Neanderthal Genes Influence Contemporary Humans’ Skull Shape, Brain Size


Individuals carrying these ancient ancestors’ DNA are more likely to have slightly elongated, rather than rounded, brains

The researchers are quick to point out that their findings don’t suggest a link between brain size or shape and behavior, but instead offer an exploration of the genetic evolution of modern brains (Philipp Gunz)

By Meilan Solly

smithsonian.com
December 14, 2018

Neanderthals may have gone extinct some 40,000 years ago, but thanks to long-ago species interbreeding, their genes live on in modern humans.

The implications of this genetic inheritance remain largely unclear, although previous studies have proposed links with disease immunity, hair color and even sleeping patterns. Now, Carl Zimmer reports for The New York Times, a study recently published in Current Biology offers yet another example of Neanderthals’ influence on Homo sapiens: Compared to individuals lacking Neanderthal DNA, carriers are more likely to have slightly elongated, rather than rounded, brains.

This tendency makes sense given Neanderthals’ distinctive elongated skull shape, which Science magazine’s Ann Gibbons likens to a football, as opposed to modern humans’ more basketball-shaped skulls. It would be logical to assume this stretched out shape reflects similarly protracted brains, but as lead author Philipp Gunz of Germany’s Max Planck Institute for Evolutionary Anthropology tells Live Science’s Charles Q. Choi, brain tissue doesn’t fossilize, making it difficult to pinpoint the “underlying biology” of Neanderthal skulls.

To overcome this obstacle, Gunz and his colleagues used computed tomography (CT) scanning to generate imprints of seven Neanderthal and 19 modern human skulls’ interior braincases. Based on this data, the team established a “globularity index” capable of measuring how globular (rounded) or elongated the brain is. Next, Dyani Lewis writes for Cosmos, the researchers applied this measure to magnetic resonance imaging (MRI) scans of around 4,500 contemporary humans of European ancestry, and then compared these figures to genomic data cataloguing participants’ share of Neanderthal DNA fragments.

Two specific genes emerged in correlation with slightly less globular heads, according to The New York Times’ Zimmer: UBR4, which is linked to the generation of neurons, and PHLPP1, which controls the production of a neuron-insulating sleeve called myelin. Both UBR4 and PHLPP1 affect significant regions of the brain, including the part of the forebrain called the putamen, which forms part of the basal ganglia, and the cerebellum. As Sarah Sloat explains for Inverse, the basal ganglia influences cognitive functions such as skill learning, fine motor control and planning, while the cerebellum assists in language processing, motor movement and working memory.

In modern human brains, PHLPP1 likely produces extra myelin in the cerebellum; UBR4 may make neurons grow faster in the putamen. Comparatively, Science’s Gibbons notes, Neanderthal variants may lower UBR4 expression in the basal ganglia and reduce the myelination of axions in the cerebellum—phenomena that could contribute to small differences in neural connectivity and the cerebellum’s regulation of motor skills and speech, the study’s lead author Simon Fisher of the Netherlands’ Max Planck Institute for Psycholinguistics tells Gibbons .

Still, the effects of such gene variations are probably negligible in living humans, merely adding a slight, barely discernible elongation to the skull.

“Brain shape differences are one of the key distinctions between ourselves and Neanderthals,” Darren Curnoe, a paleoanthropologist from Australia’s University of New South Wales who was not involved in the study, tells Cosmos, “and very likely underpins some of the major behavioural differences between our species.”

In an interview with The New York Times, Fisher adds that the evolution of UBR4 and PHLPP1 genes could reflect modern humans’ development of sophisticated language, tool-making and similarly advanced behaviors.

But, Gunz is quick to point out, the researchers are not issuing a decisive statement on the genes controlling brain shape, nor the effects of such genes on modern humans carrying fragments of Neanderthal DNA: “I don’t want to sound like I’m promoting some new kind of phrenology,” he tells Cosmos. “We’re not trying to argue that brain shape is under any direct selection, and brain shape is directly related to behaviour at all.”

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