Hair is a filamentous outgrowth of protein, found only on mammals. It projects from the epidermis, though it grows from hair follicles deep in the dermis. Although many other organisms, especially insects, show filamentous outgrowths, these are not considered "hair". So-called "hairs" (trichomes) are also found on plants. The projections on arthropods, such as insects and spiders are actually insect bristles. The hair of non-human mammal species is commonly referred to as fur. There are varieties of cats, dogs, and mice bred to have little or no visible fur. In some species, hair is absent at certain stages of life.

The primary component of hair fiber is keratin. Keratins are proteins, long chains (polymers) of amino acids.

Demodex is a genus of tiny parasitic mites which live in or near hair follicles of mammals.

Human hair

Body hair

Historically, several ideas have been advanced to describe the reduction of human body hair. All were faced with the same problem that there is no fossil record of human hair to back up the conjectures nor to determine exactly when the feature evolved. However, recent research on the evolution of lice suggests that human ancestors lost their body hair approximately 3.3 million years ago.[1]

Savanna theory suggests that nature selected humans for shorter and thinner body hair as part of a set of adaptations to the warm plains of the savanna, including bipedal locomotion and an upright posture. There are several problems with this theory, not least of which is that cursorial hunting is used by other animals that do not show any thinning of hair.

Another theory for the thin body hair on humans proposes that Fisherian runaway sexual selection played a role here (as well as in the selection of long head hair). Possibly this occurred in conjunction with neoteny, with the more juvenile appearing females being selected by males as more desirable; see types of hair and vellus hair.

The aquatic ape hypothesis posits that sparsity of hair is an adaptation to an aquatic environment, but it has little support amongst scientists and very few aquatic mammals are, in fact, hairless.

In reality, there may be little to explain. Humans, like all primates, are part of a trend toward sparser hair in larger animals; the density of human hair follicles on the skin is actually about what one would expect for an animal of equivalent size[2]. The outstanding question is why so much of human hair is short, underpigmented vellus hair rather than terminal hair.