A new study has finally provided a reason for why our brains evolved to be so large: the microbes in our gut. Specifically, the gut may have helped the body to produce enough energy over time to encourage the growth of our most important organ.
The research found that organisms with larger brains compared to body size, like humans and the squirrel monkey (our close evolutionary relative), can – thanks to their digestive microbes – consume more food and generate more energy than those with smaller brains.
It’s the first study showing how microbiota in your gut can create biological differences between animal species.
“I think we all have an innate understanding that the gut and the brain are linked. We talk about ‘feeling it in the gut’, and we know that stress and anxiety often lead to gut symptoms," first author of the study, Northwestern University Associate Professor Katherine Amato, told BBC Science Focus.
"However, this study goes even further to show how what happens in the gut may actually be the foundation that allowed our brains to develop over evolutionary time."
There are more than 100 trillion microbes in your gut, which is at least double the number of cells in your body. In total, your gut microbiome weighs around 2kg (4.4 pounds), about the same as a large pineapple.
Scientists were particularly interested in how these tiny gut organisms impacted brain size relative to body size. Humans may not have the biggest brains – that award goes to the sperm whale whose neuro-processing organ is five times bigger. However, humans have a much larger brain-to-body ratio – sperm whales are around 450 times heavier than an average US male over 20 years old.
During the experiments, detailed in journal Microbial Genomics, scientists implanted gut microbes into mice from one of three animals: two large-brained primates – humans and squirrel monkeys – and a smaller-brained primate, the macaque, an African monkey.
Researchers noticed mice with gut microbes from the larger-brained species produced more energy to power brain activity. They also ate more and created higher levels of glucose (which the body uses to fuel energy production). However, the mice with gut microbiota from the smaller-brained primate stored more energy as fat.
“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 explained.
Scientists previously believed mice with human microbes would deliver unique results. Instead, they discovered we are more similar to the other large-brained species than they thought.
“While we did see that human-inoculated mice had some differences [from mice with microbes from the other primates], the strongest pattern was the difference between large-brained primates (humans and squirrel monkeys) and smaller-brained primates (macaques),” Amato added.
Researchers are now looking to study other primates to understand how gut microbes influence brain evolution.
About our expert:
Dr Katherine Amato is an Associate Professor in biological anthropology at Northwestern University in Illinois, US. Her research focuses on gut microbiota, human evolution and primate ecology.
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