Selective Forces in the Evolution of Man

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Abstract
Recent investigations in human evolution show that the genetic changes involved in the relative growth of the brain must have occurred at a fast rate, suggesting strong evolutionary pressures. It is established that the acquisition of bipedal gait preceded the evolution of a large brain, and that tool making was practiced by early Australopithecine organisms with an essentially ape-sized brain. It is suggested that in human evolution a positive feedback relation existed between cultural and genetic change: cultural activities enable man to change his environment, and the environmental changes induced may have an effect on the adaptive values of genes, leading to changes in the direction and strength of selective pressures. They may in this way influence the genotype of the population. The fact that the cultural changes have progressed at increasingly fast rates is in agreement with this hypothesis, as is the increasingly accelerated rate of increase in presumed brain size in the early evolutionary times (Australopithecus to Peking man). The stability of human brain size in the last 200,000 years cannot be explained by a feedback mode , and may be due to a genetically or physiologically conditioned upper level of brain size.

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