ORIGIN OF THE FIRST ORGANISMS 39 1 



chemical reactions taking place in a certain sequence. Thus, 

 for the progressive evolution of our original systems the 

 important thing was not the chance entry or development 

 of any particular compound, but the appearance of a definite 

 co-ordination of the reactions which provide the constant 

 synthesis of this compound in the system in continuous inter- 

 action with the external medium. 



The more closely the chemical substances entering it from 

 the external medium resembled ingredients of the system 

 itself, the less complicated were the chains of reactions leading 

 to its synthesis and the simpler was the metabolic organisa- 

 tion. However, the system Avas correspondingly more depen- 

 dent on the constancy of the external medium and on its 

 high content of complicated organic compounds. 



Clearly the selection of our original systems and the emerg- 

 ence of the first organisms from among them must have been 

 directed towards a lessening of this dependence and the 

 formation of networks of synthetic reactions by which the 

 complicated ingredients of the system could be synthesised 

 unerringly from the somewhat diverse compounds ^vhich 

 entered it from the external medium. For this purpose the 

 compounds in question had to be ' standardised ', that is to 

 say, broken down to relatively simple and uniform fragments 

 from which any specific ingredients of the system could be 

 built up by standard methods, though using complicated 

 chains of transformations with many links. 



We do, in fact, find such a form of organisation in the 

 constructive metabolism of contemporary organisms. In 

 them, as we have pointed out several times, very simple 

 compounds of low molecular weight such as oxalic acid, 

 glycine, succinic acid, keto acids, etc., serve as starting points 

 for the synthesis of proteins, nucleic acids, lipids, porphyrins 

 and the other complicated ingredients of protoplasm. These 

 simple compounds arise as fragments split off in the course 

 of the destructive metabolism of the sugars and other sub- 

 stances which enter the cell from its surrounding medium 

 and serve as its nutrients. In the course of this destructive 

 metabolism the energ\^ needed for synthesis is liberated and 

 stored in ATP and other compounds with high-energy bonds. 

 We have already explained this with reference to glycolysis 



