Research from the US suggests primate gut microbes influence brain-related gene activity in mice, indicating a potential role in the evolution of large brains. 

The human brain is exceptionally large and energy-intensive, even among primates. Its evolution remains an open question. Scientists now focus on the gut microbiome, the community of microbes in the intestine, as it influences human biological processes like metabolism, brain signalling and development.  

A team at Northwestern University tested how transferring primate microbiomes to mice affects rodent brain activity. They used microbes from humans, squirrel monkeys (smaller-brained primates) and macaques (large-brained primates).  

After colonisation of the microbiome, the team analysed brain gene expression patterns in the germ-free mice and compared them with data from primate brains. Mice receiving microbes from humans or squirrel monkeys showed increased activity in genes linked to brain energy use and learning-related processes, mimicking the donors’ gene expression.  

In contrast, mice colonised with macaque microbes showed different gene expression patterns, including activity associated with several neurodevelopmental and psychiatric conditions.  

Katherine Amato from Northwestern University, lead author of the study published in PNAS, said: “Based on our findings, we can speculate that… if you don’t get exposed to the ‘right’ human microbes in early life, your brain will work differently, and this may lead to symptoms of these conditions.”  

Future research will explore the origins of psychological disorders and the evolutionary impact of microbes on brain physiology.