One of the most distinctive characteristics of the modern human being compared to other human species is the shape of your skull and brain. Its roundness has now been the subject of analysis by an international team of scientists, led by the Max Planck Institute for Evolutionary Anthropology (Germany).
The researchers focused on our closest extinct relatives, neanderthals, to better understand the biological bases of the modern human endocranial form.
"Our goal was to identify potential candidate genes and biological pathways related to the spherical shape of the brain," says Amanda Tilot, of the Max Planck Institute for Psycholinguistics, and co-leader of the work published in Current Biology.
In this way, the team discovered subtle variations in intracranial shape "Which probably reflect changes in the volume and connectivity of certain areas of the brain," says Philipp Gunz, a paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology and co-leader of the study.
To direct their search, the experts relied on the fact that modern humans of European ancestry carry rare fragments of Neanderthal DNA in their genomes as a result of the crossing between the two species.
So, by analyzing the cranial shape, they identified stretches of Neanderthal DNA in a large sample of modern humans, which they combined with magnetic resonance imaging and genetic information from about 4,500 people.
Thanks to the scanners, the scientists were able to detect differences in intracranial shape between the fossils of Neanderthals and the skulls of modern humans. This contrast allowed them to evaluate the cranial shape in thousands of brain MRI scans of living people.
Neanderthal genes for brain development
On the other hand, the genomes sequenced from ancient Neanderthal DNA it also allowed them to identify Neanderthal DNA fragments in modern humans on chromosomes 1 and 18, related to a less round cranial shape.
These fragments contained two genes already linked to brain development: UBR4, involved in the generation of neurons; Y PHLPP1, in the development of the isolation of myelin - a substance that protects the axons of certain nerve cells and accelerates the transmission of the nerve impulse.
"We know from other studies that complete disruption of UBR4 or PHLPP1 can have important consequences for brain development," explains lead author Simon Fisher, a geneticist at the Max Planck Institute for Psycholinguistics.
In their work, the experts found that, in carriers of the relevant Neanderthal fragment, the UBR4 gene it is slightly downregulated in the putamen, the structure located in the center of the brain that, together with the caudate nucleus, forms the striatum, and is part of a network of brain structures called the basal ganglia.
In the case of carriers of the Neanderthal fragment PHLPP1, "Gene expression is slightly higher in the cerebellum, which is believed to have a dampening effect on myelination of the cerebellum," says Fisher.
Both regions of the brain - the putamen and the cerebellum - are, according to scientists, important in movement. "These regions receive direct information from the motor cortex and participate in the preparation, learning and sensorimotor coordination of movements", emphasizes Gunz, who adds that the basal ganglia also contribute to various cognitive functions in memory, attention, planning, skill learning, and speech and language evolution.
All these Neanderthal variants they result in small changes in gene activity and make the brain shape of certain people less spherical. The researchers conclude that the consequences of transporting these rare Neanderthal fragments are subtle and only detectable in a very large sample.
Gunz et al .: "Neanderthal introgression sheds light on modern human endocranial globularity" Current Biology https://www.cell.com/current-biology/fulltext/S0960-9822(18)31470-2.
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