It isn’t just modern humans that have found giving birth painful and dangerous. Growing evidence suggests birth was difficult for our hominin relatives millions of years ago. As a result, earlier hominins like Australopithecus may have needed help to deliver their babies.
Birth is strikingly dangerous for modern humans compared with other primates. Globally, for every 100,000 births in 2017, 211 mothers died. In the worst-affected countries, such as South Sudan, the maternal mortality rate is more than five times that. Many nations have much lower rates, but that is largely due to better medical intervention, including caesarean sections – which weren’t available for most of our species’ existence.
The same isn’t true for primates including monkeys, our closest living relatives. “You’re not seeing these types of complications that you see in humans,” says Nicole Webb at the University of Zurich in Switzerland.
The long-standing explanation for the difficulty of human births is that it is caused by a combination of our large brains and the fact we walk upright on two legs. According to anthropologist Sherwood Washburn, writing in 1960, upright walking meant evolution favoured a narrower pelvis, but also a wider pelvic canal to accommodate the baby’s head – creating what he dubbed the “obstetrical dilemma”.
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Despite challenges and modifications to the idea, for many anthropologists, it still largely holds true. “In my view, the obstetrical dilemma hypothesis as Washburn framed it is still the most reasonable,” says Martin Haeusler at the University of Zurich.
It was generally thought that the obstetrical dilemma was unique to humans, or at least to the Homo genus, but new evidence suggests birth difficulties go back much further.
Webb and her colleagues have examined the birth canals of chimpanzees, which were thought to be much more spacious than those of humans. In a talk at the annual meeting of the American Association of Physical Anthropologists, held online last month, she explained that this assumption partly stems from a famous 1949 study by Adolph Schultz, which demonstrated that the entrance to the chimpanzee birth canal was almost twice as wide as the fetus’ head.
However, Webb argued that Schultz measured the wrong parts of the pelvis: chimpanzee births work differently to human births, so the tightest fits are in different places. She then described how she has made her own measurements of 29 chimpanzee pelvises and found that in the tightest spot, the fetus’ head is 85 per cent of the size of the canal: still looser than what is found in humans, but not by much.
“We looked at the more relevant dimensions of the birth canal,” says Webb. “It’s not like chimpanzees have all this space.”
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She suggested there was a shift in the evolution of great apes, whose pelvises are different from those of monkeys. Many monkeys can stretch the cartilage that joins the left and right halves of the pelvis at the front, creating a wider opening, but chimpanzees can’t do that.
“Our closest ancestors already had some of the same features [as humans], including not having so much flexibility in the front part of the pelvis to accommodate birth,” says Webb.
In another talk at the conference, Natalie Laudicina at Grand Valley State University in Allendale, Michigan, described how she had mapped the entire birth canals of five hominins: four Australopithecus specimens from three species, including the famous Lucy, and one Neanderthal.
“Every species is different,” says Laudicina. But with the exception of the 2 million-year-old Australopithecus sediba, all the species she looked at faced a tight squeeze, and she says the babies would have had to rotate to get through the birth canal. Human infants have to twist and turn to get out, which is a big part of why birth is so complicated. Laudicina says her research suggests that many earlier hominins faced similar problems and didn’t have easy births like most non-human primates.
She emphasises that the full story isn’t yet available, because the five pelvises she looked at represent pretty much all the hominin pelvises known. “It’s hard to pinpoint exactly when modern human childbirth started, because we only have one specimen per 2 million years,” she says.
But if Australopithecus had difficult and complicated births, mothers may have needed help during labour. “I think we’ve underestimated what Australopithecus [went] through,” says Webb. Other adult females, perhaps those who had already given birth, may have acted as midwives. “I think it’s something we need to entertain as a possibility,” she says.
Most primates leave their groups to give birth, suggesting they do it alone – although they also tend to do it at night, so observations are few and far between. But there have been a handful of exceptions: captive female bonobos have been seen seemingly helping other females in labour, and there are isolated reports of similar behaviour in monkeys.