Brain drove primates' evolution

The evolution of anthropoid primates, including monkeys, apes and humans, over the past 40 million years was largely driven by reorganisation of the brain, and not its size, new research has claimed.

Scientists from the University College London found that around three quarters of differences between the brains of species of monkeys and apes are due to internal reorganisation that is independent of size, debunking the theory that variation in size is the primary factor characterising anthropoid primate brain evolution.

“The brain is central to how animals adapt and modify their behaviour in a changing environment,” said lead author of the study Dr Jeroen Smaers. “What we’ve found is that in relation to the brain, species differences are mainly explained by how the brain is organised and wired internally, not how large the brain is. This suggests that brain reorganisation, not size, may have been the principal force driving brain evolution,” Smaers said.

To trace the evolutionary history of the anthropoid brain, the team collected the overall size of the brain and its internal structures for 17 anthropoid primate species.
They then mapped and compared the evolutionary changes in these structures to get an insight into how the brains of each of the species specialised while adapting to their respective environments.

“Changes in the overall size of the brain have often been proposed as the main solution to producing a more complex brain that supports more complex behaviour,” said Dr Christophe Soligo from UCL Department of Anthropology.

“A bigger brain is, however, energetically very expensive and may not always be an option. Sometimes animals do not have the choice to increase their energetic input but are still faced with a pressure to adapt. This is when reorganisation may come into play,” Soligo said.

By analysing the overall and relative size of 20 brain structures, the team also found that the prefrontal cortex - a brain area that synthesises information processed in other parts of the brain to produce complex judgements and behaviours - plays the biggest role in explaining the evolutionary changes in anthropoid primate brain organisation. “We’ve known for a while that brain reorganisation is important. But we had no idea it would explain as much variation between species as it does,” Soligo said.

“We’ve also been further able to characterise evolutionary specialisations allowing us to pinpoint what makes certain species special - such as motor learning in great apes and humans - and how far back in time specific evolutionary lineages started to evolve differently from other lineages,” added Soligo.

The study was published in journal Proceedings of the Royal Society B.