Deformed Skeletons of Pleistocene Humans May Be a Result of Inbreeding

The Pleistocene epoch was not exactly an easy time to be alive. People who lived through that period, which began about 2.6 million years ago, struggled against the chill of the last Ice Age, the threat of huge predators, and the lack of medicine. According to a new study by Washington University paleoanthropologist Erik Trinkaus, Ph.D., these hurdles placed a huge amount of stress on Pleistocene people, potentially leading, in some dire cases, to an unhealthy amount of inbreeding.

In the paper, published Monday in the Proceedings of the National Academy of Sciences, Trinkaus reports that the majority of skeletons sourced from this epoch contain “an abundance of developmental anomalies.” Among the 66 ancient individuals of various Homo species recovered from sites in the Middle East and Eurasia, he observed 75 deformities, including bowed arms, misshapen jaws, and abnormal crania. Of the possible explanations for why scientists have dug up so many abnormal skeletons from this era, one is particularly distasteful to modern humans.

Trinkaus, who tells Inverse he pursued this study because he’s interested in these humans “as people and how they managed under difficult circumstances,” investigated the ways in which these deformities were tied to the Pleistocene epoch. Approximately two-thirds of the deformities he observed in the ~200,000-year-old skeletons show up in less than one percent of modern humans, and an additional dozen could not be linked to any known modern developmental disorder. Some were later chalked up to actual medical ailments, like blood disorders or hydrocephaly, but for the most part, the seemingly high frequency of abnormal skeletons from this era can be explained by three hypotheses.

Inbreeding

In the study, Trinkaus notes that the “patterns and incidence of developmental abnormalities and anomalies” could provide insight into Pleistocene consanguinity. In other words, it’s likely that inbreeding was a common affair within these foraging populations. The offspring of consanguineous couples are known to have an increased risk for autosomal recessive disorders, a class of genetic disorders that occurs when there’s not enough genetic diversity between two people who procreate. For the roaming Pleistocene individuals, there very well could not have been enough of a local diverse population to procreate with.

“[Some] anomalies (especially dental and vertebral variants) appear to have inherited predispositions, as shown primarily through family studies,” writes Trinkaus. “Therefore, it is possible that the elevated frequency of these conditions is a product in part of high levels of consanguinity in Pleistocene populations.”

Sally Reynolds, Ph.D. is a senior lecturer in hominin palaeoecology at Bournemouth University who was not involved with this study but is also an expert on ancient humans. She agrees that inbreeding could explain the abnormally high rate of deformity.

“The study shows very interesting patterns of abnormalities at a time when there were several hominin species present, including the Neanderthals,” she tells Inverse. “It is possible that some of these abnormalities indicate small population and inbreeding effects (resulting from smaller gene pools). This is exactly the sort of evidence that you would expect to see if there are small, fragmented, stressed populations just prior to extinction.”

Health Stresses

Inbreeding aside, it’s possible that Pleistocene individuals suffered deformities simply because life was physically difficult. Trinkaus writes that “the abundance of developmental abnormalities among Pleistocene humans may have been enhanced by generally high levels of stress evident among these forging populations.”

These stresses could be things like tooth infections, traumatic lesions, and cranial injuries — the sort of stuff we would go to the doctor for. A remarkable aspect of the population that Trinkaus studied is that all of these people, save for one, survived their deformities. Still, their skeletons clearly demonstrate the cost of living with bodily hardships.

Burial Practices

Part of the reason scientists have been able to uncover so many abnormal Pleistocene bones is that special burials were present in the Middle Paleolithic and onward, increasing the odds that the remains survived to the present day. It’s difficult to determine what proportion of these people were given burials — and which type of people received them — but this must be factored in when assessing the frequency of abnormalities among Paleolithic bones.

“The presence of the developmentally (and degeneratively) unusual individuals in European Upper Paleolithic burials,” Trinkaus writes, “has suggested that differing mortuary treatment of those individuals as a result of their unusual biologies.”