Orangutan skull. Orangutans (sp. Pongo pygmaeus)comprise two tree- dwelling species of great ape with reddish-brown hair, the Borneo species (P. p. pygmaeus) and the Sumatran species (P. p. abelii). Both species are endangered. Males are approximately twice the size of females. Orangutans are the most distantly related of the great apes to humans, and are thought to have diverged from humans in evolutionary terms about 10-13 million years ago.