Obstetrical Dilemma
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 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 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 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.
Human
Humans are the only living species in the Homo genus...
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 human ancestors started to evolve into a bipedal creature
Human skeletal changes due to bipedalism
The evolution of human bipedalism approximately four million years ago has led to morphological alterations to the human skeleton including changes to the arrangement and size of the bones of the foot, hip size and shape, knee size, leg length, and the shape and orientation of the vertebral column...
. 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
Brain stem
In vertebrate anatomy the brainstem is the posterior part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves...
moved from the back of the skull to the lower side, 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
Ilium (bone)
The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.The name comes from the Latin ,...
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
Childbirth
Childbirth is the culmination of a human pregnancy or gestation period with the birth of one or more newborn infants from a woman's uterus...
, 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.