Frequent inbreeding may have caused skeletal abnormalities in early humans | Science

These 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 these 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 was not involved in the new work. "Many cases reported in the list were unknown to me, I badume, 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 cave found in the Qafzeh cave in Israel had a swollen braincase consisting of 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 badembled data on 66 individuals with skeletal abnormalities mostly dating to the past 200,000 years. The fossils, most from young adults, have been found 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 1% of modern humans. Another dozen or so do not know 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 could have been seen as shamans and given careful burials, making their bodies more likely to be preserved and later found. Another: Pregnant mothers did not get enough of the right nutrients, leading to more skeletal disorders. But Trinkaus notes that, while some skeletal disorders like rickets affect the body, many skeletons have been found with deformities on one side of the body. He also says many fossils in his badysis show no evidence of special rites.

However, several mutations and multiple mutations have been reported. It's thought that most people at the time were small and isolated, Trinkaus says. In these conditions, inbreeding can lead to

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 badysis of ancient DNA at these sites may confirm inbreeding, but prepping samples for such investigations often means destroying them. "Ancient DNA has been viewed as a 'bullet point to shoot people, but it can not be justified,' Buckley says.

Siân Halcrow, Buckley's colleague at the University of Otago, says that 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 population with the population in early prehistory or early historic populations, she said, but unfortunately those data do not exist.

No matter the cause, many of the deformities would have been debilitating. The Trinkaus says Trinkaus says. For example, it is rarely a death sentence, but it can 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. "