The large, capable human brain is a marvel of evolution, but how it evolved from the smaller primate brain into the creative, complex organ it is today is a mystery. Scientists can pinpoint when our evolutionary ancestors evolved larger brains, which roughly tripled in size when our ancestors evolved from the bipedal primates known as australopithecines.
But why this change occurred and what prompted it have remained elusive. Some believe that the use of fire and the subsequent invention of cooking provided enough nutrition for our big-brained ancestors to become masters, while a new theory points to a different kind of fire: fermentation.
The role of diet in brain evolution
Erin Hecht, one of the authors of the study recently published in Nature Communications Biology, said the key to understanding how our brains grow may well lie in what we eat and how we eat it.
"The metabolism of brain tissue is very expensive, it requires a lot of calories to keep running, and having enough energy to survive has always been a problem for most animals," said the assistant professor of human evolutionary biology. "So for the big-brained Autropis to survive, their diet had to change. Proposed theories include changes in the foods these human ancestors ate, or, the most popular, the discovery of cooking allowed them to get more available calories from any food."
But the problem with this theory is that the earliest evidence for the use of fire is about 1.5 million years ago, well after the development of the great ape brain. "The cranial capacity of our ancestors began to increase 2.5 million years ago. Conservatively speaking, our timeline has a gap of about 1 million years between the increase in brain capacity and the possible emergence of cooking technology," explained Katherine L. Bryant, one of the co-authors of the paper and currently a researcher at the Institute of Language, Communication and Brain at the University of Aix-Marseille in France. "Whatever changes occurred in their diet had to happen before their brains started to grow larger. Some other change in dietary habits must have released the metabolic constraints on brain size, and fermentation appears to satisfy this requirement."
Over the past few years, researchers have proposed other ideas, such as eating rotten meat. In the new paper, Hecht and her team propose a different hypothesis: Stored (or preserved) food fermented, and this "pre-digested" food provided a more accessible form of nutrients that fueled larger brains, allowing our larger-brained ancestors to survive and thrive through natural selection.
"This shift may have been an accident, not necessarily intentional," Hecht said. "This may be an unintended side effect of food storage. Also, over time, tradition or superstition may have led to practices that promoted fermentation or made fermentation more stable or reliable."
This hypothesis is supported by the fact that the human large intestine is proportionally smaller than that of other primates, suggesting that humans have adapted to foods that have been broken down through the chemical process of fermentation. Furthermore, fermented foods are found in all cultures and in all food categories, from wine and cheese in Europe to soy sauce and natto in Asia.
Hecht suggested that more research on the brain's responses to fermented and non-fermented foods might be helpful, as could research on smell and taste receptors, perhaps using ancient DNA. For evolutionary biologists, these are fertile areas for other researchers to exploit. (Hecht's research focuses on "how brain circuits evolve to support complex behavior," and he studies both living humans and dogs.)
As research continues, Bryant believes fermented foods have the potential to offer a wide range of benefits. She said: "This hypothesis also gives us scientists more reasons to explore the role of fermented foods on human health and maintaining a healthy gut microbiota. In recent years, there have been many studies linking the gut microbiome with physical health and mental health." "