Are Humans Furless to Thwart Parasites?

John Pickrell in England
for National Geographic News
June 17, 2003
Humans are unique among primates for our near-total bodily hairlessness. In fact, only a handful of the 5,000 or so mammals—mostly semi-aquatic species such as whales, walruses, and hippopotamuses—are not covered in dense fur.

Now, a controversial new theory suggests that human hairlessness evolved as a strategy to shed the ticks, lice, fleas, and other parasites that nestle deep in fur.

Unique human cultural adaptations such as the use of fire, shelter, and clothing allowed ancient humans the luxury of ditching their insulating hairy layer, says the study soon to appear in print in the journal Biology Letters.

"One of the most unusual things about humans is that we don't have fur," said study co-author Mark Pagel, evolutionary biologist at Reading University in England. Though humans are not literally hairless, much of our hair has become so small and fine as to render it virtually invisible.

The new theory may also explain the difference in hairiness between men and women. If having less hair leads to a reduction in parasites, then it would offer a big advantage in the race of life and might become a sexually attractive feature in a mate, said Pagel. Hair might have been retained on the head, and on the face in men, as a feature of sexual display, he said, like peacocks' tails or stags' antlers.

Hot and Hairy

The most accepted current theory argues that hairlessness was a strategy for cooling the body on the sun-baked African savanna. Researchers have suggested that hair loss occurred when hominids (bipedal primates that include recent humans, their ancestors, and related species) first left the forests. A combination of an upright posture and lack of hair might have made it easier to radiate heat back into the environment.

"Physiological equations have shown that hairlessness would have doubled the distance hominids could travel off a liter of water," said Robin Dunbar, behavioral ecologist at the University of Liverpool in England.

However, hairlessness may also be a disadvantage in terms of heat regulation, argue Pagel and his co-author Walter Bodmer of Oxford University's Weatherall Institute of Molecular Medicine in England. Hairless animals without shelter or clothing would become too cold at night, said Pagel.

The next best contender is the "aquatic ape hypothesis," which suggests that at around 8 million years ago, human ancestors underwent a semi-aquatic phase—also explaining our improved swimming abilities compared to the other great apes. The majority of hairless mammals are semi-aquatic, probably because hair offers no insulating benefits under water. However, "very little solid evidence has been presented to back up this [aquatic ape] theory," said Pagel. It's also perplexing why features acquired for life in water so many millions of years ago should have been retained, he added.

Despite its other attributes, "fur is rich repository for parasites," said Pagel. Parasites, and the diseases they carry, are a hugely important guiding force in evolution—fleas are carriers of plague, for example. "For most of the [world's animals], most of the time, the majority of death is from parasites," said Pagel.

The scientists propose that pressure imposed by parasites, in combination with man's unique intelligence, and cultural adaptations, allowed humans to shed fur along with its resident fleas, ticks, and lice. Though parasites also infect clothing, clothes can more easily be cleaned, or changed, to remove the problem.

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.

Bug-free Badge

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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Additional National Geographic Resources
Interactive Feature: Outpost: In Search of Human Origins
National Geographic magazine online: Who Were the First Americans?

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