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Why People Lost Their Fur

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#1 Bill

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Posted 17 December 2011 - 06:15 AM

Bipedalism didn’t evolve as a way for ancient humans to keep cool during the heat of the day, according to a new model published today (December 12) in Proceedings of the National Academy of Sciences. But once hominins did start walking on two feet, it ignited another change that allowed them to stay cool—the loss of body hair. The new model explains why similarly sized mammals that walk on all fours and that may tend to overheat have not given up their coats.
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“If you are already walking upright for other reasons it actually makes the advantage you get from losing hair bigger than if you were on four legs,” said David Wilkinson of John Moores University in Liverpool, who authored the study along with Graeme Ruxton of the University of Glasgow. “You are moving more of your body up above the ground and sweat evaporates more easily” than it can if you were on all fours, because more air will circulate around you, Wilkinson explained.

Wilkinson and Ruxton came to this conclusion after analyzing a mathematical model of body temperatures during activity at different times of the day for quadrupeds and bipeds with and without fur. The model is an update to a previous theory by Peter Wheeler also of John Moores University, who proposed that both hair loss and bipedalism were driven by our need to cool down. His theory was that switching from four to two feet would reduce the amount of an animal’s body in direct sun and thus increase its ability to stay cool.

But Wheeler left out a critical factor, Wilkinson said—animal movement. Stationary animals could just hang out in the shade during the peak of the day to avoid overheating, he noted, while activities such as foraging likely forced early humans into direct sunlight more often.

Taking movement into account, Wilkinson and Ruxton’s model predicted that modern human ancestors would generate much more body heat metabolically as they traveled and hunted than the sun could cause, suggesting that standing upright to avoid the sun, as Wheeler’s model proposed, would have done little to fight overheating.

“In Peter’s models, he had a nice thermal advantage to standing upright,” said Wilkinson, “but now that vanishes in our version of the model.”

The new model further showed that four-legged creatures do not shed body heat as quickly when they lose their fur, suggesting that the loss of body hair would only have been a significant advantage to ancient humans if they were already walking on two feet. Thus, Wilkinson and Ruxton argue that bipedalism arose first—for some reason other than heat loss, such as improved observation of dangers, appearing larger to predators, or freeing the hands for tool use and carrying—then hair loss began, as a way to combat overheating.
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