Researchers suggest that a taste for fermented snacks may have caused a surprising jump in the growth rate of our ancestors’ brains.
In fact, the shift from a raw diet to one that includes foods already partially broken down by microbes may have been a crucial event in the evolution of our brains, according to a prospective study by evolutionary neuroscientist Catherine Bryant of the University of Aix-Marseille in France and two American colleagues .
Human brains have tripled in size over the past two million years of evolution, while human large intestines have shrunk by approximately 74 percent, suggesting a reduced need to internally break down plant food.
We know the timeline and extent of the expansion of the human brain, but the mechanisms that allow energy to be directed to that expansion are more complex and somewhat debated.
The study’s authors put forth their “external fermentation hypothesis,” which suggests that our ancestors’ metabolic circumstances for selective brain expansion may have been driven by shifting gut fermentation to an external process, perhaps even experimenting with preserved foods such as wine, kimchi, yogurt, sauerkraut and other pickles that we still eat today.
The human gut microbiome acts as an internal fermentation machine that enhances nutrient absorption during digestion. Organic compounds are fermented into alcohol and acids by enzymes, usually produced by bacteria and yeast that live in parts of our digestive system, such as the colon.
Fermentation is an anaerobic process, meaning it doesn’t require oxygen, so like the process in our intestines, it can happen in a sealed container. The process produces energy in the form of adenosine triphosphate (ATP), which is the primary source of chemical energy that powers our metabolism.
The researchers say it’s possible that culturally transmitted ways of processing or storing food may have encouraged this function to be externalized.
Extrinsically fermented foods are easier to digest and contain more nutrients than their raw equivalents. And because there is less work for the colon if the food is already fermented, the organ’s size can decrease over time, while potentially leaving energy available for brain growth.
The brain sizes of our ancestors, australopithswere similar to those of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). The brain expansion of the human lineage is accelerating with Homooccurrence and continuation Wise man and Homo neanderthalensis.
How did our ancestors, with brains the size of chimpanzees, manage to harness the power of external fermentation?
Bryant and her team suggest that hominids with lower cognitive abilities and smaller brains may have adapted to fermentation much earlier than suggested alternative explanations for this redirection of energy from the gut to the brain, such as hunting animals and cooking on fire.
Fermentation has many benefits associated with cooked foods, such as softer textures, increased calorie content, improved nutrient absorption, and protection against harmful microorganisms.
It only needs simple storage locations, such as a cavity, cave or even a hole in the ground, and is essentially a low-level, stress-free ticket to nutritional goodness. As the researchers point out, “it can be encountered rather than requiring planning and tooling.
“Hunting, scavenging by large carnivores, and the use of fire carry their risks;” Bryant and colleagues write, “perhaps the risks of fermentation were more predictable and thus more reliably mitigated by individual and cultural learning.”
In addition to increasing the bioavailability of nutrients, external fermentation can also make poisonous foods edible, for example by removing cyanide from the common staple food cassava (Manihot esculenta).
“Forethought and mechanistic understanding are no requirements for the initial occurrence of external fermentation,” the researchers wrote. “Our early ancestors may have simply brought food back to a common location, left it there, and periodically ate a little and added more.”
Microbes from previous food items may have inoculated new food items leading to fermentation. As brains increased, humans may have developed a better understanding of fermentation.
The team emphasizes the need for empirical studies to support or refute their hypothesis, such as microbiological studies, comparative analyses, and genetic and genomic studies.
“The transfer of gut fermentation to external cultural practice may have been an important innovation in hominins,” the authors conclude, “that set the metabolic conditions necessary for selection for brain expansion to take hold.”
The study was published in Biology of Communications.
