Humans belong to the group of mammals known as primates. Most primates are arboreal. Some primate species feed on insects while others feed on fruits and other plant material. Primates have a number of adaptations that evolved in response to an arboreal existence. They have an opposable thumb for grasping limbs, a flattened face which helps with stereoscopic vision, and long limbs which rotate at the hips and shoulders for swinging through the trees.