A primate is a mammal of the order Primates (Latin: "prime, first rank"). In taxonomy, primates include two distinct lineages, strepsirrhines and haplorhines. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment. Most primate species remain at least partly arboreal. 

With the exception of humans, who inhabit every continent, most primates live in tropical or subtropical regions of the Americas, Africa and Asia. They range in size from Madame Berthe's mouse lemur, which weighs only , to the eastern gorilla, weighing over ; that is, without taking into account the weight of particular human individuals, reaching up to . Based on fossil evidence, the earliest known true primates, represented by the genus "Teilhardina", date to 55.8 million years old. An early close primate relative known from abundant remains is the Late Paleocene "Plesiadapis", c. 55–58 million years old. Molecular clock studies suggest that the primate branch may be even older, originating near the Cretaceous–Paleogene boundary or around 63–74 mya. 

The order Primates was traditionally divided into two main groupings: prosimians and anthropoids (simians). Prosimians have characteristics more like those of the earliest primates, and include the lemurs of Madagascar, lorisoids, and tarsiers. Simians include monkeys, apes and hominins. More recently, taxonomists have preferred to split primates into the suborder Strepsirrhini, or wet-nosed primates, consisting of non-tarsier prosimians, and the suborder Haplorhini, or dry-nosed primates, consisting of tarsiers and the simians.
Other than humans, what's the largest?
the eastern gorilla