Did Microbes Help Build Your Brain? The Gut Upgrade Behind Human Intelligence

Key takeaways

• When germ‑free mice were colonized with gut microbes from different primates, their brain activity shifted to resemble the species that donated the microbes.

• Microbes from large‑brained primates (humans and squirrel monkeys) boosted genes for brain energy production and synaptic plasticity, while microbes from smaller‑brained macaques drove very different patterns.

• The work suggests the gut microbiome may have quietly supported the evolution of large, energy‑hungry brains—and may still shape neurodevelopment and mental health today.

How primate microbes rewired mouse brains

Northwestern researchers started with a simple, wild question: if you give the same animal gut microbes from different primate species, will their brains start to function differently? They used germ‑free mice—animals raised without any microbes of their own—and transplanted gut microbiota from two large‑brained primates (humans and squirrel monkeys) and one smaller‑brained primate (macaques).

After eight weeks of living with these new microbial communities, the mice went through brain analyses. The result: clear, species‑specific shifts in brain gene activity that tracked with the donor’s brain size and wiring, not the mouse’s own genome.

Mice that received microbes from large‑brained primates showed higher expression of genes involved in energy metabolism and synaptic plasticity—the core processes that let neurons fire efficiently and rewire with learning. Those given microbes from smaller‑brained macaques showed a different pattern entirely.

Microbes as a hidden support system for big brains

Big brains are expensive tissue: they demand enormous amounts of fuel to build and run.
Earlier work from the same lab had already shown that gut microbes from larger‑brained primates can squeeze more metabolic energy out of the diet when transferred into mice.

This new study goes a step further by showing that the microbiome’s influence is not just about calories; it reaches into the brain itself. For mice carrying human or squirrel monkey microbes, gene programs supporting mitochondrial energy production and plastic synapses were more active, creating a brain environment that is better set up for learning and adaptation.

When the team compared these mouse brain gene signatures with actual human and macaque brain data, they found striking overlaps. In essence, the mice’s brains began to look molecularly more like the primate donor brains, even though their DNA never changed.

Links to neurodevelopmental differences

The researchers also noticed something more unsettling. Mice colonized with microbes from smaller‑brained primates showed brain gene expression patterns that overlapped with signatures seen in conditions like ADHD, schizophrenia, bipolar disorder, and autism.

Previous human studies have repeatedly linked gut microbiome differences with these conditions, but most of that evidence has been correlational. Here, the transplant experiments suggest a more causal role: changing the microbial community alone was enough to push developing brains toward or away from gene programs associated with altered neurodevelopment.

The authors suggest that if developing human brains are exposed to the “wrong” mix of microbes at critical windows, their wiring and function may shift in ways that show up later as cognitive or behavioral differences.

What this means for brain‑health and longevity thinking

This study adds weight to the idea that the microbiome is not just about digestion, immunity, or metabolic health—it is part of the architecture that builds and maintains the brain. Gut microbes appear to help set the brain’s energy budget and plasticity potential, and they may also influence how vulnerable neurodevelopment is to going off track.

In practical terms, we are not at the point of prescribing “large‑brain” microbiome cocktails. But the findings support protecting the microbiome as a central pillar of long‑term brain health: supporting microbial diversity with fiber‑rich, minimally processed foods; avoiding unnecessary antibiotics; and paying particular attention to early‑life exposures when brains are most programmable.

Taken together with other gut–brain axis work, this raises a provocative possibility: part of what we think of as “human intelligence” and cognitive resilience may have been co‑authored by the microbes we carry—and those microbial partners may still be one of the most leverageable levers for protecting brain function across the lifespan.

References:

1. Alex R. DeCasien, Jacob E. Aronoff, Elizabeth K. Mallott, Sahana Kuthyar, Sriram Chitta, Brian T. Layden, Maria L. Savo Sardaro, Stanton Gray, Lawrence E. Williams, Emma R. Liechty, Hyo M. Lee, Won Lee, James P. Curley, Christopher W. Kuzawa, Katherine R. Amato. Primate gut microbiota induce evolutionarily salient changes in mouse neurodevelopment. Proceedings of the National Academy of Sciences, 2026; 123 (2) DOI: 10.1073/pnas.2426232122