The new theory might also explain the trait in the only other virtually hairless terrestrial mammal, says the study. Buck-toothed, pink, naked mole rats live in underground colonies. This not only means that the chance of parasite transmission is very high, but also means that the temperature remains relatively constant. This sheltered environment may have also allowed them to lose their fur, suggest Pagel and Bodmer.
The parasite theory also might explain the difference in amount of hair between the sexes, which the other theories have been unable to account for, say the authors.
"Hairlessness would have allowed humans to convincingly 'advertise' their reduced susceptibility to parasitic infection, and this trait therefore became desirable in a mate," says the study.
Facial and head hair may have been retained due to their importance in sexual attraction. Women may have less, because in our species males exert more sexually selective pressure, or are choosier about looks, than females, said the study. In addition, pubic hair could be important in enhancing transmission of pheromonal signals. Pheromones are subconscious chemical signals used by many species to communicate with or attract mates.
"This is a compelling and elegant theory," said Cristophe Soligo of the Human Origins research group at The Natural History Museum in London, England. The problem with the heat-regulation theory of hair loss is that no other savanna mammals developed the same adaptation, said Soligo. The use of clothes and shelter, however, sets us apart from other animals, and might explain why we alone were able to shed our fur and our parasites, he said.
However, other researchers are not convinced. At this stage, the evidence from "a great deal of modeling using standard physiological equations," is too strong to deny, and more data would be required to back up the new theory, said Dunbar. "The parasite argument would certainly help to reinforce hairlessness, but I am yet to be convinced that it would explain the evidence as we see it," he said.
This "new theory is not a significant challenge to pre-existing theories," argued physicist Lia Amaral at the University of Sao Paulo's Institute of Physics in Brazil. Amaral has studied the thermodynamics of hair loss. "No relevant comparison with the great apes, our hairy near-relatives, is made, [in this study]," said Amaral. "Parasites may have had a role as an additional benefit [of hairlessness], but certainly not as the main selective pressure."
Though Pagel and Bodmer have not yet produced data to back up their idea, they suggest it can be tested. Research should focus on comparing amount of body hair in people living in regions of the world with low and high levels of external parasites, said Pagel, and confirming existing anecdotal evidence that parasite loads are higher on hairy parts of the body.
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