We humans are often known as “naked apes”. It might seem like a deserved nickname; after all, we lack the lush coats of body hair that chimps, bonobos and gorillas have in abundance. But we are not naked. We actually have the same density of body hair as other apes of our size, but ours is largely fine and colourless rather than thick and dark. We are coated with a layer of short, fine hair, known technically as vellus hair and colloquially as peach fuzz.
Many scientists have speculated about why we humans have lost a thick coat of body hair. But very few of them have offered answers to an equally mysterious question: why have we kept our vellus coat? The fine hairs aren’t very good at preserving body heat, and they don’t make us more or less sexually attractive. They look like the results of a half-hearted evolutionary stab at becoming hairless. Some have suggested that they have no role at all.

But Isabelle Dean and Michael Siva-Jothy from the University of Sheffield have an intriguing possible answer: they think that the vellus hairs help us to spot parasites.

The duo recruited 29 student volunteers and drew a rectangle of Vaseline on both their forearms, after first shaving one of them. While the volunteers looked away, Dean and Siva-Jothy placed a bedbug within the rectangles.  They waited until the bug started to extend its bloodsucking snout before plucking it off. They had to intervene quicker if the arms were shaved. With vellus hair, the bedbugs took between 22 and 26 seconds to find a good place. Without the hair, they searched for just 18-19 seconds.

The students also found felt the bugs more quickly when the hairs were around. They had to use a clicker every time they felt something crawling about, and they did so twice as often for their unshaved arms than their naked ones. And the hairier they were, the better they were at detecting the bugs. These results show that vellus hair makes it easier for people to feel the footfalls of parasites on their skin, while also making it harder for the parasites to start feasting.

“This is a welcome paper,” says Mark Pagel, who studies human evolution at the University of Reading. In 2003, Pagel and Walter Bodmer promoted a century-old hypothesis in a paper simply titled: “A naked ape has fewer parasites”. Early humans probably lived in large groups within a fixed base, which would have made perfect staging posts for parasites to breed and spread. (It may not be a coincidence that humans are the only species of primate to suffer from fleas.) We adapted by losing our thick hair – our finer fuzz not only harbours fewer parasites, but makes any stragglers easier to find.

Pagel says, “This new paper supports our idea that we lost our coarse hair because it provides a refuge for ectoparasites [those that live on the skin – Ed]. By using the same volunteer in both conditions, the authors controlled for individual differences that might affect the results.” Markus Rantala, a parasite specialist from the University of Turku in Finland, has argued for the same hypothesis. “The paper really made my day when I read it,” he says. “I have often wondered why we still have fine body hair on our legs and arms. This study provides a nice explanation.”

Dean and Siva-Jothy suggest that hair loss did help to reduce the risk of parasites, but that total loss would have made things worse. There’s an optimal length of fuzz where parasites are easiest to spot, and we might have hit upon it.

Obviously, this is far from the only reason why humans might have lost our thick body hair. Darwin suggested that hairy humans would have risked overheating in the open African savannah, and naked skin cools more easily through sweating. He also suggested that hairlessness might have made our ancestors more sexually attractive to each other. These ideas could co-exist quite happily with the notion that the need to avoid the bites of parasites helped to drive the evolution of the semi-naked ape.

Reference: Dean & Siva-Jothy. 2011. Human fine body hair enhances ectoparasite detection. Biology Letters http://dx.doi.org/10.1098/rsbl.2011.0987

*(The methods note that they used a Gillette Mach 4 to shave the arms – scientists are delightfully meticulous in their note-keeping).

Photo of vellus hair by Svdmolen

More on human evolution:

• What is the point of pruney fingers?
• Did humans trade guts for brains?
• Did sex with Neanderthals and Denisovans shape our immune systems? The jury’s still out
• Control Altered by Deletion – is lost DNA behind our bigger brains and spineless penises?
• Human ancestors carved meat with stone tools almost a million years earlier than expected
• Genetic neoteny – how delayed genes separate human brains from chimps
• A burst of DNA duplication in the ancestor of humans, chimps and gorillas
• Did a gene enhancer humanise our thumbs?

December 13th, 2011 by Ed Yong in Animals, Evolution, Human evolution | 7 comments | RSS feed | Trackback >