It was erroneously supposed that this form was related to hylobates.
"Darwin and Modern Science"
A.C. Seward and Others
The long-armed apes occasionally use their arms like crutches, swinging their bodies forward between them, and some kinds of hylobates, without having been taught, can walk or run upright with tolerable quickness; yet they move awkwardly, and much less securely than man.
"The Descent of Man and Selection in Relation to Sex, Vol. I (1st edition)"
Charles Darwin
This curious arrangement, so unlike that in most of the lower mammals, is common to the gorilla, chimpanzee, orang, some species of hylobates, and even to some few American monkeys.
"The Descent of Man and Selection in Relation to Sex, Vol. I (1st edition)"
Charles Darwin