Around a million years ago, our lineage went through a tipping point. In the span of only a couple hundred thousand years — an eye bat in the grand scheme of evolutionary history — the brains of our ancestors doubled in size. This phenomenal evolution has been credited to the introduction of new stone-making technology that enabled early hunter-gatherers to catch more game.

But a new study suggests that all this brain power wasn’t fueled by meat alone. Cooking starchy, carb-rich food also likely played a major role, according to researchers at Harvard University and the Max Planck Institute. The evidence lies in some of the oldest oral bacteria analyzed by science thus far.

The team harvested bits of dead bacteria from between the teeth of Neanderthals and modern humans who lived more than 10,000 years ago before the advent of agriculture, as well as chimps, gorillas, and howler monkeys. In total, the researchers analyzed DNA from bacteria preserved on the teeth of 124 individuals, including a Neanderthal who lived more than 100,000 years ago in present day Serbia, at Pešturina Cave.

The oral biome of both Neanderthals and ancient humans closely resembled each other, suggesting both groups more or less ate the same thing. What stood out was the presence of an unusual group of Streptococcus oral bacteria that is associated with an enzyme that frees sugars from starchy foods. This bacteria wasn’t present in the teeth of chimps and other primates.

Since this kind of sugar-loving bacteria was so abundant, the researchers believe that Neanderthals and humans must have inherited these microbes from a common ancestors who lived more than 600,000 years ago.

Previously, other studies showed that Neanderthals included grasses, tubers, and (cooked) barley in their diet. Now, these findings point to an abundant sugar-rich diet from another angle: ancient oral bacteria.

The amylase enzyme that binds to the strep bacteria is more efficient at breaking down cooked food, meaning our ancestors cooked food at least as far back as 600,000 years ago.

“For human ancestors to efficiently grow a bigger brain, they needed energy-dense foods containing glucose,” molecular archaeologist Christina Warinner of Harvard and the Max Planck Institute for the Science of Human History told Science. “Meat is not a good source of glucose.”

Ultimately, the great expansion of the brain of our lineage is likely owed to multiple factors. More meat helped, so did strachy foods, with cooking binding them all together in a more digestible manner that freed up more energy for brain development.

The findings appeared in the Proceedings of the National Academy of Sciences.