What makes us uniquely human? In recent decades, evidence for many supposedly uniquely human traits – such as communicative behavior, tool use, and self-recognition – has been observed in non-human animals. Still, many believe there must be something unique about the human brain.

One proposal is that the human brain is unique in its "lateralization," the asymmetric specialization of functions on one side of the brain or the other. Previous comparisons with chimpanzees have suggested that their brains do not exhibit the same degree of lateralization as the human brain.

But evidence for this view was limited, primate brains being rather hard to come by. To test the hypothesis, researchers from the Max Planck Institute for Evolutionary Anthropology and the University of Vienna have developed a novel method for modeling the brains of non-human primates such as chimpanzees, orangutans, and gorillas. 

The researchers turned to skulls. They used endocasts – models of the brain based on the imprints they leave in our skulls – to compare the relative asymmetries in the brains of different primate species. The research revealed that there are similar patterns of asymmetric lateralization in other primate species as well, though less in chimpanzees than in gorillas or orangutans, which are more distantly related to us. 

Interestingly, however, asymmetry varied more among human individuals than among individuals of the other primate species. This is presumed to be the result of the human brain’s great plasticity (the brain’s ability to change) and greater variation in human lifespan development. The human brain, it seems, inherited a lateralized structure common to other primates, which has since further evolved along with specifically human cognition and behavior.

These findings speak to our shared ancestry with other primates and animals. At the same time, they refine our understanding of human evolution. But further questions remain about the cognitive or behavioral functions of lateralization of non-human primates’ brains, and even about the brains of other animals.