The microorganisms in our gastrointestinal tract–the gut microbiome can exert a profound influence on the human body, and scientists are learning more about exactly how certain microbes can impact us. There is a direct link from the gut to the brain, known as the gut-brain axis, which is mediated by the vagus nerve. Gut microbes can generate neurotransmitters that can influence brain activity. And a new study has now suggested that gut microbes could have had a significant effect on how humans evolved larger brains.
Humans have long been known to have the largest brains of any primate, and our brains need a lot of energy to function. There is still we don’t know about how this evolution happened, and how the energy demands of the brain were met during that evolution. But new research has indicated that the gut microbiome can have a big influence on how the brain functions in various species of primates. The findings have been reported in the Proceedings of the National Academy of Sciences (PNAS).
"Our study shows that microbes are acting on traits that are relevant to our understanding of evolution, and particularly the evolution of human brains," said senior study author Katie Amato, an associate professor of biological anthropology at Northwestern University.
Previous work by this showed that microbes from larger-brained primates can generate more metabolic energy when they are transplanted into mice. In this work, they transferred gut microbes from humans, squirrel monkeys (large-brained primates), or macaques (that have smaller brains), into mice that did not carry any gut microbes at all (so-called germ-free mice).
After eight weeks of these changes, the scientists determined that brain function had changed in the mice carrying gut microbes from larger-brained primates, compared to mice that received gut microbes from smaller-brained primates. There was more activity from genes that are related to energy production and neural plasticity in mice with gut microbes from primates with larger brains, while this increased gene activity was not as significant in mice that received gut microbes from primates with smaller brains.
"What was super interesting is we were able to compare data we had from the brains of the host mice with data from actual macaque and human brains, and to our surprise, many of the patterns we saw in brain gene expression of the mice were the same patterns seen in the actual primates themselves," Amato said. "In other words, we were able to make the brains of mice look like the brains of the actual primates the microbes came from."
In mice with microbes from primates with smaller brains, there was also an increase in gene expression that is related to mental issues like ADHD, autism, bipolar disorder, and schizophrenia. The gut microbiome had been related to these issues before, but how gut microbes may be contributing to them is still unclear.
"This study provides more evidence that microbes may causally contribute to these disorders—specifically, the gut microbiome is shaping brain function during development," Amato said. "Based on our findings, we can speculate that if the human brain is exposed to the actions of the wrong microbes, its development will change, and we will see symptoms of these disorders, i.e., 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."
Now, more work has to be done to confirm these findings and to learn more about how gut microbes may be impacting human brains, and whether gut microbes can be manipulated to improve human brain function.
Sources: Northwestern University, Proceedings of the National Academy of Sciences (PNAS)
