Humans are closest to Old World monkeys, which are part of the Simiiformes variety of primate species. Old World monkeys include baboons, macaques, and brief and colobus monkeys, and they are differentiated from the New World primates like marmosets, tamarins, and South American monkeys.
Old and New World monkeys share certain traits, like prehensile tails and color vision, but Old World monkeys have characteristics that make them more closely related to humans, including reduced snouts and complex social behaviors.