During human birth, the fetus typically navigates a tight, convoluted birth canal by flexing and rotating its head at various stages. This complex process comes with a high risk of birth complications, from prolonged labor to stillbirth or maternal death. These complications were long believed to be the result of a conflict between humans adapting to walking upright and our larger brains.
Dilemma between walking upright and larger brains
Bipedalism developed around seven million years ago and dramatically reshaped the hominin pelvis into a real birth canal. Larger brains, however, didn’t start to develop until two million years ago, when the earliest species of the genus Homo emerged. The evolutionary solution to the dilemma brought about by these two conflicting evolutionary forces was to give birth to neurologically immature and helpless newborns with relatively small brains — a condition known as secondary altriciality.
A research group led by Martin Häusler from the Institute of Evolutionary Medicine at the University of Zurich (UZH) and a team headed up by Pierre Frémondière from Aix-Marseille University have now found that australopithecines, who lived about four to two million
— source University of Zurich | May 10, 2022