28 evolution: the ages and tomorrow 



the-last step is in preparation. Amino acids combine to form 

 proteins in a complex "peptide linkage." The acid group 

 of one amino acid reacts with the amino group of another, 

 leaving an acid and amino group free, a condition that leads 

 to indefinite repetition and the enormous molecular weight 

 and complexity of the proteins. The modern chemist has no 

 difficulty synthesizing the amino acids and can even pro- 

 duce a "protein-like" substance; but the great complexity 

 of the true protein is too much for his present techniques. 



All this activity in the early seas, as Oparin has worked 

 out, was producing colloids of greater and greater com- 

 plexity, proteins, fats, and carbohydrates in profusion. Elec- 

 trically active groups of opposite charge appeared, mutually 

 precipitating to form droplets of a complex mixture called a 

 coacervate, which absorbs water on its surface to form 

 membranes and hence is a primitive beginning of individ- 

 uality. The formation of these membranes, which are essen- 

 tial to the birth of an organism, can be studied by the 

 chemist in various inorganic and organic colloids. Living 

 protoplasm is a colloidal solution of various organic sub- 

 stances, complex emulsions consisting of minute electrically 

 charged particles suspended in water and held apart by the 

 repulsive forces between the charges. In pure water which 

 is a poor electrical conductor, the particles of an inorganic 

 colloid (gold, for instance) are held apart by the repulsive 

 charges. If salt is added and the conductivity of the water is 

 increased, the particles will begin to lose their charges and 

 will form larger and larger aggregates until the gold is 

 precipitated out of the colloid. The same precipitation will 

 occur if two gold colloids of opposite charge are mixed. 



There is a very marked difference in the behavior of an 

 organic colloid. In this case the molecules of the carbon 

 compound have such a strong affinity for water that the 

 colloidal particles always gather around themselves con- 

 centric layers of water molecules which form a "water 

 membrane." If salt is added to a gum arable colloid, the gum 



