Human Self-Domestication

...the hypothesis of human self-domestication has recently been revived as a possible explanation of changes of human physical traits since the late Pleistocene. These changes include the reduction of body size and decrease in skeletal robusticity, modifications in cranial and dental features including reduction in cranial capacity, shortening of the facial region of the skull and maleruption of teeth, and reduction in sexual dimorphism. In contrast to earlier biological writings, other domestication-associated features observed in animals such as an increased variation in skin colour, increasing fat storage, earlier sexual maturation and activity, and reduction in motor activity are not discussed with respect to human self-domestication in recent accounts [1]. It is indeed plausible to assume that these changes could have taken place due to the creation of an artificially protective environment after humans adopted a more sedentary lifestyle in the Neolithic period, thereby relaxing natural selection pressures.

Similarly, the idea that foetalisation and domestication could be related, has recently been highlighted in a seminal paper comparing anatomical features and behaviour of apes and humans [3]. The authors argue that changes in social structures of early humans, compared to our closest living relatives, the chimpanzee, could have favoured the selection against aggression, and that such selection was accompanied by a reduction of sexual dimorphism in humans and the retention of juvenile characteristics in body shape and behaviour. Interestingly, a parallel development has been proposed in the bonobo, which displays more neotenic physical features and is much less aggressive compared to the common chimpanzee [3].

From a biological perspective the greatest dispute with regard to physical changes in anatomically modern humans akin to domestication pertains to a slight but measurable decline of brain volume from around 1,400 cm3 to roughly 1,300 cm3, which could be interpreted in further support of the human self-domestication hypothesis. However, this decline in stature was accompanied by a reduction in body size such that the allometric brain-body relation remains unchanged [22]. In contrast to humans, domesticated animals show a large disproportionate decline of brain size by up to 30%, especially of the sensory perceptual centres, compared to their wild ancestral species, yet no such pronounced decline has convincingly been demonstrated in any human population.