In what might have been a parody of a human evolution just-so story, but apparently isn’t, we learn from “Why People Lost Their Fur”(The Scientist, December 12, 2011), Ruth Williams reporting, “The need for ancient humans to keep cool during the day might explain their lack of body hair but not why they walked on two feet:”
“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.
However,
Markus Rantala of the University of Turku, Finland, who was not convinced by Ruxton and Wilkinson’s model, offered another theory. “My personal opinion is that only selection caused by ectoparasites is able to explain the origin of human nakedness in a satisfactory way,” he said in an email. As humans began to live in fixed home bases and in close quarters, many parasites such as lice and fleas would have flourished. “As the ectoparasite burden on hominids increased, having fewer parasites may have become more important for survival than a warm fur coat,” Rantala said. Less body hair makes ectoparasites easier to spot and pick off.
So it was the bugs after all, then. In truth, no one knows how we discovered ourselves to be upright and naked. The most allusive story available is that the latter came as a shock:
Then the eyes of both of them were opened, and they realized they were naked; so they sewed fig leaves together and made coverings for themselves.