The secret to our big brains might be in our gut

In a controlled lab experiment, researchers implanted microbes from two large-brain primate species (human and squirrel monkey), and one small-brain primate species (macaque), into mice.

Their findings showed the mice with microbes from large-brain primate species produced and used more energy, while those with microbes from the small-brain species stored more energy as fat.

The data is the first to show gut microbes from different animal species shape variations in biology between animal species and supports the hypothesis that gut microbes might influence evolution by changing how an animal's body works.

The study offers a new perspective on human evolution, particularly the evolution of our large brains.

The findings will be published in the journal Microbial Genomics on Dec. 2.

Prior studies have compared the influence of genes and the environment on primates with bigger and smaller brains. However, there are very few studies comparing how different primates use energy. Even less information is available on how metabolism develops in different primate species.

"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," said the study's first author Katherine Amato, associate professor of anthropology at Northwestern.

"Variation in the gut microbiota is an unexplored mechanism in which primate metabolism could facilitate different brain-energetic requirements," Amato said.

After introducing the gut microbes into microbe-free mice, the researchers measured changes in mouse physiology over time, including weight gain, fat percentage, fasting glucose, liver function and other traits. They also measured differences in the types of microbes and the compounds they were producing in each group of mice.

The researchers expected to find microbes from different primates would lead to differences in the biology of the mice inoculated with them. They also expected mice with human microbes to have the greatest difference in biology from mice with microbes from the other two species.

"While we did see that human-inoculated mice had some differences, the strongest pattern was the difference between large-brained primates (humans and squirrel monkeys) and smaller-brained primates (macaques)," Amato said.

The mice given microbes from the humans and squirrel monkeys had similar biology, even though these two larger-brained primate species are not close evolutionary relatives of one another. This suggests something other than shared ancestry -- likely their shared trait of large brains is driving the biological similarities seen in the mice inoculated with their microbes.

"These findings suggest that when humans and squirrel monkeys both separately evolved larger brains, their microbial communities changed in similar ways to help provide the necessary energy," Amato said.

In future studies, the researchers hope to run the experiment with microbes from additional primate species varying in brain size. They would also like to collect more information on the types of compounds the microbes are producing and gather additional data on the biological traits of the hosts such as immune function and behavior.