The evolution of large brains in humans and other primates is a subject that has fascinated scientists for centuries. Brains are metabolically expensive organs; they consume a significant amount of energy, even when the body is at rest. The increased energetic demands associated with larger brains present a challenge for animals, particularly primates, that have evolved such brains. For years, researchers have sought to understand how our human ancestors managed to meet the growing energy requirements for brain development and maintenance as brain size expanded. A new study from Northwestern University offers a fresh perspective on this question, suggesting that the microbiome — the community of microorganisms residing in the gut — may have played a crucial role in helping larger-brained primates, including humans, meet their high metabolic needs.
In the study, the researchers explored the potential link between gut microbes and energy usage in primates with different brain sizes. Gut microbes are known to aid in digestion by breaking down complex food molecules and producing essential metabolites that influence the host’s metabolism. The study posited that variations in the microbial communities between different primate species could help explain differences in energy utilization and storage, thereby influencing the evolution of brain size.
To test their hypothesis, the Northwestern team conducted an experiment in which they transplanted gut microbes from three distinct primate species into mice. The primates chosen for this experiment included two species with relatively large brains — humans and squirrel monkeys — and one species with a smaller brain size — the macaque. The researchers chose these species because they represent a range of brain sizes, and they were interested in comparing how microbes from these species would affect the metabolism and energy use of the host organisms.
The results were striking. Mice that were inoculated with gut microbes from the larger-brained species — humans and squirrel monkeys — exhibited higher energy production and usage. In contrast, mice that received microbes from the macaque, the species with a smaller brain, stored more energy as fat. These findings suggest that gut microbes could influence how an animal’s body handles energy, potentially playing a role in the evolution of energy expenditure required for the maintenance of larger brains.
This is the first study to demonstrate that gut microbes can help explain biological differences between species, particularly in how energy is metabolized and utilized. The discovery provides compelling evidence for the hypothesis that gut microbiota may have had a significant influence on evolutionary processes by affecting metabolic functions and energy balance in animals, including primates. According to Katherine Amato, the study’s lead author and associate professor of anthropology at Northwestern, variations in the gut microbiota could help explain how primates, especially humans, adapted to support their larger brains over time.
The findings also suggest that the gut microbiome may have played an unrecognized role in human evolution, particularly when it comes to the development of the large, energetically costly human brain. Previous studies have focused on the roles of genes and the environment in shaping the brain and body, but the idea that gut microbes might influence metabolism and energy requirements is relatively new and unexplored. This study adds another layer to the growing body of evidence that the microbiome is more than just a passive bystander in the body’s processes.
To better understand these changes, the researchers measured various physiological changes in the mice over time. These included weight gain, fat percentage, fasting glucose levels, liver function, and other metabolic traits. The study also examined differences in the types of microbes present in the mice, as well as the compounds these microbes produced, to explore how these changes might have influenced the host’s metabolism.
As expected, the researchers observed notable differences in the physiology of the mice depending on the source of their gut microbes. The most significant differences were seen between mice inoculated with microbes from large-brained primates and those from small-brained species. Mice that received microbes from humans and squirrel monkeys exhibited similar metabolic patterns, despite the fact that these two primate species are not closely related evolutionary. This similarity hints that the shared trait of large brain size, rather than shared ancestry, might be responsible for the biological patterns observed in the inoculated mice. This suggests that the evolution of larger brains in both humans and squirrel monkeys may have driven similar changes in their gut microbiomes to meet the increased energy demands of their brains.
Amato emphasized the importance of these findings: “These results 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.” This insight opens up new avenues for understanding how the evolution of brain size in primates, particularly humans, may have been influenced by the microbiome.
The study’s findings could have far-reaching implications for how we understand the relationship between gut microbes, metabolism, and evolution. While previous research has shown that gut microbiota can influence individual health outcomes, such as obesity and metabolic disorders, this study underscores the potential evolutionary significance of these microbes. It suggests that the diversity of microbial communities could have been a contributing factor to the way in which primates evolved to meet the energetic demands of their large brains.
Looking ahead, Amato and her colleagues hope to expand their research by including additional primate species that vary in brain size. They also plan to delve deeper into the types of compounds that these gut microbes produce and how these compounds influence various aspects of metabolism, immune function, and behavior in the host species. By understanding how different microbial communities impact the biology of primates, researchers could gain new insights into how these organisms, including humans, evolved and adapted to their energetic challenges over time.
The study also raises interesting questions about the broader role of the gut microbiome in shaping human health. If the microbiome played a significant role in the evolutionary process, it stands to reason that disruptions to the microbiome in modern humans could have unforeseen consequences for metabolic health, brain function, and even mental health. Researchers are increasingly recognizing the need to study the microbiome not only as a component of human physiology but also as a key player in human evolution.
The study is published in the journal Microbial Genomics.
Source: Northwestern University