We know the left and right side of our brain are specialized for cognitive abilities like language (left hemisphere) and the right hand. That functional lateralization is reflected by morphological asymmetry too. The left and right hemispheres differ subtly in brain anatomy, distribution of nerve cells, connectivity and even neurochemistry.
It can be seen 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 of our functional lateralization but which aspects of brain asymmetry are uniquely human? It's difficult to know because our closest biological relatives, chimpanzees, are rarely available for brain dissection. For a new study, researchers developed methods to extract brain asymmetry data from skulls, which are easier to access.
The team found that the magnitude of asymmetry was about the same in humans and most great apes. Only chimpanzees were, on average, less asymmetric than humans, gorillas, and orangutans.
Humans, chimpanzees, gorillas, and orangutans (from left to right) have differently shaped endocasts and brains (see top row). But they share an asymmetry pattern, as visualized in the bottom row. 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. Credit: Simon Neubauer, CC BY-NC-ND 4.0
The patterns of asymmetry demonstrate that not only humans, but also chimpanzees, gorillas, and orangutans showed the asymmetry pattern previously described as typically human: the left occipital lobe, the right frontal lobe, as well as the right temporal pole and the right cerebellar lobe projecting more relatively to their contralateral parts. "What surprised us even more," says Philipp Mitteroecker, a co-author of the study, "was that humans were least consistent in this asymmetry with a lot of individual variation around the most common pattern."
The authors interpret this as a sign of increased functional and developmental modularization of the human brain. For example, the differential projections of the occipital lobe and the cerebellum are less correlated in humans than in great apes. This finding is interesting because the cerebellum in humans underwent dramatic evolutionary changes and it seems that thereby its asymmetry was affected as well.