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.
Body hair
P.E. Wheeler of the Department of Biology at Liverpool Polytechnic said quadrupedal savannah mammals of similar volume to humans have body hair to keep warm while only larger quadrupedal savannah mammals lack body hair, because their body volume itself is enough to keep them warm. Therefore, Wheeler said humans who should have body hair based on predictions of body volume alone for savannah mammals evolved no body hair after evolving bipedalism which he said reduced the amount of body area exposed to the sun by 40%, reducing the solar warming effect on the human body.