Professor Jean-Jacques Hublin, Max Planck Institute for Evolutionary Anthropology, Germany
Dr Laura Gruss, Radford University, USA
Professor Mark Maslin, University College London, UK
Primate pelvic anatomy and implications for birth
Professor Wenda Trevathan, New Mexico State University, USA
The pelvis performs two major functions for terrestrial mammals. It provides somewhat rigid support for muscles engaged in locomotion and, for females, it serves as the birth canal. The result for many species, and especially for encephalized primates, is an “obstetric dilemma” whereby the neonate often has to negotiate a tight squeeze in order to be born. For most monkey species, birth is a challenge that occasionally results in death when the size of the neonate is too large to successfully deliver. (Great ape females have spacious birth canals and give birth to small neonates, providing an exception to the pattern of difficult births in primates.) On top of what was probably a baseline of challenging birth, locomotor changes in the human lineage resulted in even more potential complications. Few adaptations in human evolution have had greater impact on human biology and culture than bipedalism. Almost every part of the human skeleton was altered, as well as aspects of cardiovascular, circulatory, respiratory and endocrine function. One of the most profound changes occurred in the birth process and the state of infant development at the time of birth. Negotiation of the bipedal pelvis requires a series of rotations, the end of which has the infant emerging from the birth canal facing the opposite direction from the mother. This pattern, strikingly different from what is typically seen in monkeys and apes, places a premium on having assistance at delivery. Furthermore, due to constraints provided by maternal metabolic limits to gestating the energetically expensive human fetus, as well as anatomical, placental, and immunological factors, human infants are born with slightly more than a quarter of the brain size they will achieve in adulthood, approximately half that of most other primates. The high degree of dependency at birth and an inordinately slow rate of growth of human infants and children place demands on mothers and other caretakers that appear to far exceed those of other mammals, including our closest primate relatives.