Did hyperactivity evolve as a survival aid for nomads?

Impulsivity and a short attention span may be the bane of every parent with a hyperactive toddler, but those same traits seem to help Kenyan nomads keep weight on.

A gene mutation tied to attention deficit hyperactive disorder (ADHD) is also associated with increased weight among a chronically undernourished group of nomads called the Ariaal. Notably, the mutation offers no such benefit to a cousin population that gave up the nomadic lifestyle in the 1960s.

The nomads' active and unpredictable life centred on herding might benefit from spontaneity, says Ben Campbell, an evolutionary anthropologist at the University of Wisconsin at Milwaukee, US, who was involved in the new study.

"If you are a nomad then you ought to be little more impulsive than if you are settled," he says. "You should be a little quicker on the trigger."

Different lifestyles

The Ariaal are an isolated group of nomads who wander around northern Kenya, herding cows, camel, sheep and goats. Encouraged by Christian missionaries in the 1960s, some members settled in the same region and started relying on agriculture for some of their food.

The nomads and the settled groups still interact and intermarry, but they live drastically different lifestyles. "The nomads are always doing something. They are always walking to herd their animals," Campbell says, while settled Ariaal tend to be sedentary.

A previous study found that nomadic cultures around the world tend to have the same mutations, which determines the brain's response to a pleasure-delivering chemical called dopamine and is linked to impulsivity and ADHD.

Campbell and his colleague Daniel Eisenberg, of Northwestern University in Evanston, Illinois, US, looked for the mutation in 87 settled and 65 nomadic Ariaal men.

About a fifth of the men from each group had the mutation. However, their physiques differed. Nomads with the mutation, which is in the gene called DRD4, tended to have slightly higher body-mass indexes and more muscles than nomads without the mutation – though both would be considered undernourished by Western standards. No such difference existed in the settled Ariaal.

Lean times

Why the mutation isn't more common is a mystery, says Eisenberg. Another study found the impulsive variation in about 60% of native South Americans, but only 16% of Caucasian Americans. "It might be that there is a niche for a few people with more impulsive behaviour, but when there are too many of them those niches are filled," he says.

Also unexplained is how a gene linked to ADHD promotes greater body weight in nomads, and not village dwellers. Campbell speculates that a short attention span and penchant for risk taking could benefit nomads who don't know where the next meal will come from.

However, the mutation could also make food more gratifying, or it might affect how the body converts calories to kilograms. "We really don't know," Campbell says.

The mutation "predisposes you to be more active, more demanding, and not such a pleasant person," says

Henry Harpending, an anthropologist at the University of Utah in Salt Lake City, also in the US. "You probably do better in a context of aggressive competition." In other words, in lean times, violent men may feast while passive men starve.

Journal reference: BMC Evolutionary Biology (DOI: 10.1186/1471-2148-8-172)

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