The key to human evolution may have been at the back of our minds all along – literally. Some of the biggest biochemical differences between human brains and those of other primates are found in the cerebellum, a region at the rear of the brain that has often been overlooked in evolutionary studies.
The finding adds to growing evidence that changes to the cerebellum have been crucial for the origin of the human mind.
All backboned animals have a cerebellum, which is involved in controlling movement.
“It’s not really associated with much that’s uniquely human,” says Elaine Guevara at Duke University in North Carolina. Instead, neuroscientists seeking to explain the evolution of our brains have tended to focus on the cortex, the thick outer layer of the forebrain – especially the prefrontal cortex, which underpins our ability to consciously decide what to do.
In recent years, some neuroscientists have argued that the cerebellum has changed more than thought during human evolution, and that these changes may have been crucial.
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Guevara and her colleagues studied the cerebellum and prefrontal cortex at the molecular level. They took brain samples from humans, chimpanzees and monkeys called rhesus macaques, and extracted DNA from both brain regions.
The team looked to see which parts of the DNA had small molecules called methyl groups attached. Methylation is an example of a so-called epigenetic influence on our genes. Patterns of methylation reflect which genes have been active and inactive during an animal’s life, and they vary between body parts and between species.
Guevara’s team found that the pattern of methylation in human DNA was different to that in chimps and macaques. Crucially, the difference between species was greater in the cerebellum than in the prefrontal cortex, suggesting there had been more changes there during our evolution.
It isn’t clear what the methylation changes did, says Guevara. But there are intriguing clues. Some of the genes where the cerebellar methylation patterns were different are known to be involved in changing the strengths of the connections between neurons, a process thought to be important for learning.
Some of these are also associated with neurodevelopmental differences such as autism and neuropsychiatric conditions such as schizophrenia, both of which may be unique to humans or at least more common in humans, says Guevara.
Journal reference: PLoS Genetics, DOI: 10.1371/journal.pgen.1009506