Mammals are the only animals with hair. The body hair of mammals has several uses. Hair helps in a mammal's thermoregulation, camouflage (patterns and coloration), defense (porcupine spines), skin protection, absorption and reflection of sunlight, sexual dimorphism, attracting mates, and is also a sensory structure (sinus hairs = whiskers). Mammals that are labeled "hairless" (rhino, pigs, dolphins, whales, hippos, elephants, etc) actually have very short fine hair.

Mammal hair, although complex, contains three basic structures: root, follicle, and the shaft. It was theorized that it evolved scales of reptiles. Fossil records of both scales and hair preserves in amber (especially in Baltic and Dominican ambers) do not sufficiently support this theory but could classify the Order, Family and Genus of the specimen.

The evolution of hair from fur is not yet known. One speculation is that the evolution from fur to hair occurred because the latter is an excellent heat insulator.

Much like the chick and egg scenario (which occurred first), the relationship between warm-bloodedness (endothermy) and the hair evolution from fur has not been discovered. It was debated whether hair evolved because the mammal's need for warmth because of it warm-bloodedness or endothermy occurred because of the occurrence of hair.

Hair is well-preserved. It has been preserved in ambers, fossils, ice, and tar pits. Hair can resist chemical decomposition because of its keratin components. Keratin cuticles cover the hair. Keratin is a protein and as such, hair is a valuable evidence in the field of Forensics. Hair has also been a valuable tool in studying human evolution since its structural features remain unchanged over the years.