Why modern humans have round heads

Ever since researchers first got a good look at a Neanderthal skull in the 1860s, they were struck by its strange shape: stretched from front to back like a football rather than round like a basketball, as in living people. But why our heads and those of our ice age cousins looked different remained a mystery.

Now, researchers have found an ingenious way to identify genes that help explain the contrast. By analyzing traces of Neanderthal DNA that linger in Europeans from their ancestors' trysts, researchers have identified two Neanderthal gene variants linked to slightly less globular head shape in living people, the team reports this week in Current Biology. The genes also influence brain organization, offering a clue to how evolution acting on the brain might have reshaped the skull. This "very important study" pinpoints genes that have a "direct effect on brain shape and, presumably, brain function in humans today," says paleoanthropologist Chris Stringer of the Natural History Museum in London, who was not a part of the work.

Cradle a newborn and you'll see that infants start life with elongated skulls, somewhat like Neanderthals. It's only when the modern human brain nearly doubles in size in the first year of life that the skull becomes globular, says paleoanthropologist Philipp Gunz of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. He and his colleagues analyzed computerized tomography scans of modern human and Neanderthal skulls to develop a "globularity index" of human brains.

To explore the underlying differences in brain tissue, they applied that index to MRI scans from 4468 people of European ancestry whose DNA had been genotyped. The team identified two Neanderthal DNA fragments that were correlated with slightly less globular heads. These DNA fragments affect the expression of two genes: UBR4, which regulates the development of neurons, and PHLPP1, which affects the development of myelin sheaths that insulate axons, or projections of neurons.

The Neanderthal variants may lower URB4 expression in the basal ganglia and also lead to less myelination of axons in the cerebellum, a structure at the back of the brain. This could contribute to subtle differences in neuronal connectivity and how the cerebellum regulates motor skills and speech, says senior author Simon Fisher of the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands. But any effects of the Neanderthal genes in living people would be slight because so many genes shape the brain.

Tying Neanderthal DNA to brain scans in living people is an "innovative and exciting approach" because "soft tissue in the brain is impossible to access from the fossil record," says anthropologist Katerina Harvati of the University of Tübingen in Germany. She'd like to see the findings confirmed in more people.

Indeed, Gunz and Fisher plan to delve into the UK Biobank, a giant database of British people's health records and DNA. They hope to use Biobank brain scans to find more genes and to explore how Neanderthal brains would have functioned. "The Neanderthal DNA that remains in us can help us think about what their brains were like," says geneticist Tony Capra of Vanderbilt University in Nashville.

Scans of skulls show modern human infants start out with elongated heads—somewhat like Neanderthals—but they round out in adulthood.