In the biological world, getting smarter is not all fun and games. Having a bigger and more powerful brain requires more resources, which creates a disadvantage: you need to eat more. Does your bigger brain help you eat more and stay safe? Then it’s an evolutionary advantage — otherwise, it’s not.

The cost of thinking

“Brain tissue is metabolically expensive,” says Erin Hecht, assistant professor of human evolutionary biology at Harvard, and one of the study authors, for the Harvard Gazette. “It requires a lot of calories to keep it running, and in most animals, having enough energy just to survive is a constant problem.”

For early humans, this gamble paid off. They started evolving bigger brains and eventually this led to our species dominating the planet (and you having to go to work every day). Scientists have a pretty good idea of when this happened. It started with the bipedal primates we now call Australopithecines some 2.5 million years ago. It wasn’t long (in evolutionary time) before their brains tripled in size. But it’s less clear exactly why this happened.

The traditional theory says it was fire. Fire opened up new cooking avenues and enabled humans to get more calories from their food (as well as consume previously inedible foods). But this theory has a timing issue: the brains started growing way before the first evidence of fire.

“Our ancestors’ cranial capacity began increasing 2.5 million years ago, which conservatively gives us about a 1-million-year gap in the timeline between brain size increasing and the possible emergence of cooking technology,” said Katherine L. Bryant, another of the paper’s co-authors and currently a researcher at Aix-Marseille Université in France. “Some other dietary change must have been releasing metabolic constraints on brain size, and fermentation seems like it could fit the bill.”

In fact, you’d expect that whatever spurred this brain increase happened beforehand.

But what was it?

An evolutionary pickle

In the new research, Hecht and Bryant suggest a different hypothesis: cached, fermented food. Basically, they say that our ancestors started saving up more and more food, this food was fermented, and became an accessible form of nourishment.

The first argument that supports this hypothesis is that the human large intestine is proportionally smaller than that of our primate relatives. This suggests that we’ve adapted to foods that were already broken down or “pre-digested” — which is exactly what fermented food is, in a way.

The second argument is that this could have also happened by accident. Early hominins might have simply cached food and left it there and it became fermented and preserved.

Fire and fermentation are both ways people have learned to make food better, but it was probably easier for our ancestors to stumble upon fermentation by accident. Fire doesn’t just happen every day, so the chances for early humans to figure out they could cook with it weren’t that frequent. Sometimes, they might have found food cooked by a wildfire and realized it tasted better, but it would take a special moment of realization to start cooking on purpose.

On the other hand, fermentation happens all the time because tiny organisms like bacteria and fungi are always around us and can start changing our food without us doing anything. Unlike with fire, people didn’t need a sudden insight to take advantage of fermentation; it was a more natural discovery.

Simply put, our ancestors didn’t need a “Eureka” moment with fermentation — it could have just happened by accident at first.

“This was not necessarily an intentional endeavor,” Hecht posited. “It may have been an accidental side effect of caching food. And maybe, over time, traditions or superstitions could have led to practices that promoted fermentation or made fermentation more stable or more reliable.”

An intriguing hypothesis

Researchers also compiled an extensive list of preserved fermented foods from the present. They propose that these may offer clues regarding fermentation practices in the past. From kimchi to yogurt, vinegar, or hikari, various cultures on Earth have developed ways to preserve food by fermentation. The particular taste predisposition towards fermented foods also has a cultural component. A flavour that one culture cherishes as a delicacy may be considered completely unpalatable in others.

However, overall, people seem to have a preference for fermented foods. This is a bit strange when you consider that fermented food is basically spoiled food that we still like. Still, it does fit with the idea of humans evolving with an evolutionary preference towards fermented food.

But there’s still a healthy degree of speculation to this hypothesis. Researchers want to look at ancient human DNA and see whether there are any changes linked to the consumption or taste/olfactory appreciation of fermented food.

Nonetheless, the team says that this is good reason to investigate fermented foods and their link early hominin evolution.

“This hypothesis also gives us as scientists even more reasons to explore the role of fermented foods on human health and the maintenance of a healthy gut microbiome,” she said. “There have been a number of studies in recent years linking gut microbiome to not only physical but mental health.”

The study was published in Nature Communications Biology.