Microbes in our guts, the fuel of human intelligence ?

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It is certainly one of the most captivating and complex questions that the ;#8217;humanity has been trying to answer for centuries: understand the origins of our intelligence. How did our species develop a brain of such complexity, capable of creating art, science and technology??

This fundamental question, which has mobilized philosophers and researchers for decades, today finds a small element of response in the depths of our digestive system. A study published on December 2 in Microbial Genomics, has just established for the first time a direct link between the intestinal microbiota and the extraordinary development of our cerebral organ over the course of evolution.

Metabolism: one of the secret ingredients of intelligence

The human brain consumes nearly 20% of our daily energy, a proportion without equivalent in the animal kingdom, with the exception of large mammals such as elephants, whales and dolphins. A real metabolic burden! How could our ancestors have developed and maintained such an energy-intensive organ??

Previous research has focused on genetic and environmental aspects, neglecting a key player: metabolism. As a reminder, metabolism refers to the set of biochemical processes by which living organisms convert nutrients into energy and matter necessary for their growth and maintenance.

That's why the researchers behind this study set out to study metabolic variations (changes in how quickly the body converts food into energy) between different primate species to understand what distinguishes species with large brains from those with larger brains.

The Gut, Our Second Brain

The team led by Katherine Amato at Northwestern University therefore developed an experimental approach to decipher the influence of the gut microbiota (all microorganisms, mainly bacteria) on brain development. To do this, the researchers developed a microbial transplantation protocol to highlight the interactions between intestinal bacteria and the metabolism of their host.

The researchers selected three species of primates with distinct brain characteristics: Humans (Homo sapiens) and Squirrel Monkeys (Saimiri boliviensis), with large brains, and Macaques (Macaca mulatta), with a more modest brain. The choice of these species is not insignificant: it allows us to explore how different evolutionary trajectories have shaped intestinal microbial communities in relation to brain development.

Transplanting their respective microbiotas into axenic mice – animals raised in sterile conditions, devoid of any intestinal flora – is in itself an important methodological innovation. Indeed, it allows us to isolate with great precision the specific influence of the microbiota on the physiology of the host, by eliminating confounding variables that could cloud the interpretation of the results.

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The team then monitored several physiological parameters: weight variations, changes in body composition, glycemic fluctuations and liver parameters. This battery of measurements serves to demonstrate how microbial communities modulate the energy metabolism of their host.

Results: intestinal bacteria produce compounds that profoundly modify the metabolism of the organism. These molecules are involved in the regulation of glucose and the distribution of energy reserves. The microbiota thus appears as a metabolic regulator, directing energy either towards brain development or towards fat reserves. Biochemical analyses were used to map the diversity of molecules produced by these different microbial populations, each primate species having a unique bacterial community.

Amato explains: “These results show that the evolution towards larger brains, in humans as well as in squirrel monkeys, was accompanied by comparable transformations in their microbiotas, aimed at meeting increased energy needs“. . This microbial specificity would therefore be the key to the development, for certain species, of particularly energy-intensive brains.

Mice, monkeys and humans: the same link between brain and microbiota

The results show a marked difference between species in brain size. Mice given bacteria from humans or squirrel monkeys – both species with large brains – showed more efficient use of energy. In contrast, those given bacteria from macaques – which have smaller brains – stored more energy as fat.

Quite uniquely, mice carrying human bacteria show striking similarities with those carrying squirrel monkey bacteria, although these two species are genetically distant. This discovery indicates that the development of a large brain has favored the emergence of intestinal bacteria capable of providing the necessary energy, this, independently of the genetic heritage of the species.

If the basis of intelligence is multifactorial (genetics, education, socio-economic environment, etc.) the synergy between bacteria and the brain therefore appears as an important piece of the puzzle. Although there is still much to discover – as is often the case when looking at this specific topic – our current knowledge suggests that our gut microbiota could thus be considered as a true ” second brain “, influencing our thoughts, emotions and behaviors.

• There is a link between the intestinal microbiota and the development of large brains in humans and some primates.
• Intestinal bacteria produce molecules that optimize the energy needed by the brain, influencing its evolution in the process.
• The microbiota would have evolved in parallel with our brains to meet exacerbated energy needs, independently of genetic heritage.

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