Hominin Migration: Bipedalism, Sexual Conflict, and Spreading Homo in a Phase Transition Model

Anthropogenic prehistory may be divided into two subsequent phases, the first one from the Last Common Ancestor (LCA) between humans and great apes till the emergence of the genus Homo, and the second phase onwards from then. Until hominins appeared, LCA had lived predominantly on trees, while Homo finally lived only on the ground. After LCA and before Homo, the intermediate bimodal transition period from 7 to 2 Myr BP was coined by the emergence of systematic bipedal gait, breaking the previously uniform quadrupedal locomotion symmetry. By contrast to versions of the common savannah hypothesis, this paper suggests an alternative fictitious scenario of periodic migration between alternatingly inhabitable arboreal refuges. Possibly caused by regional climate change, yet tree-climbing hominins were additionally forced to genetically develop speedy and efficient bipedal locomotion for survival during their temporary but extended regular excursions across open territory. Increasingly upright locomotion resulted in offspring’s early weaning, and in turn in the emergence of childhood with enhanced lethal risks for toddlers. Related selective pressure caused transformations of reproductive traits from gradual sexual selection in apes to undulating sexual conflicts in hominins. Between LCA and Homo, consistent with fossil evidence, the evolutionary bimodal transition phase, with forelimbs for climbing and hind limbs for running, did not necessarily require enlarged brains for advanced mental capabilities, nor specific communication or new forms of social cooperation such as those successively found in Homo. After emigration from arboreal habitats, allotted food access caused previous promiscuous mating being replaced by sexual pair bonding with exclusive paternity of Homo. Assumingly, broken spatial and temporal environmental symmetry had induced related symmetry breaking of hominin behaviour, their anatomic structures and reproduction habits, with consequences lasting to the present day.