It is certainly one of the most captivating and complex questions that humanity has been trying to answer for centuries: understand the origins of our intelligence. How did our species develop such a complex brain, capable of creating art, science and technology?
This fundamental question, which has mobilized philosophers and researchers for decades, today finds a small element of an answer. in the depths of our digestive system. A study published on December 2 in Microbial Genomicshas 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 almost 20% of our daily energy, a proportion without equivalent in the animal kingdomwith the exception of large mammals such as elephants, whales and dolphins. A real metabolic burden! How were our ancestors able to develop and maintain such an energy-intensive organ?
Previous research has focused on genetic and environmental aspects, neglecting an essential player: metabolism. As a reminder, we mean by metabolism 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 the rate at which the body converts food into energy) between different primate species to understand what distinguishes species with developed brains from others.
The intestine, our second brain
The team led by Katherine Amato at Northwestern University therefore developed an experimental approach intended to decipher the influence of the intestinal microbiota (all microorganisms, mainly bacteria) on brain development. To do this, 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 : Human (A wise man) and the Squirrel Monkey (Bolivian Saimiri), with large brains, and the macaque (Mulatto macaque), with a more modest brain. The choice of these species is not trivial: it allows us to explore how different evolutionary trajectories have shaped intestinal microbial communities in relation to brain development.
The transplantation of their respective microbiota into axenic mice – animals raised in sterile conditions, devoid of any intestinal flora – constitutes in itself an important methodological innovation. Indeed, it makes it possible to isolate with great precision the specific influence of the microbiota on the physiology of the host, by eliminating confounding variables. which could confuse the interpretation of the results.
-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 which profoundly modify the body’s metabolism. 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 adipose reserves. Biochemical analyzes 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 both humans and squirrel monkeys, was accompanied by comparable transformations in their microbiota, aimed at meeting increased energy needs. “. . This microbial specificity would therefore be the key to development, for certain species, particularly energy-intensive brains.
Mice, monkeys and men: the same link between brain and microbiota
The results reveal a marked difference between species depending on the size of their brains. Mice that received the bacteria from humans or squirrel monkeys – both large-brained species – showed more efficient use of energy. On the other hand, those who received the bacteria from macaques – with smaller brains – stored more energy in the form of fat.
Quite a singular fact: mice carrying human bacteria have 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 favored the emergence of intestinal bacteria capable of providing the necessary energy, regardless 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 brain therefore appears to be an important piece of the puzzle. Although there is still much to discover – this is often the case when we focus on this specific topic – our current knowledge suggests that our intestinal microbiota could thus be considered as a real “ second brain »influencing our thoughts, emotions and behaviors.
- A link exists between the intestinal microbiota and the development of large brains in humans and certain primates.
- Intestinal bacteria produce molecules that optimize the energy needed by the brain, influencing its development in the process.
- The microbiota would have evolved in parallel with our brains to meet increased energy needs, independently of genetic heritage.
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