Evolution of Brain Networks

The brain is an intricate organ responsible for dealing with our thoughts, feelings, and behaviors. The human brain tripled in size during recent evolution, while the rest of our body stayed more or less the same size. This remarkable change has inspired brain researchers to investigate why such brain expansion took place, and how it affects the way our brain works. In this thesis, we investigated the organization of the brain’s wiring by looking at the connections between each part of the brain and how they form a complex network — the connectome. We compared the human connectome to that of other primate species, including chimpanzees, gorillas, rhesus monkeys, and capuchin monkeys. We found that all these connectomes have important aspects in common: They are wired in an efficient way and contain ‘shortcuts’ between important brain regions for quick communication of information. At the same time, they show subtle differences in their wiring organization that are closely linked with their brain size. For example, larger brains have relatively less long-range connections, likely because long connections take up more and more space in larger brains. Larger brains also show more specialization of connectivity, especially between the left and right sides of the brain. The findings of this thesis help our understanding of human brain evolution by showing the relationships between brain size and connectome organization in a wide range of monkeys and apes. Our work suggests that primate brains are wired according to a shared connectome blueprint, with key aspects of connectome organization found in all the studied species. Brain size and adaptations to specific environments form the variations on this conserved blueprint. In the case of humans, our big brains may have paved the way for extensive specialization of brain connectivity, leading to specialized brain functions such as language and highly developed cognition.