As an Order of mammals, primates are most closely related to three other groups of mammals, tree shrews (Scandentia) from Asia, flying lemurs (Dermoptera) from Asia and the extinct Plesiadapiformes from the latest Cretaceous through Eocene of North America and Eurasia. It is not clear at present which of these is the sister taxon of Primates. However, the very diverse plesiadapiforms have long been regarded as early primates and were the most primate-like mammals from the early part of the Cenozoic Era. Several theories have been proposed to describe the behaviors of the earliest primates that selected for the unique features of primate anatomy, including our grasping hands and feet with nails rather than claws, and relatively large, forward-facing orbits surrounded by a bony ring. These include the Visual Predation Theory, the Angiosperm Predation Theory, and the Nocturnal Leaping Theory.
