3D models of the skulls from a modern human and from a Neanderthal
Courtesy of Kimberly Anne Plomp
A skull abnormality that squeezes the lower brain, often causing headaches and other neurological problems, might be part of our genetic inheritance from Neanderthals.
People with Chiari malformations have a smaller and flatter base of the skull around the area where it connects to the spine. As a result, part of the brain – the cerebellum – is squashed into the spinal canal in the neck.
Type 1 Chiari malformations, the mildest form, are thought to affect up to 1 in 100 people. They can cause symptoms like headaches, neck pain, sleep apnoea and numbness, but some people never show signs at all.
About 15 years ago, Yvens Barbosa Fernandes, a neurosurgeon at the State University of Campinas in Brazil, noticed that the base of his Chiari patients’ skulls resembled those he had seen in Neanderthal specimens in European museums, especially in the mild slope of the occipital bone, where the cerebellum rests. While Neanderthal brains were larger than modern humans’ are, they slanted back more from the forehead and across the base, giving them a flatter shape compared with the rounder form of modern human skulls.
In 2013, Barbosa Fernandes published a hypothesis proposing that the Chiari skull shape might have been inherited from extinct humans who interbred with Homo sapiens. “I started to think there was a lost link between anthropology and medicine in Chiari cases,” he says.
Inspired by this suggestion, Kimberly Plomp at the University of the Philippines Diliman in Quezon City and her colleagues built digital 3D models of the skulls of 46 people with Chiari type 1 and 57 without Chiari, based on their CT scans. Their detailed mathematical analyses confirmed that the Chiari-affected skulls had a smaller occipital bone with a flatter angle, and more brain compression at the base of the skull where the cerebellum sits.
Next, the team examined how those modern skulls compared with digital models of eight fossil heads from Homo neanderthalensis, Homo erectus, Homo heidelbergensis and ancient Homo sapiens.
They found that the skull bases of Neanderthal heads had remarkably similar measurements to those of modern humans with Chiari, while all the other ancient cranial bases resembled those of modern humans without Chiari. “It highlights the idea that these are Neanderthal traits – not just early traits,” she says. “It looks like this is just another way that Neanderthal genes might be influencing our health – and in this case, in a negative way.”
For Barbosa Fernandes, the study provides strong evidence in support of his theory. “It makes sense: if you have less angulation, you have less space for the modern human brain,” he says. “I didn’t have the science to prove my hypothesis. This paper is a big step closer towards that proof.”
As a next step, the team hopes to analyse the DNA of people with Chiari malformations to look for Neanderthal genes, Plomp says.
Other types of Chiari malformations – types 2 to 4 – are thought to have different causes. Type 2 is linked to a severe form of spina bifida, while types 3 and 4 are extremely rare and may be life-threatening.
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