Our gut microbes may hold the key to understanding the evolution of the human brain. Researchers have proposed that these tiny organisms could have provided the essential energy boost that allowed our brains to grow to such extraordinary sizes.
With the largest relative brain size of all primates, humans spend more total energy than expected based on their body size, the study published in the journal Microbial Genomics noted.
“Variation in the gut microbiota represents an unexplored mechanism through which primate metabolism could facilitate different brain energetic requirements,” the researchers wrote in their study.
Gut microbes use fibre and amino acids to produce short-chain fatty acids (SCFA), acetate, butyrate and propionate, all of which serve as energy sources for their human hosts. The energy is produced through metabolic processes such as appetite and satiety, carbohydrate conversion to fatty acids, cholesterol and triglyceride production and glucose-insulin metabolism.
SCFAs also interact with host metabolic processes, increasing glucose production to meet the high energy demands of large brains. The energy could also have an impact on other bodily functions like growth, maintenance and fat deposition.
Another role played by SCFAs is facilitating communication between the gut and brain, making them an important target for understanding brain evolution.
To learn more, the researchers implanted gut microbes from two large-brained primate species (humans and squirrel monkeys) and one small-brained primate (macaque) into mice.
The experiment revealed that mice with gut microbes from large-sized brains produced and used more energy, while those from the small-brain species stored more energy as fat.
“We know the community of microbes living in the large intestine can produce compounds that affect aspects of human biology — for example, causing changes to metabolism that can lead to insulin resistance and weight gain,” first author of the paper Katherine Amato, associate professor of anthropology at Northwestern University, said in a statement.
The researchers also found that mice with microbes from humans and squirrel monkeys exhibited similar biological characteristics, which could be driven by their shared trait of large brains.
The two larger-brained primate species are not close evolutionary relatives of one another. But they share two common features — they exhibit rapid brain growth after birth and have a relatively large brain for their body size. This means the two species require more energy to support the brain across the lifecycle.
Larger-brained primates tend to have higher blood glucose levels and lower body fat. The scientists found that mice given gut microbes from large-brained mammals exhibited reduced growth and body fat, suggesting that gut microbes may influence energy allocation, diverting resources away from body growth and fat storage toward the brain.
However, humans differ from other large-brained primates. While they exhibit increased glucose levels for brain energy, they also have higher body fat. While the former provides energy to the brain, the extra body fat could serve as a backup energy source for the brain, especially during periods of low food availability or increased energy demand.
Although the mice with human gut microbes did not show an increase in body fat typical of humans, the researchers speculate that this could be because of the restricted energy budgets of mice compared to humans.
“Our finding that the gut microbe helps shape host energy allocation patterns across host species suggests that microbes play an important role in scaffolding species differences in life histories, including the evolution of relatively large brains,” the researchers wrote in the study.