Intelligence is an energetically expensive luxury – as the rise of artificial intelligence has reminded us. The human brain is no exception.

Comprising roughly 2% of the body weight, it uses one fifth the body’s energy in the resting state – compared with less than half of this in non-human primates. A five-year-old child devotes 66% of the energy they need to stay alive to their brain.

How did our ancestors foot the energy bill to run their uniquely large brains? Eating more meat is often considered to be the answer. But there’s a catch. The brain relies on a form of fuel that is not present in meat: glucose.

Our new study, published today in Science, shows that carbohydrates contributed more than half our total energy requirements over four million years of evolution. It holds important clues for why we crave sweet foods today – and how we can eat more healthily.

Many anthropologists have credited meat eating as the stimulus to produce a large brain. After all, it required tools to butcher the carcass and access the fat-rich marrow inside bones.

Protein and fat in fruit and leaves are dilute, requiring hours of chewing, while animal foods are dense sources that can be devoured quickly. Bone marrow is a rich source of essential fats.

In truth, humans do not require more protein as a........

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