Simians are a group of primates that includes monkeys, apes, and humans. The term “simian” comes from the Greek word for ape, “simos”. Simians are characterized by their large brain size relative to body size, as well as their opposable thumbs.
Humans are the only extant members of the simian line, but there were many other simian species in the past. Some of these extinct simians include the giant apes Gigantopithecus and Meganthropus, as well as the smaller monkey-like Proconsul and Dolichopithecus.
Simians first appeared on Earth during the Eocene epoch, around 55 million years ago. They likely evolved from an ancestor that was closely related to lemurs and lorises. The earliest simians were small creatures that lived in trees and ate insects. Over time, they evolved into larger animals with more complex diets.
Today, there are two main groups of simians: New World monkeys and Old World monkeys. New World monkeys live in Central and South America, while Old World monkeys live in Africa and Asia. New World monkeys include species such as marmosets, tamarins, capuchins, spider monkeys, and squirrel monkeys. Old World monkeys include species such as macaques, baboons, langurs, colobus monkeys, and guenons. Humans are also classified as an Old World monkey species.
There is some debate over whether apes should be considered simians or not. Apes are very similar to humans in terms of anatomy and behavior; however, they do not have opposable thumbs (with the exception of gibbons). Additionally, some scientists believe that apes should be placed in a separate category due to their unique evolutionary history. For this reason, some sources only consider humans to be true simians .