Why do humans have such large and powerful brains compared with other primates?
A new study suggests part of the answer may lie not in our genes alone, but in the trillions of microbes living in our gut.
Researchers from Northwestern University have found that the gut microbiome — the community of bacteria living in the digestive system — can directly influence how the brain develops and functions.
Their findings, published in the journal Proceedings of the National Academy of Sciences, provide some of the strongest evidence yet that microbes may have played a role in the evolution of the human brain.
The human brain is extremely expensive to run. Although it makes up only a small fraction of body weight, it consumes a huge amount of energy.
Scientists have long wondered how mammals with larger brains evolved ways to meet these intense energy demands. The new study suggests that gut microbes may have helped solve this problem.
Previous research from the same team showed that gut microbes from large-brained primates, such as humans, are especially good at producing metabolic energy.
When these microbes were transferred into mice, they boosted energy production in the animals’ gut. This time, the researchers went a step further to see whether those microbes could also change how the brain itself works.
To test this, the team introduced gut microbes from three different primates into mice that had no microbes of their own.
Two of the donor species — humans and squirrel monkeys — have relatively large brains.
The third species, the macaque, has a smaller brain. The mice were then observed over eight weeks as their microbiomes became established.
The results were striking.
Mice that received microbes from large-brained primates showed increased activity in genes linked to energy use and synaptic plasticity, the process that allows brain cells to form and strengthen connections during learning.
In contrast, mice given microbes from the smaller-brained primates showed lower activity in these same brain-related processes.
Even more surprising was how closely the mice’s brain gene activity matched that of the original primates. In many cases, the gene patterns in the mice resembled those found in human or macaque brains, depending on which microbes they received.
In effect, the microbes made mouse brains behave more like the brains of the species they came from.
The researchers also noticed that mice with microbes from smaller-brained primates showed gene patterns linked to conditions such as autism, ADHD, schizophrenia, and bipolar disorder. While this does not mean microbes cause these conditions directly, it adds weight to the idea that the gut microbiome can influence brain development in powerful ways.
Taken together, the findings suggest that gut microbes may have helped support the growth of larger, more complex brains during evolution. They also raise important questions about early brain development in humans today. If the developing brain depends on exposure to certain microbes, disruptions to the microbiome early in life could potentially affect how the brain functions later on.
The study opens a new window into how evolution, microbes, and brain biology may be deeply connected — and how understanding that connection could one day improve mental health research and care.
