Like many people, Mary Ann Raghanti enjoys potatoes loaded with butter. Unlike most people, however, she actually asked the question of why we love stuffing ourselves with fatty carbohydrates. Raghanti, a biological anthropologist at Kent State University, has researched the neurochemical mechanism behind that savory craving. As it turns out, a specific brain chemical may be one of the things that not only developed our tendency to overindulge in food, alcohol and drugs but also helped the human brain evolve to be unique from the brains of closely related species.
A new study, led by Raghanti and published on September 11 in the Proceedings of the National Academy of Sciences USA,examined the activity of a particular neurotransmitter in a region of the brain that is associated with reward and motivation across several species of primates. The researchers found higher levels of that brain chemical—neuropeptide Y (NPY)—in humans, compared with our closest living relatives. That boost in the reward peptide could explain our love of high-fat foods, from pizza to poutine. The impulse to stuff ourselves with fats and sugars may have given our ancestors an evolutionary edge, allowing them to develop a larger and more complex brain.
“I think this is a first bit of neurobiological insight into one of the most interesting things about us as a species,” says Robert Sapolsky, a neuroendocrinology researcher at Stanford University, who was not directly involved in the research but helped review the new paper.
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Neuropeptide Y is associated with “hedonic eating”—consuming food strictly to experience pleasure rather than to satisfy hunger. It drives individuals to seek out high-calorie foods, especially those rich in fat. Historically, though, NPY has been overlooked in favor of flashier “feel good” chemicals such as dopamine and serotonin.
Raghanti only decided to investigate NPY because her previous research had shown that it was one of several compounds—most notably dopamine—that were abundant in the nucleus accumbens in the brain region her team was focused on and that also seem to play a role in our exceptional cognitive abilities. “Honestly, I was not expecting anything really interesting with neuropeptide Y,” she says. But to everyone’s surprise, Raghanti and her colleagues discovered that the neurotransmitter plays a key role in the human nucleus accumbens, specifically.
In addition to humans, the researchers looked at neuropeptide Y concentrations in the brains of our closest primate relatives, including chimpanzees, bonobos and gorillas. They found that while every species produced some of NPY, humans had far and away the most of it.
The researchers hypothesize that somewhere back in our distant evolutionary past, a genetic switch in our ancestors’ brain must have sent neuropeptide Y production into overdrive. This would have spurred the organ to seek out fatty foods, Raghanti says. Ultimately, it meant that our species became more reproductively successful as our ancestors stored away extra calories. Those increased calorie stores would also enable them to dedicate more energy to developing a larger brain.
But NPY also has a dark side: it has been linked to eating disorders and substance misuse. This could explain why so many people live with these conditions today—what was once an evolutionary boon has the potential to cause harm in a modern hyperindustrialized society.
Other researchers, however, express more skepticism about neuropeptide Y’s significance in human evolution. Benjamin Campbell, a neuroanthropologist at the University of Wisconsin–Milwaukee, points out that the paper drew its conclusion about NPY’s role in fat storage from research done in rats. But rats and humans are separated by 65 million years of evolution. “So the whole dynamic of that storage is totally different,” he says.
And the neurotransmitter is just one in a litany of brain chemicals active in a single region of the brain, Campbell says. It’s hard to pin such a complex aspect of human evolution on something so specific.
For her part, Raghanti agrees—which is why she wants to continue researching the significance of understudied neurotransmitters such as neuropeptide Y. “It’s not going to be just one neurotransmitter in one region explaining what makes humans human,” she says. “It’s going to be the whole concert.”