Structural Reductions in Homo Sapiens Microevolution: Jaws, Gracilization, Brachycephalization

Abstract

The author discusses evolutionary significance of structural reductions on the background of the competitive mechanism of natural selection constituting the core of darwinian theory. The classical explanation of evolutionary change based on the mechanism of prevalence of a “favourable” (in relation to actual environmental conditions) trait over the unfavourable one seems to be incapable of explaining certain changes occuring in the recent evolution of man. Under conditions of well developed cultural means of adaptation fitness-increasing significance of reduced masticatory apparatus, less robust skeleton or more rounded head is unclear. Morphological form of human body seems to be rather “neutral” with respect to fitness. However assumption of neutrality of any trait seems to be unfounded from theoretical vewpoint since the organism is an integrated unit and organisms form a higher-degree integrated biological unit — a population and functions of every part of an organism and all organisms in a population are interelated between themselves and related to an environment. Therefore if one is unable, to point to a direct link between a character and its function he would not assume neutrality of the character but should accept its indirect relation to some function.

The most general statement concerning evolutionary significance of every trait is that increase in fitness is obtained by reduction of energetic cost of reproduction. This reduction may be obtained either by minimzing cost of production and rearing of offspring or by reducing matter and energy input for development and maintenance of organisms already produced. Since development of human most powerful adaptive mechanism — culture has reduced direct energentic requirements of human organisms it may be hypothesized that during recent stages of evolution (H. sapiens neanderthalensis and up) reduction of body mass has been favoured by natural selection. Modern occurence of secular trend does not contradict this hypothesis since it is an ecosensitive phenomenon occuring without changes in gene pools.