Interbreeding between anatomically modern Homo sapiens and Neanderthals thousands of years ago may be responsible for Chiari Malformation Type 1, a serious and sometimes fatal neurological condition estimated to impact up to 1% of people today.
In 2013, scientists hypothesized that individuals develop Chiari Malformation Type 1 because some of their cranial development-coding genes derive from three extinct Homo species that have smaller basicrania than is typical for modern humans: Homo erectus, Homo heidelbergensis, and Homo neanderthalensis; Plomp et al. used 3D data and geometric morphometrics to evaluate this hypothesis. Image credit: Neanderthal Museum.
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.
“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.”
In 2010, scientists discovered genetic evidence that members of our species interbred with Neanderthals tens of thousands of years ago.
Living non-Africans have 2-5% of Neanderthal DNA that can be traced to these interbreeding events.
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 State University of Campinas researcher Yvens Barbosa Fernandes.
Because the modern human skull differs in several important ways from those of other hominins, Dr. 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.
In a new study, Simon Fraser University’s Professor Mark Collard, University of the Philippines Diliman’s Dr. Kimberly Plomp 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.
They 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.
“Studying archaeology and human evolution is not just interesting,” Professor Collard said.
“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 paleontological data can help us understand better.”
The study was published in the journal Evolution, Medicine, and Public Health.
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Kimberly Plomp et al. 2025. A test of the Archaic Homo Introgression Hypothesis for the Chiari malformation type I. Evolution, Medicine, and Public Health 13 (1): 154-166; doi: 10.1093/emph/eoaf009
