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. Head hair was an exception, which was a survival trait because it provides thermal insulation of the scalp from the sun, protects against ultra-violet radiation exposure, and also provides 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).