For evolving brains, a diet full of carbs

Scientists: Starches fueled evolution of our oversize brains.

You arewhat 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, scientistsmust 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 7 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.

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.

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.

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 pre-agricultural hunters from Europe reveal that people had extra copies of amylase genes long before they started farming. Thomas and his colleagues propose that the utilization of fire, not farming, gave rise to the need for more amylase.