The Obstetrical Dilemma refers to two conflicting trends in the evolutionary development of the human pelvis where the transition to walking upright (bipedal locomotion) required a decreased size of the bony birth-canal but the trend toward increased intelligence required a larger cranium, which would need a wider obstetrical pelvic area. The evolutionary natural selection that resolved these competing trends was a shortened gestational period, resulting in human infants that were dependent for care over many months after being born. Sexual differences resulted as well. Human males evolved narrower hips optimized for locomotion, whereas female hips evolved to be a wider optimization because of childbirthing needs.
The dilemma began when human ancestors 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 brainstem moved from the back of the skull towards the bottom, and the spine formed at a right angle to the jawline. These alterations 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. 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.
Famous quotes containing the words obstetrical and/or dilemma:
“In a sense the world dies every time a writer dies, because, if he is any good, he has been a wet nurse to humanity during his entire existence and has held earth close around him, like the little obstetrical toad that goes about with a cluster of eggs attached to his legs.”
—E.B. (Elwyn Brooks)
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