[Scientists are revisiting an influential theory that the
evolution of big brains made human childbirth risky.]
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SUNDAY SCIENCE: A THEORY OF CHILDBIRTH’S EVOLUTION MAY NOT BE WHAT
YOU’RE EXPECTING  
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Carl Zimmer
July 30, 2023
New York Times
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_ Scientists are revisiting an influential theory that the evolution
of big brains made human childbirth risky. _

, Nicole Rifkin

 

It’s a question on every new parent’s exhausted mind: Why are
babies born so helpless? In 1960, an American anthropologist laid out
an influential explanation rooted in human evolution.

As our early ancestors began walking upright, Sherwood
Washburn argued [[link removed]] in 1960,
they evolved a narrower pelvis to make walking long distances more
efficient. At the same time, those hominins were evolving larger
brains. And babies with big heads could get stuck in a tight birth
canal during delivery, threatening the lives of mothers and babies
alike.

According to Dr. Washburn, evolution dealt with this “obstetrical
dilemma,” as he called it, by shortening pregnancies, so that women
delivered babies before the infant brain was done growing.

Dr. Washburn’s theory was hugely influential and became a common
lesson in biology classes. “Sapiens: A Brief History of
Humankind,” a 2011 best-selling book
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presented the obstetrical dilemma as fact. Many researchers still
embrace it.

But a detailed new review
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evidence in the journal Evolutionary Anthropology threw cold water on
the idea. Anna Warrener, a biological anthropologist at the University
of Colorado Denver, argued that the evidence to date did not offer
strong support for the obstetrical dilemma, and that scientists had
not paid enough attention to possible alternatives. What’s more, the
scientist said, the idea sends a pernicious message to women that
pregnancy is inherently dangerous.

“It perpetuates a narrative of bodily incompetence,” Dr. Warrener
said.

In graduate school, Dr. Warrener did not see any reason to doubt the
obstetrical dilemma. For her dissertation, she investigated one of Dr.
Washburn’s key assumptions — that women walk less efficiently than
men do because their pelvis is wider for childbirth. But in 2015,
after studying volunteers walking on treadmills, Dr. Warrener found
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having a wider pelvis did not create a bigger demand for oxygen.

“The data came in, and I was like, Wait a minute — I may have
gotten some of the story wrong,” she recalled.

Holly Dunsworth, a biological anthropologist now at the University of
Rhode Island, also became disenchanted with the obstetrical dilemma
when she took a close look at the evidence. “I was scandalized,”
she said.

In 2012, she and her colleagues published a study
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pregnancies in humans and other primates. They found that, in general,
bigger primates tended to have longer pregnancies than smaller ones.
For their size, humans don’t have shortened pregnancies. If
anything, human pregnancies are longer than one would predict for a
primate of their size.

Since then, Dr. Dunsworth has become a strong critic of the
obstetrical dilemma, arguing that the timing of childbirth is
determined by the size of babies’ bodies, not their heads. The
birthing process begins when a fetus demands more energy than a
mother’s body can provide, she proposes. “We’re giving birth to
massive babies,” she said.

Other scientists, however, have come to the theory’s defense, while
admitting that its original conception was overly simplistic.

In a study published last month
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of researchers argued that the difference between the male and female
pelvis shows signs of natural selection acting in different
directions. While human males are bigger and taller on average than
human females, certain parts of their pelvises are relatively smaller.
The biggest differences are in the bones that surround the birth
canals in human females.

Despite these differences, the female pelvis still creates a tight fit
between a baby’s head and the birth canal, sometimes putting both
the baby and mother in peril.

“So why did natural selection not manage to kind of resolve this
situation and make birth a little less risky?” asked Nicole
Grunstra, an evolutionary anthropologist at the University of Vienna
and one of the study’s authors. “It has evolved to be an
evolutionary compromise between competing demands,” she said — in
other words, to solve an obstetrical dilemma.

But Dr. Grunstra acknowledged flaws in Dr. Washburn’s original
version of the theory. She suspected that walking may not have been
the most important factor in the evolution of the pelvis. Merely
standing upright, she said, might have put pressure on the pelvic
floor, preventing the evolution of a more spacious birth canal.

The skeptics aren’t convinced by these arguments. In her new review,
Dr. Warrener questioned whether babies getting stuck in birth canals
have posed a major threat to women’s lives. It is far more common,
she noted, for new mothers to die from blood loss or infections.

She also criticized the way in which Dr. Grunstra and other defenders
of the obstetrical dilemma make the case for their hypothesis. In her
view, they assume that every piece of human anatomy has been
fine-tuned by natural selection for a specific job.

Sometimes, Dr. Warrener said, adaptations are flukes. For example,
some of the genes that build the pelvis are also active in the
development of other parts of the skeleton. If another bone in our
body were to evolve into a new shape, the pelvis might change simply
as a byproduct — not because it was evolving for walking or
childbirth.

“I think sex differences in the pelvis have been somewhat of a red
herring,” Dr. Dunsworth said. Like other bones, the pelvis does not
have a fixed shape encoded in a genetic blueprint. Its development is
influenced by the tissues around it, including the uterus, the ovaries
and other organs. The proportions of the female pelvis may result in
part from all the organs that grow inside it.

Both Dr. Dunsworth and Dr. Warrener worry that the obstetrical dilemma
leads to a widespread notion of the female body as inescapably
defective.

“That just makes us feel like problems that need to be solved by
medicine,” Dr. Dunsworth said. That narrative may play a part in the
medicalization of childbirth in recent decades, she added.

The World Health Organization has warned
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doctors are increasingly performing unnecessary medical intervention
on mothers, while chronic disorders that can threaten maternal health
— such as high blood pressure, obesity and diabetes — get little
attention.

“The way we live now probably doesn’t lead us to meet the
challenge of childbirth as well as our bodies did when they developed
differently in the past,” Dr. Dunsworth said.

But recognizing the over-medicalization of modern pregnancy doesn’t
end the debate about its origins, Dr. Grunstra said. “That does not
in itself mean that evolutionary explanations are wrong,” she said.

_CARL ZIMMER [[link removed]] writes
the “Origins” [[link removed]] column.
He is the author of fourteen books, including “Life's Edge: The
Search For What It Means To Be Alive.”_

THE HIDDEN HARMS OF CPR
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The brutal procedure can save lives, but only in particular cases. Why
has it become a default treatment?
By Sunita Puri
The New Yorker
August 5, 2023

* Science
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* Evolution
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* Medicine
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* childbirth
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