Scientists Were Wrong About Human Pattern of Brain Asymmetry Being Unique

The shared brain asymmetry pattern is visualized on a human endocast (cast of the internal bony braincase) in lateral view (left) and from the lower side (right). This pattern includes a more backwards projecting left hemisphere and a more forward projecting right hemisphere with localized larger surface areas (orange) in one hemisphere as compared to corresponding smaller regions (blue) in the other hemisphere. It also includes differential projections of the cerebellar lobes and the temporal poles.
Credit: Simon Neubauer, CC BY-NC-ND 4.0

Brain imprints on cranial bones from great apes and humans refute the long-held notion that the human pattern of brain asymmetry is unique.

The left and right side of our brain are specialized for some cognitive abilities. For example, in humans, language is processed predominantly in the left hemisphere, and the right hand is controlled by the motor cortex in the left hemisphere. The functional lateralization is reflected by morphological asymmetry of the brain. Left and right hemispheres differ subtly in brain anatomy, the distribution of nerve cells, their connectivity and neurochemistry. Asymmetries of outer brain shape are even visible on endocasts. Most humans have a combination of a more projecting left occipital lobe (located in the back of the brain) with a more projecting right frontal lobe. Brain asymmetry is commonly interpreted as crucial for human brain function and cognition because it reflects functional lateralization. However, comparative studies among primates are rare and it is not known which aspects of brain asymmetry are really uniquely human. Based on previously available data, scientists assumed that many aspects of brain asymmetry evolved only recently, after the split between the human lineage from the lineage of our closest living relatives, the chimpanzees.

In a new paper, researchers from the Max Planck Institute for Evolutionary Anthropology and the University of Vienna measured the magnitude and pattern of shape asymmetry of endocasts from humans and apes. “Great ape brains are rarely available for study, but we have developed methods to extract brain asymmetry data from skulls, which are easier to access. This made our study possible in the first place,” says lead author Simon Neubauer.

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