Natural Selection has balanced gene frequencies to allow for this phenomenon. In the Miocene period, the climate changed and rainforests turned to savannah. Bipedalism allowed survival advantages, as pre-hominids were no longer arboreal. At the same time bipedal fossils began to show Encephalisation, a rapid increase in brain size. The larger brain contributes to higher intelligence and thus increased fitness.
With the onset of bipedalism, the shape of the pelvis had to change to allow for efficient striding gait. The pelvis became shorter and narrower to form the carrying angle and support the upper body, thus reducing the size of the birth canal.
A combination of reduced pelvis size and encephalisation created an evolutionary compromise. Effectively, all Homo sapien babies are born prematurely, before the brain is 80% of its total size, as is the case with other mammals. This allows the head to fit through the pelvis.
The infant is born with only 25% of its total brain size and is still in a period of infantile helplessness. Other internal organs are also incomplete. In a biological sense, this means the baby has not completed gestation. Aside from the Obstetric Dilemma parturition should not occur until 21 months from conception, 12 months later than is the case. Once the baby reaches 21 months, the brain reaches 80% of it’s total size and the brain growth rate slows. In the uterus, the placenta provided warmth, protection, oxygen and nutrients. The increased brain size in the species allows the mother to provide these needs externally, and the free hands resulting from bipedalism allow the baby to be carried from danger.
Modern Obstetrics and Caesarean sections have reduced deaths to mother and baby during childbirth. In primitive societies this is not the case. The larger an infant’s skull, the less likely he and his mother will survive childbirth. The average skull size for babies conceived is larger than that for babies born alive. Mothers with the gene for larger skull size and their offspring were being selectively eliminated due to complications during childbirth. Natural Selection balances gene frequencies, as when a large brained baby does survive, its increased fitness will result in higher reproduction success than most individuals. This results in a balanced population where the genotype has a larger skull than the phenotype.
Physiological features such as stretch points in the pelvis and relaxing hormones, and the fontanelles and sutures of the baby’s head have made childbirth easier, though it is still more traumatic than that of other primates.
The Obstetric Dilemma has pushed the boundaries of human physiology and evolution. Encephalisation causes neonatal, obstetric and locomotion handicaps, but the increase in brain size offers overwhelming compensatory benefits. While this evolutionary compromise does have ecological and energetic sacrifices, it results in a larger brain – culminating in a more intelligent and reproductively successful species.
