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Mardi, 29 Novembre 2011 22:03

Poop-Throwing Chimps Provide Hints of Human Origins

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Poop-Throwing Chimps Provide Hints of Human Origins

Pick up an object that’s close at hand. Throw it at something, or even someone (but gently, of course!) You’ve just reenacted what appears to be a pivotal stage in human evolution, when a propensity for projectiles shaped cognitive powers that later became language and symbolic thought.

That, at least, is one hypothesis for how humans became so smart. And now researchers have found support in chimpanzees, among whom the ability to throw goes hand-in-hand with increased intelligence and brain development.

“Imagine you’re an early hominid throwing at a rabbit. There’s increased selection for the cognitive demands of throwing, and that has some consequences for the development of the brain,” said psychologist Bill Hopkins of Emory University. “That’s where the throwing part becomes really interesting.”

In a study published in the January Philosophical Transactions of the Royal Society B, Hopkins and colleagues tracked several years’ worth of throwing behaviors in captive chimpanzees. (“If I was going to get s–t thrown at me, I was going to get something out of it,” said Hopkins.) Chimps are the closest living ancestor to humans, and the only species aside from ourselves in which throwing is regularly seen.

The researchers were especially interested in relationships between throwing, cognition and lateralization, or the way certain activities are concentrated in the left or right hemispheres of our brains. Language processing occurs in the left side, which also controls our right hands; and most people use their right hands to throw, as do chimpanzees.

While throwing at first might not seem demanding, coordinating it requires intensive, on-the-fly calculations. An equation for throwing a ball, for example, would include the distance to a target, the ball’s heaviness and the thrower’s strength. A moving target makes it even harder. Other psychologists and anthropologists have put throwing at the beginning of a cognitive cascade into higher-order thought, but Hopkins said his team is the first to test this proposition.

'If you imagine that throwing started off as left hemisphere-dominant, before the emergence of speech, then speech and language would have co-opted that side of the brain.'

From brain scans of chimps that threw most often and accurately, Hopkins found heightened development in and connections between the motor cortex, where physical actions are coordinated, and the Broca’s area, which in humans is central to speech production. Better throwing meant more sophisticated, left hemisphere-reliant brains.

“It supports the idea that these areas could have been selected for as a consequence of throwing,” said Hopkins. “If you imagine that throwing started off as left hemisphere-dominant, before the emergence of speech, then speech and language would have co-opted that side of the brain.”

In behavioral tests, true-throwing chimps also proved especially apt in social intelligence and communication. Intriguingly, they fared no better than poor-throwing chimps on physical problem-solving tests, suggesting that throwing behaviors emerged not for hunting, as is commonly assumed, but to interact with peers.

“Why did these chimps learn to throw in a captive context? I’ve never in my life seen a chimp be given a banana for throwing s–t at someone,” said Hopkins. “The reward is not something food-based. The reward is that they can control a person’s behavior. They get a pile of something to throw, and usually the person tries to run. The chimp learns, ‘If I can do this, I can have some control over the world outside my cage.’”

Hopkins also noted the story of Santino, a chimp at a Swedish zoo who, in a display of sophisticated planning skills, collects stones, hides them from zoo staff and throws them at visitors.

In future research, Hopkins plans to look more closely at neurological changes in chimps newly taught to throw. “It would be interesting to see how far they can go,” he said.

Image: A chimpanzee throws a PVC pipe at a viewer. (Hopkins et al./Philosophical Transactions of the Royal Society B)

Citation: “The neural and cognitive correlates of aimed throwing in chimpanzees: a magnetic resonance image and behavioural study on a unique form of social tool use.” By William D. Hopkins, Jamie L. Russell and Jennifer A. Schaeffer. Philosophical Transactions of the Royal Society B, Vol. 367 No. 1585, January 12, 2012.


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