Human hair growth
The three stages of hair growth are the anagen, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The rate or speed of hair growth is about 1.25 centimetres or 0.5 inches per month, or about 15 centimetres or 6 inches per year.
Long hair
Humans, horses, orangutans and lions are among the few species that may grow their head hair or manes very long. Humans are believed to have lost their fur 2.5–3 million years ago as hominids when transitioning from a forest habitat to the open savanna, as an effect of natural selection, since this development made it possible to run fast and hunt animals close to the equator without getting overheated. An exception was however head hair, which was kept to provide thermal insulation of the scalp from the sun, to protect against ultra-violet radiation exposure, and also to provide cooling (when sweat evaporates from soaked hair). The ability to grow straight hair has been observed among Homo sapiens sub-groups in less sunny regions further away from the equator. Relative to kinked Afro-textured hair, straight hair allows more UV light to pass to the scalp (which is essential for the production of vitamin D, that is important for bone development).