Symmetry Breaking of Hominin Locomotion, and Migration Feedback

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. The intermediate bimodal transition period was coined by the emergence of systematic bipedal gait, breaking the previously uniform locomotion symmetry. By contrast to versions of the common savannah hypothesis, this paper suggests an alternative fictitious scenario by which migration between seasonally inhabitable arboreal refuges, possibly caused by regional climate change, forced hominins 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 did not necessarily require advanced mental capabilities, nor specific communication or new forms of social cooperation such as those successively found in Homo. Assumingly, broken spatial and temporal environmental symmetry had induced related symmetry breaking of hominin behaviour, their anatomic structures and reproduction habits.