In a nutshell: Areas of the brain that have evolved to be much bigger in humans than in other primates act as communication hubs.

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One of the biggest differences between humans and other primates is the size of our cerebral cortex. This thick folded layer of cells, which covers much of the surface of the brain, has evolved to contain more cells and span a much bigger area in humans. Not all parts of this brain region have increased at the same rate over time, however; some areas have expanded more quickly than others during evolution.

Maintaining a greater number of brain cells takes more energy away from other important processes in the body, so these high-expanding brain regions must have a useful function – an evolutionary advantage – in humans.

Using a mix of scientific approaches, Brain Function CoE investigator Marcello Rosa and colleagues from Monash University, along with researchers in Norway, sought to discover what that function might be.

The researchers began by comparing the size of the cerebral cortex in four primate species – marmoset, capuchin and macaque monkeys and humans. This analysis identified the ‘high-expanding’ regions of the cerebral cortex – the areas that have expanded more quickly than others during evolution. It also showed more of these regions in humans than in non-human primates.

The researchers then used functional magnetic resonance imaging (fMRI) to measure the brain activity of more than 200 children and adults while they rested with their eyes closed or performed cognitive tasks. From this data, the researchers could tell which regions of the cerebral cortex were more active during the different tasks, and how those regions were connected to other areas of the brain.

To see how brain evolution is linked to brain behaviour, the researchers compared the fMRI data to the maps of the cerebral cortex. They found that the high-expanding regions connected more flexibly to other parts of the brain, with the precise connections depending on the brain activity that a specific cognitive task required. These regions were particularly active during tasks that required different types of information to be integrated.

By looking at the brain activity in people of different ages, the researchers also found that the high-expanding regions are among the last in the brain to mature – they don’t become fully functional until early adulthood. In addition to being late to appear in evolution, it seems they are late to develop in the human brain.

This research offers one explanation for why humans have evolved to devote valuable energy to the maintenance of more brain cells: to create communication hubs that can transmit information flexibly, enabling us to carry out more complicated cognitive tasks.

Next steps:
The researchers are now using higher-resolution techniques, capable of identifying the connections of single cells, to explore the connectivity of the areas of the cortex that have expanded the most. This will allow them to obtain more insight into what functions have driven the evolution of the human brain.

Sneve, M. H., Grydeland, H., Rosa, M. G. P., Paus, T., Chaplin, T., Walhovd, K., & Fjell, A. M. (2018). High-expanding regions in primate cortical brain evolution support supramodal cognitive flexibility. Cerebral Cortex. doi: 10.1093/cercor/bhy268

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