Evolution through brain asymmetry

We humans share roughly 98.5% of our genetic material with our closest living relatives, chimpanzees. This extraordinarily high level of genetic homology highlights the requirement for small but striking genetic differences between these two species, as we look and behave differently. A team of Edinburgh neuroscientists, in collaboration with colleagues from the University of Oxford as well as researchers from the United States and China, have recently contributed to elucidating the potential origin of some of the behavioural differences between the two species. The multi-center study, published in the scientific journal NeuroImage, compared magnetic resonance images of the brains of 273 humans and 70 chimpanzees. This enabled them to investigate the structure and surface of the brain whilst being minimally invasive.

Using this technique, they found some brain asymmetries that were uniquely human and could not be detected in chimpanzees. Human brain asymmetries had been previously identified, such as the “torque”, an overall anti-clockwise twist of the brain around itself with one lobe reaching further forward than the other, or side-specific asymmetries in the Sylvian fissure, a prominent wrinkle separating parts of the brain. While famous brain researcher Paul Broca hypothesized over 100 years ago that ‘Man is, out of all species, the one whose brain is the most asymmetrical’, there have been no previous large-scale interspecies comparisons between the anatomy of the human and chimpanzee brain.

While by no means all of the brain function can be ascribed to large-scale morphology (which would take us back to the age of phrenology), the brain consists of discrete areas responsible for different functions such as vision, speech – both formation and comprehension -, and auditory processing. Therefore, if one of these areas is increased in size, it is possible that the underlying brain region has gained significance in cognition throughout evolution. The unique human asymmetries described in this study could therefore potentially correlate to the emergence of distinctly human features such as language, and might fuel further research of the evolution of human behaviour.


This article was written by Chiara Herzog and edited by James Hitchen.

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