Frequent inbreeding may have caused skeletal abnormalities in early humans

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These bowed femurs were found (top to bottom) in China’s Tianyuan cave, Russia’s Sunghir burial site, and the Czech Republic’s Dolní Vĕstonice site.

Erik Trinkaus

Early humans faced countless challenges as they fanned out of Africa: icy conditions, saber-tooth cats, and, according to a new study of ancient skeletons, an unusually high number of birth defects, both debilitating and relatively inconsequential. It’s unclear why such abnormalities seem to be so common, but scientists say one strong possibility is rampant inbreeding among small hunter-gatherer groups.

“This paper represents a valuable compilation,” says Vincenzo Formicola, an anthropologist at the University of Pisa in Italy who wasn’t involved in the new work. “Many cases reported in the list were unknown to me and, I assume, to many people working in the field.”

Many human fossils from the Pleistocene (roughly 2.5 million B.C.E. to 9700 B.C.E.) have unusual features. For example, femur bones with abnormal bowing have been found from China to the Czech Republic. The skull of a toddler found in the Qafzeh cave in Israel had a swollen braincase consistent with hydrocephalus, a condition in which fluid floods the skull. And a fossilized man in Liguria in Italy had a bowed right upper arm bone but a normal left one.

By and large, these were viewed as one-off curiosities. But Erik Trinkaus, a paleoanthropologist at Washington University in St. Louis, Missouri, noticed a pattern: These skeletal deformations seemed to be suspiciously common in the fossil record.

So Trinkaus did the math. He assembled data on 66 individuals with skeletal abnormalities mostly dating to the past 200,000 years. The fossils, most from young adults, were found in sites scattered throughout the Middle East and Eurasia and represent several different species of Homo. Trinkaus then researched how common their conditions are in modern human populations.

He found that about two-thirds of the ancient abnormalities occur in less than 1% of modern humans. Another dozen or so didn’t match any known modern developmental disorder. Trinkaus ran the odds that archaeologists would have uncovered so many ancient abnormalities by chance, and he found that it would have been a “truly, vanishingly small probability.” That suggests, he reports today in the Proceedings of the National Academy of Sciences, that early humans faced some cultural or environmental pressure that led to so many deformities.

One possibility, previously proposed by other researchers: Ancient people with skeletal deformities might have been seen as shamans and given careful burials, making their bodies more likely to be preserved and later found. Another: Pregnant mothers didn’t get enough of the right nutrients, leading to more skeletal disorders. But Trinkaus notes that, whereas some skeletal disorders like rickets affect the whole body, many skeletons were found with deformities on only one side of the body. He also says many fossils in his analysis show no evidence of special rites.

However, several bodies show abnormalities consistent with known genetic mutations, and multiple individuals from at least one site exhibited several different conditions, suggesting the people might be related. It’s thought that most human populations at the time were small and isolated, Trinkaus says. In those conditions, inbreeding can lead to widespread harmful genetic mutations.

Evidence of low genetic diversity among Pleistocene humans based on ancient DNA analysis also supports this hypothesis, says Hallie Buckley, a bioarchaeologist at the University of Otago in Dunedin, New Zealand. “Of all the arguments put forward … this seems the most likely explanation.”

Further analysis of ancient DNA at these sites might confirm inbreeding, but prepping samples for such investigations often means destroying them. “Ancient DNA has become increasingly viewed as a ‘magic bullet’ to shoot at any question about past human populations, but that may not always be justified,” Buckley says.

Siân Halcrow, Buckley’s colleague at the University of Otago, says that although she appreciates Trinkaus’s thorough cataloging, his paper has several weaknesses, most notably in its estimates of how common these abnormalities are in modern people—and how common they used to be. It would be better to compare the ancient rates to later populations in prehistory or early historic populations, she says, but unfortunately those data don’t exist.

No matter the cause, many of the deformities would have been debilitating. The fact that so many survived past childhood suggests early humans must have offered each other social support and medical knowhow, Trinkaus says. For example, although hydrocephaly is rarely a death sentence thanks to modern treatment, it can easily be fatal if left untreated. “The Qefzeh child with hydrocephaly lived until about 3 or 4 years old. When you consider it lived 100,000 years ago, that’s pretty amazing.”

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