Depiction of a family of Neanderthal flint workers. Credit: Charles Robert Knight, Public domain, via Wikimedia Commons.

Prehistoric interbreeding with Neanderthals may be responsible for a modern-day brain condition, according to new research.

A new Simon Fraser University-led study reveals interbreeding between humans and their ancient cousins, Neanderthals, as the likely origin of a neurological condition estimated to impact up to 1% of people today.

The study was led by Kimberly Plomp, a recent postdoctoral fellow at SFU and Mark Collard, the Canada Research Chair in Human Evolutionary Studies and a professor in the Department of Archaeology.

Their findings suggest that Chiari Malformation Type 1, a serious and sometimes fatal neurological condition, may be linked to Neanderthal genes that entered the human gene pool through interbreeding tens of thousands of years ago.

Chiari Malformation Type 1 occurs when the back of a human’s skull is too small to properly hold the brain, causing part of the base of the brain to herniate out of the skull and into the spinal canal.

This can cause the herniated part of the brain to be pinched, leading to symptoms such as headaches, neck pain, dizziness and, in severe cases, death if too much of the brain herniates out.

Collard said: “In medicine, as in other sciences, clarifying causal chains is important. The clearer one can be about the chain of causation resulting in a medical condition, the more likely one is to be able to manage, or even resolve, the condition.

“The hypothesis needs to be tested further, but our study may mean we are one step closer to obtaining a clear understanding of the causal chain that gives rise to Chiari Malformation Type 1.”

Genetic evidence

The knowledge that our ancient human ancestors successfully mated with Neanderthals (and some other hominin species) is not new. The impacts of such interbreeding, however, are still coming to light.

In 2010, scientists discovered genetic evidence that members of our species interbred with Neanderthals tens of thousands of years ago.

It is now clear that living non-Africans have 2% to 5% Neanderthal DNA that can be traced to these interbreeding events.

We have also learned that genes from other extinct hominin species exist in the modern human gene pool due to interbreeding in the distant past.

The idea that Chiari Malformation Type 1 might be the result of other hominin genes entering the human gene pool through interbreeding was initially proposed by Yvens Barbosa Fernandes of Brazil’s State University of Campinas.

Because the modern human skull differs in several important ways from those of other hominins, Fernandes reasoned, having a skull that is influenced by the genes of other hominin species may be one of the factors that causes the malformation.

Medical imaging technology

Plomp, Collard, and their colleagues put this theory to test using modern medical imaging technology and advanced statistical shape analysis techniques to compare 3D models of skulls from living humans, both with and without Chiari Malformation Type 1, to fossils hominins, including ancient Homo sapiens, Neanderthals, Homo heidelbergensis, and Homo erectus. 

The team found that people with Chiari malformation share more shape traits in common with Neanderthals than do people without the malformation.

Interestingly, all other fossil skulls were closer in shape to humans without Chiari Malformation Type 1, indicating that the findings are not due to shared ancestry, but instead support the hypothesis that some people today have Neanderthal genes that affect their skull shape, and this skull shape results in a mismatch between the shape of the skull and shape of the modern human brain.

It’s this mismatch that results in the brain not having enough room in the skull, and thus, the brain is pushed out the only hole available, the spinal canal.

With different populations around the world having varying levels of Neanderthal DNA, the study predicts that certain populations — including those from Europe and Asia — could be at a higher risk of Chiari Malformation Type 1 than others, though further research is required to confirm this.

Collard said: “Studying archaeology and human evolution is not just interesting – it also has the potential to help us understand and, in some cases, cope with problems in the present.

“In this case, we’ve used fossils to help us shed light on a medical condition, but there are a lot of other contemporary problems that archaeological and palaeontological data can help us understand better.”

The study is published in Evolution, Medicine, and Public Health.