Encephalization and Obstetrics in Primates with Particular Reference to Human Evolution

Primate pelvic anatomy and implications for birth

“…For most primate species, the entrance and exit of the birth canal are longer in the sagittal dimension than in the transverse dimension, as noted for generalized quadrupeds above. The neonatal cranium is largest in the sagittal dimension in most primate species, including humans [2]. Furthermore, the back of the infant cranium presents the greatest dimension of the skull at birth, so that it fits best against the more roomy back of the monkey pelvis and the more roomy front of the human pelvis, as noted above.…”

“…The need for a rigid and stable pelvis for walking in mammals places limits on the size of the neonate that can successfully pass through the pelvis, and thus an upper limit on brain size at birth. This poses challenges for all mammalian orders characterized by having large brains in adulthood relative to body size, a description that fits most primates [2]. In a well-known diagram, Schultz [3] first depicted the relationship between neonatal head size and maternal birth canal as a series of rectangles (modified here as ovals) that showed a tight fit in monkeys, gibbons and humans in contrast to relative spaciousness in great apes (figure 1).…”

The Evolutionary Foundations of Preferences

“…Otherwise, we would expect evolution to simply have done away with whatever constraints might appear in our decision-making. Evolutionary psychologists routinely appeal to limits on our cognitive capabilities, finding evidence for these limits in the relatively large amount of energy required to maintain the human brain (Milton [93]), the high risk of maternal death in childbirth posed by infants' large heads (Leutenegger [83]), and the lengthy period of human postnatal development (Harvey, Martin and Brock [70]). …”

Birth, obstetrics and human evolution

“…Because the cephalopelvic constraints in monkeys 11,12 are greater than those in great apes (i.e. more similar to humans), the comparisons made herein between non-human primates and humans will use monkeys, rather than our closer relatives, the great apes, to represent non-humans.…”