Human Hair: Why Is It Only On Our Heads?

Ever wonder why humans only have a few small patches of hair on our heads, when chimpanzees - our closest biological relatives - have bodies covered with hair?

It's one of the more fascinating aspects of human hair. While most mammals sport fur coats from head to paw, humans only have thin vestiges of bodily fur - and a shock of thick, luxuriant hair right atop our heads. Sometimes, it seems, evolution has an odd sense of humor.

Explanations for how human hair got to look the way it did abound in anthropological circles; currently there's no single scientific theory to explain it. What we do know is that, sometime between six and eight million years ago, our ancestors began shedding hair like an overstressed suburbanite. Whether this was due to environmental factors or was a result of choices in behavior remains to be discovered.

The most seemingly obvious culprit is environment; human hair began disappearing because apelike pre-humans came down from the trees and into the savannah, in search of a broader spectrum of prey. Those of our ancestors who bore less fur would have had an easier time dealing with the relentless heat of the sub-Saharan savannah - meaning nature selected for hairlessness, and we gradually lost all of the hair that didn't provide shade in the sun.

The problem with this hypothesis, though, is that the African savannah also gets mighty cold at night. Any hair that made us swelter through the hot day would have also shielded us from the cold at night.

Similarly, our ancestors may have lost much of their body hair as they began to forage for food in shallow water; in doing this, they gradually lost hair in favor of increased body fat, which is a much more effective insulator in water. Unfortunately, while this may be a compelling explanation, no concrete evidence has surfaced to support it.

If evolutionary human hair loss doesn't sit at the intersection of environment and behavior, then, where does it sit? The answer may be as simple as a test given to schoolchildren every year.

In modern industrialized nations, lice are relatively rare, but in the African veldt millions of years before the invention of agriculture, they were more prevalent. Human hair is like a pantry to some parasites; while they don't feed on the follicles themselves, they hide in the thickets and feed on the flesh of the host. Prehistoric humans, without the hygienic practices and medical advances we have today, were especially vulnerable to parasites like body lice. And of course, those primates with less hair would be less vulnerable, meaning less human hair overall as the millennia progressed.

Of course, the whole matter is unsolved. Perhaps the real reason is a combination of factors; maybe a variety of hunting and foraging techniques led to a reduction in body hair. If an epidemic of lice-like parasites swept through vast populations of prehistoric humans, those with less hair would be more likely to survive. In the ensuing years, primates with less body hair may have become more attractive to potential mates, utilizing hairlessness in the same way birds use bright plumage. Fast-forward to modern times, when human hair is confined mostly to the tops of our heads and the bottoms of our faces.

The answer may be elusive. But one thing is clear: Our hair is one of our most prized biological possessions. We use it to express ourselves, to attract mates, and to run our fingers through when we're frustrated. It's a unique product of human evolution.