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Researchers: Illusion Made Throwing Critical to Human Evolution


February 7, 2011 — Can't help molding some snow into a ball and hurling it or tossing a stone as far into a lake as you can? New research from the University of Wyoming and Indiana University shows how humans, unlike any other species on Earth, readily learn to throw long distances. This research also suggests that this unique evolutionary trait is entangled with language development in a way critical to human existence.

The study, that appeared online Jan. 14 in the journal "Evolution and Human Behavior," was led by Qin Zhu, assistant professor in the UW Department of Kinesiology and Health, and Geoffrey Bingham, professor in IU's Department of Psychological and Brain Sciences. It suggests that the well-established size-weight illusion, where a person who is holding two objects of equal weight will consider the larger object to be much lighter, is more than just curious or interesting, but a necessary precursor to humans' ability to learn to throw -- and to throw far.

Just as young children unknowingly experience certain perceptual auditory biases that help prepare them for language development, the researchers assert that the size-weight illusion primes children to learn to throw. It unwittingly gives them an edge -- helping them choose an object of size and weight most effective for throwing.

"Language and throwing led to the survival of Homo sapiens, and we are now beginning to gain some understanding of how these abilities are rapidly acquired by members of our species," Bingham says.

Why is throwing so important from an evolutionary standpoint? The authors said Homo sapiens have been successful as a species because of three factors: Social organization and cooperation, language, which helps with the former factor, and the ability to throw long distance. This trio allowed Homo sapiens to "take down all the potential competition," Bingham says. It brought us through the ice ages because Homo sapiens could hunt the only major food sources available, big game such as mammoths and giant sloths.

Zhu considers throwing and language in concert, because both require extremely well-coordinated timing and motor skills, which are facilitated by two uniquely developed brain structures -- the cerebellum and posterior parietal cortex.

Bingham and Zhu recruited 12 adult men and women to perform various tests related to perception, the size-weight illusion and throwing prowess.

Another way to explain the size-weight illusion is that for someone to perceive that two objects -- one larger than the other -- weigh the same, the larger object must weigh significantly more than the smaller object. Their study findings show that skilled throwers use this illusion of "equal-felt" heaviness to select objects that they are able to throw to the farthest, maximum distance with surprising accuracy. This suggests that the proverbial illusion is not a misperception, "It is a perceptual bias that prepares for fully functional throwing ability," Zhu says.

Neanderthals, which co-existed with Homo sapiens long ago, lacked the more developed cerebellum and posterior parietal cortex.

"These brain structures have recently been found to distinguish Homo sapiens from Neanderthals," the authors say. "It is possible that this is what enabled us to beat out Neanderthals, who otherwise had the larger brains."

For more information, contact Zhu, phone (307) 766-5752, e-mail qzhu1@uwyo.edu.

Photo:
Graduate student Rashmi Ravi of India records the motion of thrower Angela Chambers of Green River, a freshman in kinesiology and physical education. They participated in a study on the role of throwing in human evolutionary development. (UW Photo)

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