We Can't Blame Obesity On Our Hunter-Gatherer Ancestors AnymoreS

One popular idea to explain the current obesity epidemic proposes that our hunter-gatherer ancestors had a fat-storing adaptation, which allowed them to survive frequent famines. Today, with our overabundance of food, this adaptation is causing us to get fat. There's just one problem with this idea — research now shows that hunter-gatherer societies aren't necessarily prone to food shortages.

The Thrifty Genotype Hypothesis

The notion that hunter-gatherer societies experience frequent periods of food shortage and even famines is pervasive across a number of scientific fields, including paleoanthropology and evolutionary psychology. This concept has even helped shape a variety of different theoretical models in everything from biology to economics — the "thrifty genotype hypothesis" is, perhaps, one of the most notable.

The thrifty genotype hypothesis (and other related theories) stems from the idea that our hunter-gatherer ancestors had an evolutionary adaption that allowed them to survive the frequent famines they must have faced. This adaptation still persists today, according to the hypothesis.

"If it were true, it would mean that some or all people have an extreme genetic predisposition to convert calories more efficiently to fat," Colette Berbesque, an evolutionary anthropologist at the University of Roehampton in the UK, told io9. "We've gotten so good at turning calories into fat for later use that we are all just getting obese because we don't have those lean times anymore."

Initially, scientists thought they had found evidence for the hypothesis when they looked at the subsistence practices of Samoans, Pima Native Americans and the Yanomami. People of these societies, they found, quickly experienced rising levels of obesity when they ditched their traditional diets. But it turned out that the groups had actually been involved in non-intensive agriculture — such as horticulture combined with hunting — for a long time, and that it was the "westernization" of their diets that led to their weight gain.

A couple of other studies compared the rates of famine and food shortages among people utilizing different types of subsistence, including agriculturalists, horticulturalists, hunter-gatherers and pastoralists. The research showed that hunter-gatherers experienced famines at about the same rate as other people. But when Berbesque took a closer look, she found that the studies didn't take into account different habitats. Half of the hunter-gatherer societies that researchers have data on live in arctic or subarctic regions, where very few agriculturalists live. What's more, it's impossible to tell an evolutionary story with just this sample, because many hunter-gatherers of the past lived in warm areas, Berbesque said.

So Berbesque and her colleagues decided to take another look at hunter-gatherer societies to see if they really do suffer from more famines than other people.

We Can't Blame Obesity On Our Hunter-Gatherer Ancestors AnymoreS

Hunter-Gatherers Vs. Agriculturalists

For their study, the team used the so-called standard cross-cultural sample, which is a massive database containing information on 186 societies across the globe, including 36 hunter-gatherer societies. The database, which is primarily a sample of preindustrial societies, is coded with thousands of different variables, including famine, diseases, marital patterns, incidences of conflict, location, rainfall, temperature, plant productivity and many others.

The researchers compared hunter-gatherer societies — defined as non-horseback hunting societies that obtain less than 10% of their nutrition from farmed goods or animal husbandry, and engage in minimal trade — with agriculturalists, based on eight different famine-related variables, such as occurrence of short-term starvation, persistence of famine and recurrence of famine. To make sure the habitats were similar, they only looked at cultures that lived in regions with an effective temperature, which is the average temperature of the warmest and coldest months, of at least 13 degrees Celsius (55.4 degrees Fahrenheit). The societies they included also had similar "net primary productivity," which has to do with the amount of new plant growth each year, Berbesque explained.

"We wanted to only statistically compare hunter-gatherers with agriculturalists that had similar habitats," Berbesque said. "Once we did that, we found that hunter-gatherers had significantly less bouts of famine."

In particular, hunter-gatherers had less famine than the agriculturalists across five of the eight variables, including occurrence of famine, severity of famine and persistence of famine. Interestingly, the hunter-gatherer societies also had lower scores for the "contingency of famine" variable. That is, the agriculturalists were more prepared for famines, which, depending on how you look at it, could mean they had to deal with food shortages more often.

Berbesque thinks there are a few reasons why contemporary hunter-gatherer societies were less prone to famines than agriculturalists. For one, they weren't tied to the land, so if they experienced a major change in their food source, they could just pick up and move to a better location. Another reason could be that they just used their land differently. It's sometimes said that preindustrial hunter-gatherers occupied marginalized habitats that weren't as productive as agriculturalists' habitats, but this idea comes from a farmer's point of view — while the land may not have been good for farming, it was still fine for other modes of subsistence.

Though the literature is rife with references to the "feast and famine" lifestyle of ancient hunter-gatherers, the study suggests it wasn't like that at all — a finding with important consequences.

"I think that the thrifty genotype, as an evolutionary story in explaining the western obesity epidemic, is misplaced," Berbesque said. Additionally, some people today adhere to diets that recommend periodic fasting, in an attempt to mirror the diets of our lean ancestors. But the study, as well Berbesque's own observations of modern hunter-gatherer societies, shows the idea behind this practice may be unfounded.

Check out the full study over in the journal Biology Letters.

Top image via Frank Vassen/Flickr. Inset image via Ian Sewell/Wikimedia Commons.