Every hair on your body has a growth cycle, with a growing phase and a resting phase. For facial hair a cycle is normally completed in four weeks, but a scalp hair may be with you for as much as six or seven years. A hair in your armpit is likely to last about six months, a leg hair for two months. Removing hair, whether through cutting, shaving, or waxing, has no effect on what happens at the root. We each grow about twenty-five feet of hair in a lifetime, but because all hair falls out at some point, no single strand can ever get longer than about three feet. Hair grows by one third of a millimeter a day, but the rate of hair growth depends on your age and health and even the season of the year. Our hair cycles are staggered, so we don’t usually much notice as our hair falls out.
II
IN OCTOBER 1902, police in Paris were called to an apartment at 157 rue du Faubourg Saint-Honoré, in a wealthy neighborhood a few hundred yards from the Arc de Triomphe in the 8th arrondissement. A man had been murdered and some works of art stolen. The murderer left behind no obvious clues, but luckily detectives were able to call upon Alphonse Bertillon, a wizard at identifying criminals.
Bertillon had invented a system of identification that he called anthropometry but that became known to an admiring public as Bertillonage. The system introduced the concept of the mug shot and the practice, still universally observed, of recording every arrested person full face and in profile. But it was in the fastidiousness of its measurements that Bertillonage stood out. Subjects were measured for eleven oddly specific attributes—height when seated, length of left little finger, cheek width—which Bertillon had chosen because they would not change with age. Bertillon’s system was developed not to convict criminals but to catch recidivists. Because France gave stiffer sentences to repeat offenders (and often exiled them to distant, steamy outposts like Devil’s Island), many criminals tried desperately to pass themselves off as first-time offenders. Bertillon’s system was designed to identify them, and it did that very well. In the first year of operation, he unmasked 241 fraudsters.
Fingerprinting was actually only an incidental part of Bertillon’s system, but when he found a single fingerprint on a window frame at 157 rue du Faubourg Saint-Honoré and used that to identify the murderer as one Henri Léon Scheffer, it caused a sensation not just in France but around the world. Quickly, fingerprinting became a fundamental tool of police work everywhere.
The uniqueness of fingerprints was first established in the West by the nineteenth-century Czech anatomist Jan Purkinje, though in fact the Chinese had made the same discovery more than a thousand years earlier and for centuries Japanese potters had identified their wares by pressing a finger into the clay before baking. Charles Darwin’s cousin Francis Galton had suggested using fingerprints to catch criminals years before Bertillon came up with the notion, as did a Scottish missionary in Japan named Henry Faulds. Bertillon wasn’t even the first to use a fingerprint to catch a murderer—that happened in Argentina ten years earlier—but it is Bertillon who gets the credit.
What evolutionary imperative led us to get whorls on the ends of our fingers? The answer is that nobody knows. Your body is a universe of mystery. A very large part of what happens on and within it happens for reasons that we don’t know—very often, no doubt, because there are no reasons. Evolution is an accidental process, after all. The idea that all fingerprints are unique is actually a supposition. No one can say for absolute certain that no one else has fingerprints to match yours. All that can be said is that no one has yet found two sets of fingerprints that precisely match.
The textbook name for fingerprints is dermatoglyphics. The plow lines that make up our fingerprints are papillary ridges. They are assumed to aid in gripping, in the way tire treads improve traction on roads, but no one has ever actually proved that. Others have suggested that the whorls of fingerprints drain water better, make the skin of the fingers more stretchy and supple, or improve sensitivity, but again no one really knows what they are there for. Similarly, no one has ever come close to explaining why our fingers wrinkle when we have long baths. The explanation most often given is that wrinkling helps them to drain water better and improves grip. But that doesn’t really make a great deal of sense. Surely the people who most urgently need a good grip are those who have just fallen in water, not those who have been in it for some time.
Very, very occasionally, people are born with completely smooth fingertips, a condition known as adermatoglyphia. They also have slightly fewer sweat glands than normal. This would seem to suggest a genetic connection between sweat glands and fingerprints, but what that connection is has yet to be determined. As cutaneous features go, fingerprints are frankly pretty trivial. Far more important are your sweat glands. You might not think it, but sweating is a crucial part of being human. As Nina Jablonski has put it, “It is plain old unglamorous sweat that has made humans what they are today.”
Chimpanzees have only about half as many sweat glands as we have, and so can’t dissipate heat as quickly as humans can. Most quadrupeds cool by panting, which is incompatible with sustained running and simultaneous heavy breathing, especially for furry creatures in hot climates. Much better to do as we do and seep watery fluids onto nearly bare skin, which cools the body as it evaporates, turning us into a kind of living air conditioner. As Jablonski has written, “The loss of most of our body hair and the gain of the ability to dissipate excess body heat through eccrine sweating helped to make possible the dramatic enlargement of our most temperature-sensitive organ, the brain.” That, she says, is how sweat helped to make you brainy.
Even at rest we sweat steadily, if inconspicuously, but if you add in vigorous activity and challenging conditions, we drain off our water supplies very quickly. According to Peter Stark in Last Breath: Cautionary Tales from the Limits of Human Endurance, a man who weighs 155 pounds will contain a little over forty-two quarts of water. If he does nothing at all but sit and breathe, he will lose about one and a half quarts of water per day through a combination of sweat, respiration, and urination. But if he exerts himself, that rate of loss can shoot up to one and a half quarts per hour. That can quickly become dangerous. In grueling conditions—walking under a hot sun, say—you can easily sweat away ten and a half to twelve and a half quarts of water in a day. No wonder we need to keep hydrated when the weather is hot.
Unless the loss is halted or replenished, the victim will begin to suffer headaches and lethargy after losing just three to five quarts of fluid. After six or seven quarts of unrestored loss, mental impairment starts to become likely. (That is when dehydrated hikers leave a trail and wander into the wilderness.) If the loss gets much above ten and a half quarts for a 155-pound man, the victim will go into shock and die. During World War II, scientists studied how long soldiers could walk in a desert without water (assuming they were adequately hydrated at the outset) and concluded that they could go forty-five miles in 80-degree heat, fifteen miles in 100-degree heat, and just seven miles in 120-degree heat.