The brains of a large group of monkeys began to change around 15 million years ago as they evolved to cope with different environments and spend more time on the ground, according to a new study from The Australian National University (ANU).   

The study compared fossil skulls dating back 36 million years to the brains of present-day monkeys.  It focused on Old World monkeys – a group which includes species like baboons, macaques and mandrills.  

The study showed the temporal lobe – the part of the brain responsible for things like memory and communication – was smaller in the early monkeys.   

According to lead author and ANU PhD candidate Alannah Pearson, this started to change around 15 million years ago, when the temporal lobe began to scale up to match an increase in brain size in most species.   

“The temporal lobe helps monkeys remember different calls from their own species, or detect sounds from dangerous predators,” Ms Pearson said.   

36 million years ago, parts of Africa were very wet and tropical, but as it became drier and tropical areas disappeared, a lot of Old World monkeys adjusted to spend more time on the ground in woodland areas. 

This change in environment meant the monkeys also had to rely heavily on the temporal lobe for things like recognising where their new territory ended.  

“Group sizes also got bigger, so they needed to communicate with more individuals and remember more faces,” Ms Pearson said.   

“A larger temporal lobe is useful for spotting individuals that aren’t part of your troop.”  

Some species, like baboons, adapted to only live mostly on the ground, while others, like the macaques, started to spend time both in the trees and on the ground.  

Ms Pearson used CT scans to create three-dimensional versions of the skulls of the various species.  They all showed similar patterns.  

“The brain doesn’t survive in fossil records, so we used a cast of the inside of the skull to end up with a 3D representation – what’s known as an endocast,” Ms Pearson said.  

The study has been published in The Journal of Mammalian Evolution. 

Top image:  Paul/

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