Our brains could have more in common with our ape cousins than previously thought, which might require us to rethink ideas on the evolution of brain specialism in our early human ancestors.
The left and right sides of our brains aren’t symmetrical; some areas on one side are larger or smaller, while other parts protrude more.
The pattern of these anatomical differences, or asymmetries, was thought to be uniquely human, originating when our brain hemispheres became specialised for certain tasks, such as processing language with the left side.
Now, it seems the pattern came first – before humans evolved. Brain pattern comparisons between humans, chimpanzees, gorillas and orangutans reveal that our brains’ left-right differences aren’t unique, but shared with great apes.
“It suggests it is an ancestral pattern that was established far earlier during evolution, before the split of human and great apes lineages,” says Simon Neubauer at the Max Planck Institute for Evolutionary Anthropology in Germany.
His team analysed skulls from 95 humans, 45 chimpanzees, 43 gorillas and 43 orangutans. Brain shape is imprinted on the inside of the skull during growth, so the team used CT scanning to detect these details in the hollow skulls and then created digital models of each brain.
Anatomical features on the left and right sides of each brain model were then marked with digital dots. When the hemispheres were superimposed, mismatching dots revealed both the pattern and magnitude of brain asymmetry.
They all shared a common pattern but it was less pronounced in chimpanzees than in the other species. This may help explain why we’ve failed to spot the deep evolutionary history of brain asymmetry previously. Earlier studies only compared human brain asymmetry with chimpanzees – which alongside bonobos are our closest living relatives. Doing so suggested our pattern of asymmetry was unique, evolving from increased brain specialisation after human and chimpanzee lineages split over 4 million years ago.
This can’t be so now that we know gorillas and orangutans share the pattern. What’s more, it is no longer evident that our early human ancestors, whose fossils show the asymmetric pattern, evolved specific functions that rely on the left or right side of the brain.
However, although we now know that other great apes do in fact show some of the brain asymmetry that we do, the exact style of the asymmetry in humans is still unique. “It’s not the pattern itself but in the variation of this pattern,” Neubauer explains. Digital models of human brains had much more variation around the shared pattern than apes, especially in the cerebellum of the brain. This could reflect increased brain modularisation, possibly for specific functions including symbolic communication, perception, emotion and decision-making.
“I applaud that they have cast the comparative net more widely than usual human-chimpanzee comparisons,” says evolutionary anthropologist Robert Barton at Durham University in the UK. “Their emphasis on differences in the cerebellum between humans and other species reinforces growing evidence that the cerebellum was much more important in human brain evolution than has generally been recognised.”
Journal reference: Science Advances , DOI: 10.1126/sciadv.aax9935
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