THE EVOLUTION OF COMPLEX BIOCHEMICAL PATHWAYS

The earliest cells arose well over 3.5 billion years ago. They were anaerobic and heterotrophic, using the carbohydrates, amino acids, and other organic compounds free in the environment in which they lived. In other words, they depended on previous abiotic synthesis of organic compounds. But, as the organisms became more abundant and more efficient at removing nutrients from the medium, they began to deplete the supply of nutrients. The rate of spontaneous formation of organic matter from inorganic raw materials was most likely never very high, and it must have taken many millions of years for a moderate supply of nutrients to accumulate. With the supply being depleted at an ever increasing rate, and the drain on available resources becoming more and more severe, the competition between organisms for available nutrients must have increased, too. Forms inefficient at obtaining nutrients perished; those more efficient survived in greater numbers. Natural selection would have favored any new mutation that enhanced the ability of its possessor to obtain or process food.

At first, the primitive organisms probably carried out relatively few complex biochemical transformations. They could obtain most of the materials they needed ready-made. But it would have been these very materials—materials that could be used directly with little alteration—that would have dwindled most rapidly. There would therefore have been strong selection for any organisms that could use alternative nutrients. Suppose, for example, that Compound A, which was necessary for the life of cells, was initially available in the medium, but its supply was being rapidly exhausted. If some cells possessed a gene that coded for an enzyme a that catalyzed synthesis of A from another compound, B, in greater supply in the medium, then those cells would have had an adaptive advantage over cells that lacked the gene. They could survive even when A was no longer available in the medium by carrying out the reaction

B ^___a___> A

But then there would have been increasing demand for free B, and the rate of its utilization would soon have exceeded the rate of its abiotic synthesis. Thus the supply of B would have dwindled, and there would have been strong selection for any cells possessing another gene that coded for an enzyme b, catalyzing synthesis of B from C. These cells would not have been dependent on a free supply of either A or B, because they could make both A and B for themselves as long as they could obtain enough C:

C^___b___>B^___a___> A

This process of evolution of synthetic ability might have continued until eventually most cells made all the A they required by carrying out a long chain of chemical reactions (i.e., a chemical pathway):

G^___f___> F^___e___> E ^___d___> D ^___c___> C^___b___>B ^___a___> A

In this way the primitive cells would slowly have evolved more elaborate biochemical capabilities. The earliest form of metabolism was almost surely glycolysis and fermentation, processes universal in living organisms today (of the three main energy-yielding systems—glycolysis, aerobic respiration, and photosynthesis—only glycolysis and fermentation are universal, occurring in all living cells).

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