Primates are an order of mammals. Most primates are characterized by well-developed binocular vision, a flattened, forward-oriented face, prehensile digits, opposable thumbs (sometimes the first and second digits on the feet are also opposable), five functional digits on the feet, nails on the tips of the digits (instead of claws), a clavicle (or collarbone), a shoulder joint allowing free movement of the arm in all directions, a tail (except for apes), usually only two mammae (or teats), relatively large development of the cerebral hemispheres of the brain, usually only one offspring born at a time, and having a strong social organization. Most species of primates are highly arboreal (that is, they live in the forest canopy), but some live mostly on the ground. Primates first evolved early in the Cenozoic Era, about 60 million years ago. The ancestral stock of the primates is thought have been small, carnivorous animals...