Monkeys are a large and diverse mammal group that includes most primates. Humans, chimpanzees (Pan troglodytes) and other apes share an ancestor with monkeys but belong to a separate group of primates that diverged from monkeys millions of years ago. Monkeys are typically smaller than apes and usually have tails, which apes lack. The monkey group doesn't include lemurs, which are another branch of the primate family tree and live in Madagascar.