MIAMI BEACH, FL - JUNE 29: Miley Cyrus poses backstage at the iHeartRadio Ultimate Pool Party Presented by VISIT FLORIDA at Fontainebleau's BleauLive in Miami featuring live performances by Pitbull, Ke$ha, Afrojack, Icona Pop, Krewella and Jason Derulo on June 29, 2013 in Miami Beach, Florida. (Photo by Gustavo Caballero/Getty Images for Clear Channel)
For flashy hairstyles, Donald Trump, Miley Cyrus and Lady Ga-Ga have made their mark in the halls of human hairdom.
But whether we have star quality hair or not, the bigger question may be: Why do we have so much hair on our heads in the first place? And why not so much elsewhere?
"Even though hair is very important to people, we know very little about why it is the way it is," says evolutionary biologist Aaron Sandel of the University of Michigan. "Hairy or bald, human hair is one of our inheritances from evolution, and we should be proud of it."
If you are sitting on the beach right now among some hairy-looking folks, you may not think so, but people are pretty darn unusual in their hairlessness, Sandel concludes in the current American Journal of Physical Anthropology. The study comparesthe hair density data on people and 52 other mammals, including apes and monkeys, taken from studies dating to the 1930s. Overall, the study finds a connection between body size and hairiness, where the larger the species, the less dense the hair, typically, in everything from rats to buffalo.
But people and their closest living cousins, chimps, are significantly less hairy than is even normal for mammals their size, something that is also true to a lesser extent for other apes, such as gorillas. "Humans really stand out for their hairlessness though, compared to everyone else," Sandel says.
People are actually covered in hair, mostly fine downy "vellus" hairs, as opposed to the thick hair covering most mammals and the tops of our heads. The dominance of these fine hairs over thicker ones draping other mammals is what gives the human form its smoother appearance. (Plus a lot of shaving and waxing for you beachgoers.)
"For humans, most people think that we became hairless due to trying to stay cool," Sandel says. Evidence from human genetics and the gene maps of lice suggests that humanity's ancestors shed their hair more than 1 million years ago, during the era of the early human species called Homo erectus. Homo erectus species lived on the African savanna, Sandel says, and "they spent a lot of time running around and sweating." In that environment, keeping cool would have offered a survival advantage, so in theory, humans might have evolved less dense hair over time, shedding body fur that would be an impediment to cooling off by sweating.
But there are monkeys that live in savanna environments today who are plenty hairy, Sandel adds. And then there is the matter of chimps, who live in jungles, being less hairy than normal. "Chimps actually have thinner hair on their heads than humans," says Sandel, who is spending the summer studying chimp behavior at Uganda's Kibale National Park. So, he suggests that some deeper evolutionary factor might be at work, one that was affecting the last common ancestor of both chimps and humans, an ape-like species that lived more than 5 million years ago.
"We know that humans are "naked apes," but Sandel shows in his new paper that chimpanzees are also less hairy than we would expect for their body size," says Penn State anthropologist Nina Jablonski, author of Skin: A Natural History. "From this, he draws the reasonable conclusion that the common ancestor of chimps and humans must have been less hairy than other apes. The question then is, 'why?'."
Perhaps the hormones that delay maturation in people, our long childhood, and then turn on hair growth in puberty in them (remember suddenly having to shave?) have given us a relatively hairless appearance as a side effect of their chemistry. Or perhaps being less hairy made people less suitable targets for ticks and other parasites, another advantage passed down to offspring.
"The short answer is that we don't know why this is the case," Jablonski says. "Is there a functional explanation in terms of enhanced heat loss, or was there a random genetic change in the common ancestor that may have led to lower body hair density? These things now need to be investigated."
Anthropologist Barbara King of the College of William and Mary in Williamsburg, Va., is skeptical of an evolutionary explanation for hairlessness that works for both chimps and people. "After all, the pattern of hair density in humans results in a unique (within primates) visual presentation," says King, the author of How Animals Grieve. "As other anthropologists have noted, we humans possess a whole 'skin canvas', a place of vibrant self-expression, that may well have played a significant role in our behavioral evolution."
Not to mention, there is also the question of the human hair adorning those celebrities (or whatever the situation is with Mr. Trump). A head full of hair might have helped our otherwise smooth ancient ancestors keep the large, calorie-burning human brain warm on cool savanna nights. Or it might be a matter of "sexual selection," a preference for striking display features seen across the animal kingdom playing a role in mating choices, which led to a nice coiffure remaining a part of human heritage. That would certainly explain the appeal of celebrity hairstyles. "But I am a little doubtful," Sandel says about that idea. "Our ancestors had a lot of problems surviving, and I'm not sure that nice hair would offer that many benefits that many years ago."
Perhaps some things in life, such as Donald Trump's hair, are just meant to remain mysteries.
Dan Vergano, USA TODAY