A tree house, tree fort or treeshed is a platform or building constructed around, next to or among the trunk or branches of one or more mature trees while above ground level. Tree houses can be used for recreation, work space, habitation, observation or as temporary retreats. Building tree platforms or nests as a shelter from dangers on the ground is a habit of all the great apes, and may have been inherited by humans. It is true that evidence of prehistoric man-made tree houses have never been found by paleoanthropologists, but remains of wooden tree houses would not remain. However, evidence for cave accommodation, terrestrial man-made rock shelters, and bonfires should be possible to find if they had existed, but are scarce from earlier than 40000 years ago. This has led to a hypothesis that archaic humans may have lived in trees until about 40000 years ago. Even today, treehouses are built by some indigenous people in order to escape the danger and adversity on the ground. In some parts of the tropics, houses are either fastened to trees or elevated on stilts to keep the living quarters above the ground to protect occupants and stored food from scavenging animals. The Korowai, a Papuan tribe in the southeast of Irian Jaya, live in tree houses, some nearly 40 metres (130 ft) high, as protection against a tribe of neighbouring head-hunters, the Citak.