Effect of Human Brain Size on Labor and Bipedalism

The real obstetrical dilemma came long after Lucy and her colleagues became extinct, when there was a sudden upsurge in brain expansion. About 1.5 million years ago, die adult hominid brain went from the Australopithecine size of 400 cubic centimeters to 750 cubic centimeters in a species called Homo habilis, the first member of our genus. In other words, the brain just about doubled in size. A mere million years later. the hominid brain doubled once again until it reached its present average size of 1200 cubic centimeters. This is quite sudden in evolutionary terms.

What was the effect of this voluminous increase in infant brain size on the birth process? The architecture of the pelvis, once it adapted to bipedalism, remained about the same for three million years. Obviously, much larger brains were not going to slide easily through a pelvis that had been designed for efficient bipedalism and small-brained newborns. The problem is architectural—the pelvis was designed as a scaffold for bipedal musculature and is as wide as it can be while still allowing women to walk efficiently. There is no way to retrofit the pelvis to accommodate a larger-brained infant. And so compromise had to come from the infant, and it did. First, there is a biological limit placed on infant brain development. Like all primates, human infants are born with a brain that takes up about 12 percent of their body weight; although destined to be highly encephalized—that is, proportionally biggerbrained as adults than other primates—yet start out with the relatively same-sized brain as any other primate. Again, we make up for this by an extremely fast rate of brain growth after birth. Second, prior to birth the bones of the skull are not fused and there is thus lots of room for smashing those bones together and molding the head as it squeezes through the pelvic passage. The "soft spots" on infant skulls, more correctly called "fontanels," are the areas where various skull bones meet—-often you can see the pulse of a heartbeat through the thin cranial membranes. In nonhuman primates these spots are almost totally fused, but in humans, they remain wide and flexible. The resulting 'cone-head" shape of the human newborn's head is simply nature's way of squeezing a child out with little damage to brain tissue.


And so we have the miracle of modern human birth—a painful, twisted journey that squeezes the infant head like Play-Doh and causes mothers unbelievable pain. And we also have all the pieces for the answer to why our infants are born so helpless. They are born with unfinished brains because the pelvis simply cannot be any wider or any bigger. If it were, women couldn't walk. Painful childbirth and helpless babies are an evolutionary compromise between selection for bipedalism, which came first. and later adult brain expansion.

Anthropologists Karen Rosenberg and Wenda Trevathan point out that the consequences are not just mechanical, they are also behavioral and social. The tight fit and tortuous route for human birth causes long and difficult labor for both the mother and the child. This trauma affects how the mother feels after birth, both physically and mentally. And infants come out rather exhausted and battered from this ordeal as well. The more than difficult process might account for the difference in style between human and nonhuman primate birth. A birth in a colony of Barbary macaques that was witnessed by two researchers, Vivika Ansorge and Kurt Hammerschmidt, was described as comparatively quick but not without pain.^° The mother, following the troop to the sleeping trees for the night, stopped several times and did stretches with her legs. a sort of dance that signaled something odd going on. She squatted. repeatedly touched her genital areas, and emitted low vocalizations which the researchers described as "moans." Eventually, she reached behind with her right arm and scooped up the baby that was coming out between her legs. She held it to her chest and it yelped. Within minutes the mother, rather dazed to be sure, moved on. There are very few descriptions of primate births because animals most often give birth at night or early in the morning. Humans, too, most often give birth in the early hours, but are hardly ever alone. Rosenberg and Trevathan suggest that the idea of attending a birth is actually an evolved strategy of our species that is necessary because human mothers are less equipped than monkey mothers to help in the process. The mother is in greater pain, the birth takes longer, and the infant comes out face down. She needs someone to catch the infant and clear its air passages. She needs someone to hand her the baby and later pull on the placenta if need be. Wenda Trevathan calls this "obligate midwifery," suggesting that we need to have attendants because the evolution of bipedalism and big babies left us no choice. And so our birthing is not just a biological event, but a social event as well. It relies on the help of family and friends, and emphasizes how important interpersonal interaction is to the human species, even when one of us first appears.


Painful labor is a compromise between our large brain size and the ability of a woman to give birth mechanically.

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/home and garden/home furnishings (0.285356)
/health and fitness/disease (0.277626)

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Childbirth (0.989830): dbpedia | freebase
Brain (0.910703): dbpedia | freebase | opencyc
Human brain (0.720796): dbpedia | freebase
Human (0.686439): dbpedia | freebase | opencyc
Primate (0.626326): dbpedia | freebase | opencyc
Infant (0.557450): dbpedia | freebase | opencyc
Nervous system (0.545319): dbpedia | freebase | opencyc
Skull (0.517056): dbpedia | freebase | opencyc

 Our Babies, Ourselves: How Biology and Culture Shape the Way We Parent
Books, Brochures, and Chapters>Book:  Small , Meredith (1999-05-04), Our Babies, Ourselves: How Biology and Culture Shape the Way We Parent, Anchor, Retrieved on 2011-06-29
Folksonomies: parenting pregnancy babies infancy parenthood


14 JUN 2011

 Labor Memes

Memes about dealing with labor.
Folksonomies: pregnancy labor
Folksonomies: pregnancy labor
04 SEP 2011

 Why Evolution is True

Memes that support the Theory of Evolution