From Wikipedia, the free encyclopedia

The Obstetrical Dilemma refers to the evolutionary development of the human species through a number of biological changes, specifically the shifting of the females' pelvic bones, thereby shortening the fetal incubation period.

The dilemma began when Homo sapiens started to evolve into a bipedal creature. Over this period of transformation, a number of structures in the body changed size, proportion or location in order to accommodate bipedal locomotion. The foramen magnum and the brain stem moved from the back of the skull to the lower side, and the spine formed at a right angle to the jawline. These alterations to allow a person to stand upright and face forward without placing undue strain on the neck or back. To help support the upper body, a number of structural changes were made to the spine and pelvis. The ilial pelvic bone shifted forward and broadened, while the ischial pelvic bone shrank, narrowing the pelvic canal.

These changes were occurring at the same time as humans were developing larger craniums. Therefore, in order to successfully undergo childbirth, the infant must be born earlier and earlier, thereby making the child increasingly developmentally premature.

Today, human infants are born with only 25% of their brains fully developed, compared to chimpanzees, whose offspring are born with 45-50% brain development^{[citation needed]}. Since human infants are born so early, they spend more than a year in a nearly incapacitated state. Therefore, human infants depend on their parents much more and for much longer than other primates.

This heightened reliance forced a shift in human society. The constant needs of the child forces the father to remain with the child's mother in order to ensure his offspring's survival. This is known as the division of labor and it is credited as the beginning of family structure and kinsharing.

• Bipedalism and human birth: The obstetrical dilemma revisited