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

The first humans were faced with countless challenges when they came out of Africa: icy conditions, saber-toothed cats and, according to a new study on ancient skeletons, abnormally high numbers of congenital malformations, both debilitating and relatively unimportant. It is not clear why such abnormalities appear to be so common, but scientists say a strong possibility is endemic consanguinity among small hunter-gatherer groups.

"This document is a valuable compilation," says Vincenzo Formicola, an anthropologist at the University of Pisa in Italy, who did not participate in this new work. "Many of the cases reported in the list were unfamiliar to me, and I suppose there are many who work on the ground."

Many Pleistocene human fossils (about 2.5 million Central Europe to 9700 BC) have unusual characteristics. For example, bones of the femur with abnormal curvature have been discovered from China to the Czech Republic. The skull of a child found in Qafzeh Cave in Israel had a puffy puzzler compatible with hydrocephalus, a condition in which a fluid flooded the skull. And a fossilized man in Liguria in Italy had a rounded right arm bone but a normal left bone.

Overall, they were considered unique curiosities. But Erik Trinkaus, a paleoanthropologist at Washington University in St. Louis, Missouri, noted a pattern: these skeletal deformities seemed strangely common in the fossil record.

So Trinkaus did the math. He collected data on 66 people with skeletal abnormalities dating back mainly to the last 200,000 years. The fossils, mostly from young adults, have been found in scattered sites throughout the Middle East and Eurasia and represent several species of Homo. Trinkaus then studied how common their conditions are in modern human populations.

He found that about two-thirds of ancient anomalies occur in less than 1% of modern humans. Another dozen or so did not correspond to any known modern development disorder. Trinkaus feared that archaeologists had accidentally discovered many ancient anomalies of development, and he discovered that it would have been a "very small probability." Proceedings of the National Academy of Sciences, that early humans faced cultural or environmental pressure that led to so many deformities.

A possibility, previously proposed by other researchers: old people with skeletal deformities could have been considered shamans and buried with care, which would make their body more likely to be preserved and found later. Another: pregnant mothers did not have enough good nutrients, which led to more skeletal disorders. But Trinkaus notes that while some skeletal disorders such as rickets affect the whole body, many skeletons have been found with deformities on one side of the body. He also says that many fossils in his analysis show no evidence of special rites.

However, several bodies have abnormalities consistent with known genetic mutations, and several individuals from at least one site presented with several different conditions, suggesting that people might be related. It was thought that most of the human populations of the time were small and isolated, says Trinkaus. In these conditions, consanguinity can lead to many harmful genetic mutations.

Evidence for low genetic diversity in Pleistocene humans, based on an ancient DNA analysis, also confirms this hypothesis, said Hallie Buckley, a bioarchaeologist at the University of Otago at Dunedin, New Zealand. "Of all the arguments put forward … this seems the most likely explanation."

Further analysis of old DNA at these sites may confirm consanguinity, but preparing samples for such surveys often means destroying them. "Old DNA is increasingly seen as a magic bullet for answering all questions about human populations in the past, but that may not always be right," says Buckley.

Siân Halcrow, Buckley's colleague at the University of Otago, says that although she appreciates Trinkaus' careful cataloging, her paper has several weaknesses, including her estimation of the frequency and frequency of these abnormalities in men. modern. It would be better to compare earlier rates to more recent prehistoric populations or early historical populations, she says, but unfortunately these data do not exist.

Regardless of the cause, many malformations would have been debilitating. The fact that so many children survived during their childhood suggests that the first humans had to offer mutual support and medical know-how, explains Trinkaus. For example, although hydrocephalus is rarely a death sentence because of modern treatment, it can easily be fatal if left untreated. "The Qefzeh child with hydrocephalus lived for about 3 or 4 years. When you think she lived 100,000 years ago, it's really amazing. "