For Evolving Brains, a ‘Paleo’ Diet of Carbs

You are what you eat, and so were your ancient ancestors. But figuring out what they actually dined on has been no easy task.

There are no Pleistocene cookbooks to consult. Instead, scientists must sift through an assortment of clues, from the chemical traces in fossilized bones to the scratch marks on prehistoric digging sticks.

Scientists have long recognized that the diets of our ancestors went through a profound shift with the addition of meat. But in the September issue of The Quarterly Review of Biology, researchers argue that another item added to the menu was just as important: carbohydrates, bane of today’s paleo diet enthusiasts.

In fact, the scientists propose, by incorporating cooked starches into their diet, our ancestors were able to fuel the evolution of our oversize brains.

Roughly seven million years ago, our ancestors split off from the apes. As far as scientists can tell, those so-called hominins ate a diet that included a lot of raw, fiber-rich plants.

After several million years, hominins started eating meat. The oldest clues to this shift are 3.3-million-year-old stone tools and 3.4-million-year-old mammal bones scarred with cut marks. The evidence suggests that hominins began by scavenging meat and marrow from dead animals.

At some point, hominins began to cook meat, but exactly when they invented fire is a question that inspires a lot of debate. Humans were definitely making fires by 300,000 years ago, but some researchers claim to have found campfires dating back as far as 1.8 million years.

Cooked meat provided increased protein, fat and energy, helping hominins grow and thrive. But Mark G. Thomas, an evolutionary geneticist at University College London, and his colleagues argue that there was another important food sizzling on the ancient hearth: tubers and other starchy plants.

Our bodies convert starch into glucose, the body’s fuel. The process begins as soon as we start chewing: Saliva contains an enzyme called amylase, which begins to break down starchy foods.

Amylase doesn’t work all that well on raw starches, however; it is much more effective on cooked foods. Cooking makes the average potato about 20 times as digestible, Dr. Thomas said: “It’s really profound.”

Cooking would have made wild tubers much more nutritious to humans, he noted, “which is not to be sniffed at, especially if you’re a very hungry Pleistocene hunter-gatherer.”

Another clue to the importance of carbohydrates, Dr. Thomas said, can be found in our DNA. Chimpanzees, our closest living relatives, have two copies of the amylase gene in their DNA. But humans have many extra copies — some people have as many as 18. More copies of the amylase gene means we make more of the enzyme and are able to derive more nutrients from starches, Dr. Thomas said.

When scientists first discovered the extra genes, they hypothesized that our improved production of amylase evolved at the dawn of agriculture several thousand years ago. As wheat and other starchy crops became staples, the argument went, natural selection favored people with more amylase.

But recent studies of the DNA of preagricultural hunters from Europe reveal that people had extra copies of amylase genes long before they started farming. Dr. Thomas and his colleagues propose that the invention of fire, not farming, gave rise to the need for more amylase. Once early humans started cooking starchy foods, they needed more amylase to unlock the precious supply of glucose.

Mutations that gave people extra amylase helped them survive, and those mutations spread because of natural selection. That glucose, Dr. Thomas and his colleagues argue, provided the fuel for bigger brains.

The fossil record shows a drastic acceleration in the size of hominin brains starting roughly 800,000 years ago. Today our outsize brains use up as much as a quarter of the calories we burn.

Other experts said that Dr. Thomas and his colleagues have marshaled a lot of compelling evidence for the importance of carbohydrates in human evolution. But they were not ready to embrace the detailed scenario the scientists lay out in the new paper.

“They may be right, but it’s not a slam-dunk,” said Greg Wray, an evolutionary biologist at Duke University. “The timing is not well resolved enough.” There is still considerable uncertainty about when fire was invented and when amylase gene copies evolved, Dr. Wray noted.

Even Dr. Thomas isn’t ready to dispense dieting tips based on this theory of evolution.

“I think evolutionary biology can have a lot to say about food and health,” said Dr. Thomas. “But nutrition is so incredibly complex, and we’ve only scratched the surface.”