A primate that has a round head, a mobile neck, a large brain, a mobile and largely hairless face, with small ears, eyes directed forwards, no moist muzzle, and an undivided upper lip, belonging to the infra-order Simiiformes that also includes apes and humans. New World and Old World monkeys diverged evolutionarily about 40 million years ago. New World (platyrhine) monkeys comprise five families with flat noses and nostrils that open to the sides, and some have prehensile tails. Old World (catarrhine) monkeys have nostrils that open downwards; none have prehensile tails and some lack tails. There are about 260 known species.