August 19 2003 Tuesday SCIENCE DESK Why Humans and Their Fur Parted Ways By NICHOLAS WADE NYT One of the most distinctive evolutionary changes as humans parted company from their fellow apes was their loss of body hair But why and when human body hair disappeared together with the matter of when people first started to wear clothes are questions that have long lain beyond the reach of archaeology and paleontology Ingenious solutions to both issues have now been proposed independently by two research groups analyzing changes in DNA The result if the dates are accurate is something of an embarrassment It implies we were naked for more than a million years before we started wearing clothes Dr Alan R Rogers an evolutionary geneticist at the University of Utah has figured out when humans lost their hair by an indirect method depending on the gene that determines skin color Dr Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology in Leipzig Germany believes he has established when humans first wore clothes His method too is indirect it involves dating the evolution of the human body louse which infests only clothes Meanwhile a third group of researchers resurrecting a suggestion of Darwin has come up with a novel explanation of why humans lost their body hair in the first place Mammals need body hair to keep warm and lose it only for special evolutionary reasons Whales and walruses shed their hair to improve speed in their new medium the sea Elephants and rhinoceroses have specially thick skins and are too bulky to lose much heat on cold nights But why did humans the only hairless primates lose their body hair One theory holds that the hominid line went through a semi aquatic phase witness the slight webbing on our hands A better suggestion is that loss of body hair helped our distant ancestors keep cool when they first ventured beyond the forest s shade and across the hot African savannah But loss of hair is not an unmixed blessing in regulating body temperature because the naked skin absorbs more energy in the heat of the day and loses more in the cold of the night Dr Mark Pagel of the University of Reading in England and Dr Walter Bodmer of the John Radcliffe Hospital in Oxford have proposed a different solution to the mystery and their idea if true goes far toward explaining contemporary attitudes about hirsuteness Humans lost their body hair they say to free themselves of external parasites that infest fur blood sucking lice fleas and ticks and the diseases they spread Once hairlessness had evolved through natural selection Dr Pagel and Dr Bodmer suggest it then became subject to sexual selection the development of features in one sex that appeal to the other Among the newly furless humans bare skin would have served like the peacock s tail as a signal of fitness The pains women take to keep their bodies free of hair joined now by some men may be no mere fashion statement but the latest echo of an ancient instinct Dr Pagel s and Dr Bodmer s article appeared in a recent issue of The Proceedings of the Royal Society Dr Pagel said he had noticed recently that advertisements for women s clothing often included a model showing a large expanse of bare back We have thought of showing off skin as a secondary sexual characteristic but maybe it s simpler than that just a billboard for healthy skin he said The message No fleas lice or ticks on me is presumably concealed from the conscious mind of both sender and receiver There are several puzzles for the new theory to explain One is why if loss of body hair deprived parasites of a refuge evolution allowed pubic hair to be retained Dr Pagel and Dr Bodmer suggest that these humid regions dense with sweat glands serve as launching pads for pheromones airborne hormones known to convey sexual signals in other mammals though not yet identified in humans Another conundrum is why women have less body hair than men Though both sexes may prefer less hair in the other the pressure of sexual selection in this case may be greater on women whether because men have had greater powers of choice or an more intense interest in physical attributes Common use of depilatory agents testifies to the continuing attractions of hairlessness especially in human females the two researchers write Dr David L Reed a louse expert at the University of Utah said the idea that humans might have lost their body hair as a defense against parasites was a fascinating concept Body lice spread three diseases typhus relapsing fever and trench fever and have killed millions of people in time of war he said But others could take more convincing There are all kinds of notions as to the advantage of hair loss but they are all just so stories said Dr Ian Tattersall a paleoanthropologist at the American Museum of Natural History in New York Causes aside when did humans first lose their body hair Dr Rogers of the University of Utah saw a way to get a fix on the date after reading an article about a gene that helps determine skin color The gene called MC1R specifies a protein that serves as a switch between the two kinds of pigment made by human cells Eumelanin which protects against the ultraviolet rays of the sun is brown black pheomelanin which is not protective is a red yellow color Three years ago Dr Rosalind Harding of Oxford University and others made a worldwide study of the MC1R gene by extracting it from blood samples and analyzing the sequence of DNA units in the gene They found that the protein made by the gene is invariant in African populations but outside of Africa the gene and its protein tended to vary a lot Dr Harding concluded that the gene was kept under tight constraint in Africa presumably because any change in its protein increased vulnerability to the sun s ultraviolet light and was fatal to its owner But outside Africa in northern Asia and Europe the gene was free to accept mutations the constant natural changes in DNA and produced skin colors that were not dark Reading Dr Harding s article recently as part of a different project Dr Rogers wondered why all Africans had acquired the same version of the gene Chimpanzees Dr Harding had noted have many different forms of the gene as presumably did the common ancestor of chimps and people As soon as the ancestral human population in Africa started losing its fur Dr Rogers surmised people would have needed dark skin as a protection against sunlight Anyone who had a version of the MC1R gene that