Compared to other large primates, humans have unusually big brains. Now, scientists have suggested that the large size of our brains may have been influenced by the gut microbiome. The microbes in the gut are known to affect the human body and brain, and produce bioactive chemicals including neurotransmitters. This study has found that the trillions of microbes in the human gut, which generate lots of metabolic energy that is used to make glucose, may have had a significant influence on our brain evolution.
The brain requires lots of energy, so mammals with bigger brains need to produce enough power to grow and maintain those brains, and keep them functioning properly. This research, which was reported in the journal Microbial Genomics, has indicated that gut microbes helped our ancestors meet those energy needs.
In this work, the researchers used a mouse model that did not have any gut microbes (so-called 'germ-free' mice), and transferred gut microbes from primate species into those mice. There were two different mouse models of these gut microbes: one mouse model carried gut microbes from two primate species with larger brains: humans and squirrel monkeys. The other mice carried gut microbes from macaques, which are primates with smaller brains.
The researchers analyzed metrics such as fat percentage and fasting glucose in the mice, as well as the secondary metabolites generated by the gut microbes over time.
The mice with large-brain primate gut microbes were shown to consume more food, generate and use more energy, but put on less body fat compared to mice with small-brain primate gut microbes, which converted more energy to fat for storage.
This research has indicated that variations in animal body shapes could be influenced by gut microbes, and shown that gut microbes might be affecting evolution by changing an animal's physiology.
While many studies have analyzed the genetic and environmental factors that may have affected the evolution of primate brains, very few have investigated how energy and metabolism may impact this process.
“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,” noted first study author Katherine Amato, an 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.”
Now the scientists want to learn more about microbes in other primates, and understand more about the kinds of compounds that are being created in the gut that may affect human biology or behavior.
Sources: Northwestern University, Microbial Genomics