Did Rampant Inbreeding Contribute to Early Humans’ High Rate of Skeletal Deformities?

A new analysis of 66 early humans’ remains has revealed an astounding 75 instances of skeletal abnormalities, from bowed femur and arm bones to misshapen jaws, dwarfism, and a swollen braincase consistent with hydrocephalus, a condition characterized by the buildup of fluid inside the skull.

This surprisingly high rate of birth defects is likely representative of ancient populations as a whole, paleoanthropologist Erik Trinkaus of Washington University in St. Louis writes in a new paper published in the Proceedings of the National Academy of Sciences. As he tells Science magazine’s Michael Price, the odds of uncovering so many abnormalities in such a small sample size simply by chance are “truly, vanishingly small.”

Instead, Trinkaus argues that the skeletons—which date to around 200,000 years ago and were unearthed in regions as far ranging as China, the Czech Republic, Italy and Israel—bear witness to the widespread cultural and environmental pressures faced by our Pleistocene predecessors.

Perhaps pregnant mothers failed to follow a healthy diet, leaving their offspring susceptible to skeletal disorders like rickets. Maybe individuals exhibiting abnormalities were given more elaborate burials, upping the chances of their preservation and future rediscovery. (Although it’s worth noting, Cosmos’ Andrew Masterson says, that Trinkaus hasn’t found evidence of different burial practices employed for those with or without defects.) It's also possible life as a hunter-gatherer was just ceaselessly challenging: As the study notes, “The abundance of developmental abnormalities among Pleistocene humans may have been enhanced by the generally high levels of stress evident among these foraging populations.”

But the most probable culprit is rampant inbreeding amongst ancient populations, according to Hallie Buckley, a bioarchaeologist at New Zealand’s University of Otago who was not involved in the new study. Given the limited size and relative isolation of early human communities as evidenced by the low level of genetic diversity seen in previous studies of ancient DNA, Buckley tells Price that “this seems the most likely explanation.”

Some of the abnormalities Trinkaus spotted are inherited conditions, making them more likely to manifest amongst the offspring of closely-related individuals. Many persist to this day, the study explains, popping up in recent human samples as “unusual but not exceptional” conditions. Others are “extremely rare” in contemporary populations and therefore unlikely to show up in the fossil record.

Based on comparisons with modern humans, Trinkaus found that the chances of identifying more “typical” abnormalities amongst early populations were around five percent. The chances of identifying one of the rarer abnormalities were as low as 0.0001 percent.

“The chances of finding them in combination, or collectively in evidence in every set of remains to date uncovered and reliably dated, is astronomical,” Masterson summarizes for Cosmos.

The presence of deformities in ancient remains isn’t surprising in and of itself, he adds, but becomes significant in lieu of the sheer number seen in just 66 sets of remains.

Still, Siân Halcrow, a bioarchaeologist at the University of Otago who was not involved in the study, tells Science’s Price that extrapolating estimates of abnormality incidence amongst early humans by drawing on similar figures in modern populations could prove problematic. A better approach would be to compare the ancient rates evident across the sample with data from prehistoric or early historic populations—an arduous task complicated by the fact that such data pools don’t actually exist yet.

Trinkaus wasn’t able to directly pinpoint the causes of the 75 abnormalities identified, but as he concludes in the study, there were likely an array of factors—not just one—at play: “A substantial number of these abnormalities reflect abnormal or anomalous developmental processes, whether as a result of genetic variants altering developmental processes or as the products of environmental or behavioural stress patterns altering expected developmental patterns.”

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's senior associate digital editor, history.