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Primates make up a biological order, a tier of taxonomic classification significantly above species but below class and phylum. Primates are also a clade, meaning they descend from a common ancestor, which is thought to have lived more than 65 million years ago, when dinosaurs still roamed the earth. They are classified into three main groups: New World monkeys, small primates that live in the Americas; Old World monkeys and apes, which live exclusively in Africa, except for humans which live most everywhere, and orangutans which live in Indonesia and Malaysia; and prosimians, the most primitive primates. The most well-known prosimian is the lemur, which lives on Madagascar, though other prosimians can be found in small quantities in Southeast Asia.
Primates used to be divided into simians and prosimians. Simians are the larger, more human like primates such as apes and monkeys, while prosimians are smaller and more closely resemble rodents. Later it was found that the family Tarsiidae (tarsiers), previously labeled prosimians, was genetically closer to the simians, and therefore got lumped into the same suborder with them. So, the order Primates consists of two suborders - suborder Strepsirrhini, the non-tarsier prosimians, and suborder Haplorrhini, the tarsiers, monkeys, and apes.
The suborder Haplorrhini is further divided into two infraorders - Tarsiiformes (tarsiers) and Simiiformes (Old and New World monkeys). Simiiformes is broken down into two parvorders - Platyrrhini (New World monkeys) and Catarrhini (Old World monkeys). Platyrrhini contains over 125 unique species, including howler, spider and wolly monkeys, night and owl monkeys, tamarins, and many more. Catarrhini is divided into two superfamilies, Cercopithecoidea (Old World monkeys, about 135 species) and Hominoidea (gibbons and humans, about 20 species). There are over 378 primate species currently recognized, with a few new species being discovered per year.
Because it was only relatively recently that it was realized that tarsiers are more closely related to the simians, the older classification, which divides Primates into the suborders Prosimii and Anthropoidea, can still be found in many textbooks and web sites on the Internet. There is still disagreement among primatologists as to what the true classification should be, but the Strepsirrhini/Haplorrhini division is the direction in which acceptance is moving.
The Old World monkeys and New World monkeys parted ways about 45 million years ago. The New World monkeys crossed the Bering land bridge during an ancient Ice Age, bringing the Primates to two entirely new continents. During this time, prosimians could also be found over a much wider geographic region than they can be found today, and included large portions of Europe and Asia. Competition with simians forced many prosimian species into obscurity or extinction.
About 25 million years ago, the Old World monkeys (Cercopithecidae) diverged from the apes and gibbons (Hominoidea). The gibbons ("lesser apes") separated from the apes and humans ("great apes") about 18 million years ago. The great apes consist of gorillas, chimpanzees, orangutans, and humans, the species that are the most obviously human like. Because of their significant intelligence, there are groups in many countries that say all great apes should be regarded as persons, with certain basic rights like freedom from being experimented on. The most famous of all primate species, of course, is the familiar Homo sapiens sapiens.