Brain tissue is among the most energetically costly in the body. As a result, larger-brained mammals require more energy to support brain growth and maintenance. But exactly which biological changes allowed human ancestors to meet the very high needs for energy as they evolved larger brains has remained unclear. A new Northwestern University study points to the role of gut microbes, tiny living organisms in our digestive system that help break down food and produce energy.
What they found
The researchers found that 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.
What's new
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, which was published in Microbial Genomics, offers a new perspective on human evolution, particularly the evolution of our large brains.
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 Katherine Amato, first author of the study and 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.
How they did it
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.
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.
Surprising similarities
The researchers expected to find that 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 the two 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.
What's next
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.
