Great ape brains have a feature that we thought was unique to humans

Our brains may have more in common with our ape cousins than previously thought, which may force us to rethink ideas about the evolution of brain specialization in our early human ancestors.

An asymmetrical model
The left and right sides of our brain are not symmetrical; some areas on one side are larger or smaller, while other areas protrude further.

It was thought that the pattern of these anatomical differences, or asymmetries, was only human - when our brain hemispheres specialized for certain tasks, such as processing language with the left side.

Now it seems that this pattern came first - before humans evolved. Comparisons of brain patterns between humans, chimpanzees, gorillas and orangutans reveal that the left-right differences in our brains are not unique, but shared with the great apes.

An Ancestral Model Earlier in Evolution
"This suggests that it is an ancestral model that was established much earlier in evolution, before the division of human and ape lineages," says Simon Neubauer at the Max Planck Institute for Evolutionary Anthropology in Germany.

His team analysed the skulls of 95 humans, 45 chimpanzees, 43 gorillas and 43 orangutans. The shape of the brain is imprinted inside the skull during growth, so the team used CT scans to detect these details in the skulls and then created digital models of each brain.

The anatomical features on the left and right sides of each brain model were then marked with numerical dots. When the hemispheres were superimposed, incompatible dots revealed both the pattern and the extent of brain asymmetry.

They all shared a common pattern, but it was less pronounced in chimpanzees than in other species. This may help explain why we have been unable to identify the deep evolutionary history of brain asymmetry before. Previous studies have only compared human brain asymmetry to chimpanzees - who, along with bonobos, are our closest living relatives.

This suggested that our pattern of asymmetry was unique, evolving from increased brain specialization after the division of human and chimpanzee lineages more than 4 million years ago.

A shared model
This can no longer be the case now that we know that gorillas and orangutans share the same pattern. Furthermore, it is no longer evident that our early human ancestors, whose fossils show this asymmetrical pattern, developed specific functions that depend on the left or right side of the brain.

However, although we now know that other great apes do in fact show some brain asymmetry, the exact style of asymmetry in humans is still unique . "It's not the pattern itself, but the variation in that pattern," says Neubauer. Numerical models of the human brain showed much more variation around the shared pattern than monkeys, especially in the cerebellum.

This could reflect an increased modularization of the brain, perhaps for specific functions such as symbolic communication, perception, emotion and decision-making.

The cerebellum was more important in the evolution of the human brain.
"I applaud the fact that they have cast this comparative net more widely than the usual human-chimpanzee comparisons," says evolutionary anthropologist Robert Barton at the University of Durham in the United Kingdom. "Their focus on differences in the cerebellum between humans and other species reinforces the growing evidence that the cerebellum was much more important in the evolution of the human brain than has been generally recognized. »