Our brain is asymmetrical: both the halves of the brain differ slightly from each other both anatomically and functionally. Until now, this asymmetry has been the unique selling point of humans, but now researchers have refuted it. Using skull analyzes on gorillas, chimpanzees and orangutans, they found that these great apes have the same extent and pattern of asymmetry as we do. There seems to be a division of labor between the hemispheres of our closest relatives among the primates. In contrast to the great apes, the pattern of asymmetry in humans is more marked by individual differences.
The left and right side of our brain look alike at first glance. However, if you look closer, there are some differences. The two hemispheres differ in the subtleties of the anatomy, in the distribution of the nerve cells and also in their connections. The asymmetries of the outer shape of the brain are even visible on the inside of our skull bones. Among other things, the left occipital lobe protrudes further than the right one and lies in a correspondingly deeper depression in the bone. But our brain halves also differ in functionality. Some of our tasks are mostly done on the left, others on the right. For example, in most people speech processing takes place mainly in the left brain.
View of the skulls of gorillas, chimpanzees and co
According to popular belief, functional asymmetry is an important prerequisite for the great performance of the human brain – it is only this division of the brain that gives us our cognitive abilities. That is why this type of lateralization was considered unique human. Other mammals show little or no anatomical asymmetry. But what about our closest relatives, the great apes? Because comparative studies among primates have so far been rare, it is not known which aspects of brain asymmetry are really typically human. So far, scientists have assumed that many aspects of this lateralization only developed after the separation of the human lineage from the lineage of our closest living relatives.
Simon Neubauer from the Max Planck Institute for Evolutionary Anthropology in Leipzig and his colleagues have now examined for the first time whether this assumption is correct. “Apes brains are rarely available for studies, but we have developed methods to extract brain asymmetry data from skulls that are available in large numbers,” explains Neubauer. “This is what made our study possible in the first place.” For their study, the researchers examined and measured the inside of the skulls of 228 people, chimpanzees, gorillas and orangutans. They evaluated more than 900 different measuring points and determined whether there were any deviations between the right and left half of the skull.
Asymmetry also in the great apes
The evaluations showed: Contrary to previous assumptions, the apes’ brains are also asymmetrical. They show the same pattern of anatomical right-left differences as previously described by humans. For example, the left occipital lobe and the right anterior major lobe also protrude further in the great apes than the respective counterparts of the other half of the brain, as the researchers report. The extent of this asymmetry was also approximately the same in humans and in most great apes. Only chimpanzees showed slightly smaller differences on average than humans, gorillas and orangutans. “The average pattern of asymmetry is therefore shared by humans and great apes – even though it was previously considered to be unique human,” stated Neubauer and his colleagues. In their view, the new results suggest that the lateralization of the brain is not a “reinvention” of humans, but that the common ancestors of humans and apes already had such an asymmetry of the brain. “This pattern of asymmetry developed even before the human lineage came into being,” explains Neubauer’s colleague Philipp Gunz.
Interestingly, however, there was still a striking deviation: “What surprised us even more was that people were least consistent in this asymmetry,” reports co-author Philipp Mitteröcker from the University of Vienna. In humans, there is therefore a larger individual range in the pattern of anatomical right-left differences. In the great apes, the asymmetries of the occipital lobe and the cerebellum were mostly closely linked. In humans, on the other hand, these varied independently of each other, as the researchers report. They interpret this as an indication that the human brain works more modularly than that of the great apes: individual brain areas are more independent in their function and development than in gorillas, chimpanzees and the like.
Source: Simon Neubauer (Max Planck Institute for Evolutionary Anthropology, Leipzig) et al., Science Advances, doi: 10.1126 / sciadv.aax9935