Human brains are bigger and better than any of our closest living or dead non-human relatives in relation to body weight. Scientists say we have fish and frogs to thank for this.

When early humans started to fish, they also began feeding their hungry brains.

The arrival of language and tool-making tend get all the credit for the big-brain phenomenon.

But before language or tools, a healthy diet was a brain's first fertilizer, said Stephen Cunnane, a metabolic physiologist at the University of Sherbrooke in Quebec.

"Something had to start the process of brain expansion, and I think it was early humans eating clams, frogs, bird eggs and fish from shoreline environments," Cunnane said.

Cunnane presented his research here Saturday at the annual meeting of the American Association for the Advancement of Science.

Baby food

Three-quarters of a human infant's energy goes straight to the brain.

Given that babies are helpless, that sounds like a lot to spend on an organ that is cognitively useless and does little to ensure a child's survival, Cunnane said.

But human babies have extra energy to feed their brains. Unlike other primates, human newborns are born with baby fat. That lovable chub stores the energy needed to quench a baby's ravenous brain.

The fatter the baby, the healthier its brain, the thinking goes.

A diet that included fish and shellfish — and particularly frogs and eggs — would have provided ancient humans, and their fattening babies, with the best source of nutrients and minerals to foster brain development.

Still today

Even today, many people are dependent on shore-based foods. And it's possible, Cunnane speculates, that diets which aren't based on the ancient tradition put us at grave risk.

Deficiencies in iodine and iron — minerals rich in a fish diet — can lead to cognitive degeneration. That's why companies added iodine to salt starting in the 1920s.

"We're still vulnerable when we're not consuming that vitamin-rich diet," Cunnane told LiveScience. "I think we're seeing it today in neurodegenerative diseases like Alzheimer's. If you take away the fuel, the brain suffers."

So what would happen if we fatten up skinny chimp babies? A natural chimpanzee diet is low in brain food.

If scientists fed them fish, Cunnane said, their brains might grow.

However, he added, "We'd never see the results. The experiment would take tens of thousands of years of evolution. But I think there would be a change in chimp brains."

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