly the beginnings of chemical evolution, start- 

 ing with the molecules of the primitive atmosphere being subject to 

 a primitive photosynthesis, using the far ultraviolet or radiation from 

 the radioactivity of the earth's crust to transform them. The earliest 

 molecules on the surface of the earth were those shown on the top row 

 of Fig. 8. particularly methane, ammonia, and water. If these mole- 

 cules are subjected to radiation of energy great enough to break the 

 bonds of carbon-carbon, carbon-hydrogen, hydrogen-hydrogen, nitro- 



H 

 H-6 0=C=0 



Carbon 

 Water dioxide 



H 



H-C-H 



i 



H 



Methane 



H 

 i 



H 



Hydrogen 



N-H 



I 



H 



Ammonia 



9 



H-C-OH 



H 



H-C-C-OH 



i 



H 



H H 

 «■ ■ ■ it ^. 



HO-C-C-C-C-OH 



i i 



H H 



Formic acid Acetic acid Succinic acid 



FIG. 8. Primeval and primitive organic molecules. 



\f o 

 H-C-C-OH 



H-N 



Glycine 



gen-hydrogen, hydrogen-oxygen, which can be done by ionizing 

 radiation, 30 such as the beta rays of potassium-40 which are plentiful 

 in the earth's crust, or with ultraviolet light of wavelengths shorter 

 than 2,200 A, 31 then the atoms which are so formed may reorganize 

 to form more complex molecules, a few of which are shown on the 

 bottom row of Fig. 8. You already recognize these molecules as being 

 the present-day substrate materials (formic acid, acetic acid, succinic 

 acid, and glycine) upon which all living organisms operate. Glycine, 

 shown here, is the only nitrogen-containing compound in the bottom 

 row of Fig. 8, and it is the simplest of the amino acids, of which the 

 proteins are constructed. By exchanging one of the carbon-bound 

 hydrogen atoms of the glycine for any of a group of other atoms, some 

 20 different amino acids can be built up. 



