Scientists have figured out why we love carbs

(CNN) — The origin of modern humans’ long-standing love affair with carbs may predate our existence as a species, according to a new study.

A once prevailing stereotype of ancient humans feasting on mammoth steak and other hunks of meat helped foster the idea of a protein-heavy diet that was necessary to fuel the development of a large brain.

But archaeological evidence in recent years has challenged this view, suggesting that humans long ago developed a taste for carbohydrates, roasting things such as tubers and other starch-laden foods that have been detected by analyzing bacteria lodged in teeth.

The new research, published in the journal Science on Thursday, offers the first hereditary evidence for early carb-laden diets. Scientists traced the evolution of a gene that enables humans to digest starch more easily by breaking it down into simple sugars that our bodies can use for energy. The study revealed these genes duplicated long before the advent of agriculture.

This expansion may even go back hundreds of thousands of years, long before our species, Homo sapiens, or even Neanderthals emerged as distinct human lineages.

Researchers based at The Jackson Laboratory in Farmington, Connecticut, and the University of Buffalo in New York state analyzed the genomes of 68 ancient humans. The study team focused on a gene called AMY1, which allows humans to identify and begin breaking down complex carbohydrate starch in the mouth by producing the enzyme amylase. Without amylase, humans would not be able to digest foods such as potatoes, pasta, rice or bread.

Humans today have multiple copies of this gene, and the number varies from person to person. However, it has been tricky for geneticists to piece together how and when the number of these genes expanded — a reflection of when eating starch likely became advantageous for human health.

“The main question that we were trying to answer was, when did this duplication occur? So that’s why we started studying ancient genomes,” said the study’s first author Feyza Yilmaz, an associate computational scientist at The Jackson Laboratory.

“Previous studies show that there’s a correlation between AMY1 copy numbers and the amount of amylase enzyme that’s released in our saliva. We wanted to understand whether it’s an occurrence that is corresponding to the advent of agriculture. This is … a hot question,” she said.

A genetic opportunity

The team found that as far back as 45,000 years ago, hunter-gatherers — whose way of life predated agriculture — had an average of four to eight copies of AMY1, suggesting that Homo sapiens had a taste for starch long before the domestication of crops shaped human diets.

The research also revealed duplication of the AMY1 gene existed in the genomes of Neanderthals and Denisovans, an extinct hominin first discovered in 2010 about whom relatively little is known. The presence of multiple copies of the gene in three human species suggests that it was a trait shared by a common ancestor, before the different lineages split, according to the study.

That finding means archaic humans had more than one copy of AMY1 as far back as 800,000 years ago.

It’s not clear exactly when the initial duplication of AMY1 took place, but it likely happened at random. The presence of more than one copy created a genetic opportunity that provided humans with an advantage for adapting to new diets, especially those rich in starch, as they encountered different environments.

The analysis also showed that the number of AMY1 copies a person carries increased steeply in the past 4,000 years — likely favored by natural selection as humans adapted to the starch-rich diets resulting from the shift from a hunter-gatherer lifestyle to agriculture and farming grains.

The study “provided compelling evidence” of how the molecular machinery for converting difficult-to-digest starches into easily accessible sugars evolved in humans, said Taylor Hermes, an assistant professor in the department of anthropology at the University of Arkansas, who wasn’t involved in the research.

What’s more, the new research bolsters the emerging theory that it was carbs, rather than proteins, that provided the energy bump necessary for the increase in human brain size over time, he noted.

“The authors finding that an increased copy number of the amylase gene, which results in a greater ability to break down starch, may have emerged hundreds of thousands of years before Neanderthals or Denisovans gives more credit to the idea that starches were being metabolized into simple sugars to fuel rapidly growing brain development during human evolution,” Hermes said.

“While I think more testing with higher-quality ancient human genomes is warranted, I was surprised that the authors were able to detect multiple copies of amylase genes in Neanderthals and Denisovan genomes that have been previously published,” Hermes added. “This shows the value in continuing to mine the genomes of our human ancestors for important medical and physiological records.”

It is challenging to understand how individual genes varied over time in populations, and the study is “extremely impressive,” said Christina Warinner, John L. Loeb Associate Professor of the Social Sciences and Anthropology at Harvard University.

“We know that dietary shifts have played a central role in human evolution … but reconstructing these events that took place thousands, hundreds of thousands, and even millions of years ago is daunting,” Warinner, who wasn’t involved in the research, said.

“This study’s genomic sleuthing is helping to finally time stamp some of those major milestones, and it is revealing tantalizing clues about humanity’s long love affair with starch.”