Turning Brain Evolution on its Head

CT/MRI of a human (left), chimpanzee (center), and gorilla (right)

J.L. Alatorre Warren, UZH

The modern human brain is three times as large as those of our closest living great ape relatives, and the human skull is distinctly large and globe-shaped, compared to other primates. A recent imaging study of the heads of great apes has concluded that human brain size and structural features, such as the part of the cortex where complex language developed, evolved relatively independently from the human braincase.

Paleoanthropologist José Luis Alatorre Warren and colleagues at the University of Zurich in Switzerland and Georgia State University in Atlanta, Georgia, used a combination of previously gathered computed tomography (CT) and magnetic resonance imaging (MRI) data to investigate the heads of forty-one humans, twenty-four chimpanzees (Pan troglodytes), and two gorillas (Gorilla gorilla). This allowed the researchers to detail the topography of the species’ brain surface, as well as the inside and outside of the braincase. They primarily focused on mapping the relative locations of certain folds and furrows in the brain and of specific bony seams in the braincase. Charting the variation in these features both within and among species helped the researchers reconstruct the primates’ evolutionary histories.

The researchers concluded that the brain followed its own evolutionary trajectory, independent of that of its braincase. For example, they found that the shifting boundaries of structures in the human brain’s frontal lobe, likely tied to the rearrangements leading to complex language, did not reflect discernable changes in the adjacent bony regions. Instead, major changes in the braincase, such as the opening for the spinal cord pushing forward, are better explained by humans adapting to walking upright on two legs. Furthermore, our especially rounded human skull is not mirrored in the shape of the corresponding portion of the brain.

The results are a cautionary tale about making inferences about our ancestors’ brains based on their fossil skulls. “Evolutionary changes in the braincase do not directly inform changes in the brain,” says senior author Christoph P.E. Zollikofer. (Proceedings of the National Academy of Sciences)