CONTINUING HUMAN EVOLUTION: BODIES, BRAINS AND THE ROLE OF VARIABILITY

Abstract

The Darwinian revolution in biological thinking gave priority to individual variation and rendered typological ideas and rigid classifications inappropriate for the study of life. Large biological variability of humans is a fact that must be accepted. In morphological characteristics such as body weight or brain size one half or more of the total variance within the species is produced by differences between individuals within the same populations, slightly less than one quarter by sexual dimorphism, and only about one quarter results from differences between populations. A large portion of variability is heritable; however, environmental influences during individual life play an important role. Perception of human variation is biased by prejudice. This is seen in the ascribing of a larger role to gender differences than really exists. Patterns of human variability change through time. In the recent past (last 10 000 years) cranial capacity underwent a decrease by more than 10%, head shape changed substantially during the process called brachycephalization (horizontal rounding of the braincase) and stature fluctuated. During the last century in some countries human stature increased while in others it remained unchanged or even decreased. Reasons for these trends are not clear but for at least some of them the operation of natural selection is strongly suggested. Opportunity for the operation of natural selection decreased regularly in the recent past and concurrently variability of biological characteristics increased. Ethnic classifications based on biological criteria are invalid because of large individual variability and incessant multidirectional diachronic changes. ‘Kierkegaard once stated that life can only be understood backward, but it must be lived forward. However, it can only be lived forward with knowledge and appreciation of the past.’ (F. E. Poirer, 1977)