How women evolved for childbirth

Eric Hopton for – Your Universe Online

5 million year old adaptation still causing problems

Data from a 1980’s industrial “crash test dummy” study has provided evidence of evolutionary adaptation in human childbirth. New research has identified a complex link between pelvis structure and head size and shows how the human body evolved to ease childbirth.

Compared to other primates, human mothers and babies have a particularly hard time during delivery. This is a throwback to the time when upright walking evolved 4-5 million years ago. The human pelvis adapted to the new type of movement. Much later, our brains became much larger. That, of course, meant the head size of neonates also increased, but they were still being be delivered through pelvises which had adapted to upright walking.

This evolutionary legacy left the modern human with restricted space in the birth canal during childbirth that can have severe consequences. Women in developing countries, without modern medical care or cesarean sections during birth, still suffer from high mortality due to childbirth, and this is partly due to this pelvic adaptation problem.

Large heads run in the family

This new study by scientists from the Universities of Oslo and Vienna shows how the dimensions of head, stature, and pelvis in a human body are linked in a complex way to help ease the “tight fit” childbirth problem. These links have not been recognized before.

Mothers with large heads usually give birth to neonates with large heads. The researchers found that females with a large head have birth canals that can better accommodate those large-headed neonates.

Barbara Fischer, an evolutionary biologist at the Universities of Oslo and Vienna, along with Philipp Mitteroecker, an anthropologist at the University of Vienna, analyzed 3D data of the human pelvis. The data came from an older and rather unusual source.

“The motivation for the US-American researchers who collected these pelvis data in the 1980s was an industrial one. They wanted to improve the design of crash test devices and car seats to increase vehicle safety,” said Fischer. A large number of human pelvises were measured.

Fischer and Mitteroecker found a complex association between the shape of the human pelvis, body height, and head size which helps to ease the “obstetric dilemma.” Their results show that the dimensions of head and height do not vary independently, but instead they are linked to pelvis shape.

“We found out that women with large heads, compared to women with small heads, possess a birth canal that is shaped in a way that neonates with large heads can pass it easier,” explains Fischer. The sacrum is shorter in these women and it leaves more space in the outlet of the birth canal, which is beneficial for birth.

On average, short women have harder births than taller women, and carry a higher risk that the fetus will not fit through the birth canal. Fischer and Mitteroecker show in their study that shorter women possess a rounder birth canal, probably an adaptation to the stronger selection pressure at birth in these women. Despite the identified patterns, the authors clarify that the individual risk for a difficult birth depends on various environmental influences along with genetic factors.


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