The history of sports extends back to the Ancient world in 7000 BCE. The physical activity that developed into sports had early links with warfare and entertainment. Study of the history of sport can teach lessons about social changes and about the nature of sport itself, as sport seems involved in the development of basic human skills (compare play). As one delves further back in history, dwindling evidence makes theories of the origins and purposes of sport more and more difficult to support. As far back as the beginnings of sport, it was related to military training. For example, competition was used as a mean to determine whether individuals were fit and useful for service. Team sports were used to train and to prove the capability to fight in the military and also to work together as a team (military unit).