March 23, 2012
Matt Damon is so afraid of snakes that he wept on the set of "We Bought a Zoo," according to a report this week in People magazine. Co-star Scarlett Johansson says she watched Damon "cry like a baby and rock back and forth when the snakes were spread all over the set. He was pretty terrified."
Damon's reaction may be unusual, but his fear isn't. A new study suggests that all humans have a "genetic phobia" of snakes, due partly to a long evolutionary history in which pythons preyed on people.
Published in the Proceedings of the National Academy of Sciences, the study draws its conclusions from the Agta Negritos, a tribe of hunter-gatherers in the Philippines. Their complex rivalry with local pythons may offer a window into ancient human-snake relations, according to study author Thomas Headland, an anthropologist at Southern Methodist University. Headland spent 24 years with the Agta, and now uses his insight to reveal the intertwined history of people and snakes.
"I'm not sure if anyone till now has demonstrated with data just how dangerous large snakes must have been to early hominin species in the ancient past," writes Headland, who co-authored the paper with Cornell University herpetologist Harry Greene. "It was a lucky break that I happened to be living with [Agta Negritos] for most of the past half-century, where I stumbled on this startling phenomenon of intense symbiosis interactions between the Agta and giant pythons."
The data, he adds, "support the theory that we have a genetic phobia" of snakes.
The Agta have been in decline lately as modern society invades their rural habitat, but Headland lived among them in the 1960s, '70s and '80s, when their culture was more intact. He was there to study their language and lifestyle, but he also took note of prevalent python attacks: 26 percent of the adult men he interviewed had been attacked by a snake at some point, and almost all had scars from python bites. Tribe members also recalled six fatal python attacks from 1934 to 1973 — which amounts to one "traumatic python incident" every two to three years.
The victims included a 4-year-old girl and her 3-year-old brother, both of whom were killed in 1973 when a reticulated python slid into their thatched hut. (A third sibling was saved when their father returned and found the snake, killing it with a bolo knife.) Another python had eaten an adult male a few years earlier, finally relinquishing him when the man's son located the snake and cut it open to retrieve the body for burial.
Scientists have long known that humans are adept at spotting snakes, even when they're hidden, and this may help explain why. Studies of the amygdala, a small brain region that handles fear response, have shown the human brain reacts to seeing a snake even before it has cognitively processed the image — i.e., we can be scared of a snake before we know it's there. This would be adaptive for early humans if they routinely faced mortal danger from snakes, as Headland's study suggests they did.
"I can easily entertain as logical the hypothesis that human fear of spiders and scorpions is genetic, as is our fear of snakes. These fears are a human universal and, I think, different from the fear we may have when a robber holds a gun on us," Headland writes. "Men have it just as much as women, and certainly the Agta people have it, at least for snakes. We learned to be afraid of cars from our parents. But our fear of snakes and spiders seems to be not learned, but genetic."
The human-snake relationship isn't quite that simple, though. The Agta are also known to hunt pythons for food, and Headland himself once saw a 23-foot snake carcass that Agta hunters had killed, yielding 55 pounds of meat. They share many favorite foods, too, including deer, wild pigs and monkeys, often placing them in direct competition.
It's thus no surprise the Agta have an innate distrust of snakes, but Headland suggests this wariness became embedded in human genes long ago — the Agta, he argues, are simply one of the last societies where it's still widely useful for survival.
Lynne Isbell, an anthropologist at the University of California-Davis, tells ScienceNow the study reveals an "evolutionary arms race" between people and snakes. The need to see camouflaged snakes likely improved human eyesight over time, she says, while our growing intelligence may have driven snakes to evolve more sophisticated camo. "This paper uncovers a unique and complex natural relationship that the authors effectively argue has been in existence for millions of years. I especially like it because it may help to explain the paradoxical response of humans to snakes: We are both strongly repelled by and attracted to snakes."
Of course, that fear ceases to be adaptive if it makes you clam up like Matt Damon. And in most cases, it's no longer needed anyway, as Isbell points out: "I doubt such data would be possible to collect now that there are few human populations remaining that live so intimately with the land in the tropics where giant snakes also live."
Even if our fear of snakes is an outdated relic, though, it can still occasionally be relevant....