What makes the human brain unique? We compared it with monkeys and apes to find out

Scientists have long tried to understand the human brain by comparing it to other primates. Researchers are still trying to understand what makes our brain different to our closest relatives. Our recent study may have brought us one step closer by taking a new approach – comparing the way brains are internally connected.

The Victorian palaeontologist Richard Owen incorrectly argued that the human brain was the only brain to contain a small area termed the Hippocampus minor. He claimed that made it unique among the animal kingdom and, he argued, the human brain was therefore clearly unrelated to other species. We’ve learnt a lot since then about the organisation and function of our brain, but there is still much to learn.

Most studies comparing the human brain to that of other species focus on size. This can be the size of the brain, size of the brain relative to the body, or or the size of parts of the brain to the rest of it. However, measures of size don’t tell us anything about the internal organisation of the brain. For instance, although the enormous brain of an elephant contains three times as many neurons as the human brain, these are predominantly located in the cerebellum, not in the neocortex that is commonly associated with human cognitive abilities.

Until recently, studying the brain’s internal organisation was painstaking work. The advent of medical imaging techniques, however, has opened up new possibilities to look inside the brains of animals quickly, in great detail, and without harming the animal.

We used publicly available MRI data of white matter, the fibres connecting parts of the brain’s cortex. Communication between brain cells runs along these fibres. This costs energy and the mammalian brain is therefore relatively sparsely connected, concentrating communications down a few central pathways.

The connections of each brain region tell us a lot about its functions. The set of connections of any brain region is so specific that brain regions have a unique connectivity fingerprint.

In our study, we compared these connectivity fingerprints across the human, chimpanzee and macaque monkey brain. The chimpanzee is, together with the bonobo, our closest living relative. The macaque monkey is the non-human
primate best known to science. Comparing the human brain to both species meant we could not only assess which parts of our brain are unique to us, but also which parts are likely to be shared heritage with our non-human relatives.

Much of the previous research on human brain uniqueness has focused on the prefrontal cortex, a group of areas at the front of our brain linked to complex thought and decision making. We indeed found that aspects of prefrontal cortex had a connectivity fingerprint in the human that we couldn’t find in the other animals, particularly when we compared the human to the macaque monkey.

But the main differences we found were not in the prefrontal cortex. They were in the temporal lobe, a large part of cortex located approximately behind the ear. In the primate brain, this area is devoted to deep processing of information from our two main senses: vision and hearing. One of the most dramatic findings was in the middle part of the temporal cortex.

The feature driving this distinction was the arcuate fasciculus, a white matter tract connecting the frontal and temporal cortex and traditionally associated with processing language in humans. Most if not all primates have an arcuate fasciculus but it is much larger in human brains.

However, we found that focusing solely on language may be too narrow. The brain
areas that are connected via the arcuate fasciculus are also involved in other cognitive functions, such as integrating sensory information and processing complex social behaviour. Our study was the first to find the arcuate fasciculus is involved in these functions. This insight underscores the complexity of human brain evolution, suggesting that our advanced cognitive abilities arose not from a single change, as scientists thought, but through several, interrelated changes in brain connectivity.

While the middle temporal arcuate fasciculus is a key player in language processing, we also found differences between the species in a region more at the back of the temporal cortex. This temporoparietal junction area is critical in processing information about others, such as understanding others’ beliefs and intentions, a cornerstone of human social interaction.

In humans, this brain area has much more extensive connections to other parts of
the brain processing complex visual information, such as facial expressions and
behavioural cues. This suggests that our brain is wired to handle more intricate
social processing than those of our primate relatives. Our brain is wired up to be
social.

These findings challenge the idea of a single evolutionary event driving the
emergence of human intelligence. Instead, our study suggests brain evolution happened in steps. Our findings suggest changes in frontal cortex organisation occurred in apes, followed by changes in temporal cortex in the lineage leading to humans.

Richard Owen was right about one thing. Our brains are different from those of other species – to an extent. We have a primate brain, but it’s wired up to make us even more social than other primates, allowing us to communicate through spoken language.