Researchers' take on left-brain might not be right, study explains

The long-held notion that the human pattern of brain asymmetry is unique has now been challenged by scientists. The team found the same asymmetry pattern in chimpanzees, gorillas, and orangutans.

The left and right side of our brain is 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 the morphological asymmetry of the brain.

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.

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.

Lead author Simon Neubauer said: "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."

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.

They also investigated the pattern of asymmetry and could demonstrate that not only humans, but also chimpanzees, gorillas, and orangutans showed the asymmetry pattern previously described as typically human.

"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.

The finding of a shared asymmetry pattern but greater variability in humans is intriguing for the interpretation of human brain evolution.