For eccrine glands — the source of watery perspiration — density in humans was about 10 times higher than that of chimps or macaques. This finding adds to earlier research comparing proportions of eccrine with apocrine glands. Old World monkeys like macaques have equal parts eccrine and apocrine glands. Our closer relatives, chimpanzees and gorillas, bear roughly two eccrine for every one apocrine gland. And humans: almost percent eccrine. In hot conditions, most people can easily sweat one liter per hour or 12 liters a day.
Our highest recorded rates, about 3. And because humans lack thick fur, this sweat can freely evaporate, drawing heat away from the body.
Thus, it seems nearly bare skin and abundant eccrine glands are partnered adaptations for keeping us cool. Strengthening this link, eccrine glands and hair follicles, which derive from the same embryonic cell layer, are influenced by shared genetic pathways. In a study Kamberov and colleagues showed mouse feet grew varying proportions of eccrine glands or hairs, depending on activity levels of the En1 gene during development. These effects of this gene are thought to be very similar in humans.
Most researchers agree with the premise that fur-loss and eccrine gland-gain coevolved for thermoregulation during human evolution. The obvious next question, then, is: When exactly did that occur? Certainly after our last common ancestor with chimpanzees and before the origins of Homo sapiens. So that narrows it to sometime between about nine million and , years ago. Pubic hair may have been retained for its role in enhancing pheromones or the airborne odors of sexual attraction.
Sign up for our email newsletter. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Mark Pagel, head of the evolutionary biology group at the University of Reading in England and editor of The Encyclopedia of Evolution , fills us in: We humans are conspicuous among the 5, or so mammal species in that we are effectively naked.
However, the so-called body-cooling hypothesis fails to explain some aspects of human body hair patterns, such as why men tend to be hairier than women. Of note, humans are covered with tiny and colorless vellus hairs, except on the palms, soles of the feet, lips and nipples, Kamberov previously told Live Science. Hormones that emerge during puberty can transform some of these vellus hairs into longer, colored terminal hairs. But aside from this sometimes scruffy body hair, long hair tends to grow only on our heads.
The second theory, known as the aquatic ape hypothesis, proposes that ancient humans spent a lot of time in water. Fur weighed them down while swimming, so they gradually lost their hair. However, there is no evidence that humans spent a significant amount of time in water during the evolutionary past, so Pagel finds this hypothesis hard to believe. It also fails to explain why humans didn't evolve to gain their fur back after leaving the waterside. Pagel proposed the third theory, the ectoparasite hypothesis, in a study published in the journal Proceedings of the Royal Society B: Biological Sciences.
Ectoparasites are parasites that live on the outside of the host's body. Human hair is decidedly strange. Most of our body hair is so wispy and short as to be almost invisible, though in some places it is coarse and curly. Our head hair is almost uniquely long and flamboyant. We are pretty much the only animal to have hair that grows continuously for many years, and also to suffer the indignity of going bald.
No wonder our relationship with our hair is a tangled one. Human hair comes in two basic types: terminal hair, which grows on the scalp, eyebrows and eyelashes, and vellus hair, which is found everywhere else.
Beyond that, the main difference between hair types is how long they grow for before the follicle runs out of steam. This is what determines their length and thickness. Hair follicles go through cycles of growth and dormancy. During the growth phase the hair grows continuously at about 0. But at some point the hair-producing cells die off and growth stops. The hair falls out and the follicle goes dormant for around six months before sprouting new hair-producing cells and entering a new growth phase.
The length of the growth phase is controlled by hormones. Leg hairs grow for about two months, which is why they are short and fine.
0コメント