Why are we the naked ape?

RIGHT from the start of modern evolutionary science, why humans are hairless has been controversial. "No one supposes," wrote Charles Darwin in The Descent of Man, "that the nakedness of the skin is any direct advantage to man: his body, therefore, cannot have been divested of hair through natural selection."

If not natural selection, then what? Despite his book's title, Darwin had little to say about human origins. He intimated that his strongest supporter, Thomas Huxley, had already dealt extensively with that question, and contented himself with a couple of pages about how we came to differ from our nearest relatives, the African apes. He concluded that the loss of body hair was due to sexual selection: that men (or more specifically, he implies, women) became hairless to attract a mate.

That explanation has not stood the test of time. Of all the thousands of mammal species, it is hard to believe that the males of just one species would develop an arbitrary preference for balder-bodied females, or that in just one species of primate it was the male's preference that decided the issue. If a man of Darwin's genius could not have come up a more convincing solution than that, some key factor must have been missing from the narrative.

For most of the past century it was assumed that the problem had been solved. Raymond Dart, the anthropologist who recognised the significance of the famous Taung baby's skull in 1924, began promoting the idea that while the apes' ancestors stayed in the trees, our ancestors moved onto the open plains. There the males became hunters, got overheated in the chase, and shed body hair to cool down.

The problem with that theory is that no other mammal has resorted to this method of cooling down. Hair insulates animals against the sun by day as well as against the cold by night. The hominid females are not thought to have become overheated hunters, so they would merely have suffered the downsides of hairlessness - being cold at night, more prone to abrasions, and having no fur to provide a handhold for infants to cling to. Yet they ended up even more hairless than the males.

Dart's solution, while the front-runner for more than 50 years, failed to win everyone over. In 1970, Russell W. Newman from the US Army Research Institute of Environmental Medicine argued in Human Biology that hominids could never have evolved on the plains with their "unique trio of conditions: hypotrichosis corpus, hyperhydrosis, and polydipsia". In other words, too little hair, too much sweat, and a need to drink little but often. Newman's paper ran counter to contemporary beliefs and was largely ignored.

William Montagna, the most indefatigable student of primate skin of his generation and then at the Oregon Regional Primate Research Center, regretfully reported in 1972 that after years of research his investigations had "failed to explain the unique feature of man's skin - his almost complete nakedness. We are left with the major objective... unattained."

In 1977, Stephen Jay Gould weighed in with his neoteny hypothesis: nakedness was part of an evolutionary strategy for acquiring larger brains. Our brains grow very fast just before we are born. By protracting that stage, rapid brain growth can continue, and along with the larger brains, we retained other characteristics of more juvenile primates - flatter faces, bigger heads and eyes, and naked skin. The idea was widely debated, but one weakness was that the time before birth is the only time when our bodies really are covered with hair. Moreover, we only retain fetal characteristics selectively, and only if useful to adults. We grow out of having short bandy legs and no teeth. We would also grow out of having a naked skin if it proved to be a disadvantage.

Other minority views included the idea put forward by a correspondent of Darwin's, who suggested that the hominids shed their hair because naked skin was less likely to harbour ticks and other noxious parasites. Darwin rejected this on the grounds that all other apes and savannah animals had faced the same problem with parasites. The idea has been revived variously but Darwin's doubts stand.

One proposal which few considered worth examining came from Alister Hardy at the University of Oxford. His field was marine biology, and he had noted that human skin was not only naked but lined with a layer of fat, and that this was true of a number of aquatic mammals. His article (New Scientist, 17 March, 1960, p 642) asked simply: "Was Man more aquatic in the past?" But this was rejected out of hand, largely because the savannah theory was considered unchallengeable.

Towards the end of the century, however, doubts were raised about the savannah theory, after closer examination of the fossilised flora and fauna linked with early hominid fossils. In 1997, Kaye Reed of Arizona State University in Tempe, deduced that the habitat consisted of varied landscape of bush, shallow wetlands, gallery forest, and medium-density woodland. It is now generally accepted that the earliest bipeds lived in wooded, even forested habitats.

hat proved bipedalism did not evolve on the savannah, but since hair does not fossilise, it is hard to establish when the hominids began to lose it. In 2006, Nina Jablonski, an anthropologist at Penn State University, wrote a book, Skin: A natural history, in which she proposed that after the bipeds finally moved to the savannah - and nobody doubts some eventually did - their profuse sweating created the need to shed body hair to allow the sweat to evaporate. She calculated such a hominid could have sweated up to 13.6 litres a day. But she did not explain why the equally sweaty patas monkey did not lose its hair in the same environment, nor how the hominids could have drunk so much in a hot, dry landscape.

More recently, investigations have changed tack as some scientists decided the most pressing need was no longer to speculate on the causes of nakedness, but to establish when it evolved. Palaeontologists cannot tell us this, so different specialists have become involved.

In 2004, geneticists Alan Rogers, Stephen Wooding and Dave Iltis, then of University of Utah, Salt Lake City, wrote a paper about the melanocortin 1 receptor (MC1R), which makes a protein affecting the colour of skin and hair. It is possible to date when this allele came into existence, and they suggest since it conferred a measure of sun-resistance, it must have been selected for when our ancestors first became naked and needed such protection. They date it no later than 1.2 million years ago - earlier than most scientists would estimate.

Three years later, a parasitologist at Florida Museum of Natural History in Gainesville called David L. Reed showed that differences in the DNA of lice can tell us much about their hosts. Human head lice are related genetically to lice on chimpanzees, and the two species began to diverge as humans and chimps diverged. But crab lice in human pubic areas are more closely related to those of gorillas so must have crossed over later, in a chance encounter. This led Reed to hypothesise our ancestors had already lost their body hair - except in pubic and axillary areas - 3.3 million years ago. That's an even bigger leap backwards.

None of this affects Hardy's theory that humans lost their body hair during an aquatic interlude. But his theory claims to offer one explanation for a whole range of enigmatic human features: bipedality, hairlesslessness, fat layer, descended larynx, loss of olfaction, and so on. If it turns out that the big two - bipedality and nakedness - arose at roughly the same time, that might shift the balance of probability some little way toward Hardy.

Only one thing is certain: the question is not going to go away. Any scenario which fails to tie up this loose end will continue to be less than satisfying. It will always be haunted by the suspicion that something in the story of our emergence is still missing.
Profile

Elaine Morgan was a screenwriter, winning BAFTAs for her work for British TV. She became fascinated by the idea of a semi-aquatic phase in our evolution, and in 1972 wrote The Descent of Woman to pave the way for an academic book by Alister Hardy (never written). Some think the aquatic idea merits more research but many palaeontologists regard it as a non-starter. Morgan spoke at TED 2009.