 Humans , horses , and <unk> are among the few species that may grow their head hair very long . Humans are believed to have lost their fur 2 @.@ 5 – 3 million years ago 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 kinky 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 ) . 
