Young orang-utan with its mother

Primate brains: how do humans compare?

Natalie Hunter investigates what our brains have in common with those of our closest living relatives, non-human primates

The human brain is one of the most complex objects in the known universe. It contains billions of neurons and connections, and our understanding of how it produces complex behaviours is incredibly limited. But how did the human brain evolve, and what is it that allows us to do things other species are not able to? To answer these questions, we need to look at the brains of our closest living relatives, non-human primates.

Humans as apes

Humans are great apes, along with orang-utans, gorillas, chimpanzees and bonobos. The non-human great apes – and especially our closest living relatives, chimps and bonobos – have extraordinary mental capacities. Studies have shown that chimps may understand that others have minds and knowledge different from their own, known as ‘theory of mind’. For a long time many believed that only humans were capable of this. Research now shows that many species may also have theory of mind, including crows and elephants, although consensus on this is lacking. Chimps also display incredibly good working memory, outperforming humans on memory tests – which you can see in this video.

However, there are some human skills that appear to be unique to us. One obvious example is our language abilities. Another is our ability to create incredibly complex technologies by building on knowledge passed down from previous thinkers, a concept known as ‘cumulative culture’. We are also able to maintain deeper social bonds with a larger number of individuals than our closest living relatives. What is it about human brains that allows for these skills?

Unique features of the human brain

First of all, the human brain is very large. Primates in general have larger brains than other mammals; for example, a capuchin monkey’s brain will be around three times larger than that of a similarly sized cat. However, human brains are on another level: ours are six or seven times larger than would be expected of a mammal with our body size.

However, the species with the largest brain-to-body-size ratio is not Homo sapiens, but the tree shrew, suggesting that size is not everything. Perhaps more important is which parts of the brain are large. Humans have a very large neocortex (the layer on top of the brain that is associated with higher cognitive functions), though this is a trait of all apes and monkeys, and compared to them, the human neocortex is not particularly large.

So what makes the human brain special? Strangely, it may come down to the problem of childbirth and brain size, known as the ‘obstetrics dilemma’. Humans walk on two legs, meaning our pelvises are narrower and straighter than most quadrupedal (four-legged) mammals. This means that there is less room for babies to exit during childbirth, making it incredibly dangerous without help and modern medicine, for both mothers and children.

The human brain at birth is already as large as it could possibly get at that stage – any larger and childbirth would be impossible. Evolution has produced a number of mechanisms to make up for this, such as human babies’ skulls not being fully fused to allow the skull to be safely deformed during birth. But what this also means is that after birth the human brain is allowed to continue growing, doubling in size during the first few years of life and continuing to grow throughout childhood. In non-human apes, the gaps between the skull bones are far smaller and fuse much sooner than in humans, allowing for less growth.

Human brains also display more folding of the surface, known as ‘convolutions’, than other apes. This increases the surface area of the brain without affecting size or volume, meaning that more connections can be made in the human brain per unit of volume.

All of these differences may account for some of the variation in human and non-human ape skills and behaviours, however this field of research is ongoing; some believe we have a specific ‘human cognition module’ in our brain that sets us apart and which is yet to be uncovered.

Genetic variation

Finally, we share around 98.8 per cent of our DNA with chimpanzees – could such a small proportion of our genetic code account for all of the differences between us and them? We don’t really know, but researchers have found that sometimes single genes can have very big impacts on the development of the brain.

For example, one study found that the insertion of a single human gene into mice embryos caused them to produce a significantly larger number of neurons, leading to the characteristic brain convolutions normally only seen in large-brained mammals (especially humans). And the unique human FOXP2 gene appears to play a significant role in human language. Examples such as these may go some way to explaining how such little genetic difference can produce such vast behavioural and physical discrepancies.

The question of how these differences evolved is a hot topic in anthropology and evolutionary neuroscience, and there is very little consensus as to how and why human brains have developed as they have. We’re increasingly finding that the other great apes are capable of a wide range of complex tasks. For example, we used to think only humans used tools, but we now know chimps, bonobos, orang-utans, capuchins, macaques and other primates do too. The more we study other great apes, the smaller the gap between them and us seems to be.

Lead image:

A young orang-utan huddling with its mother. Orang-utans – one of the great apes – will stay with their mothers until they are a few years old, forming a strong bond.

Eric Kilby/Flickr CC BY

References

Further reading

About this resource

This resource was first published in ‘Inside the Brain’ in November 2017.

Topics:
Genetics and genomics, Neuroscience, Psychology
Issue:
Inside the Brain
Education levels:
16–19, Continuing professional development