The largest sample of early catarrhines come from the Fayum depression in present-day Egypt. Today this region is arid desert, but during the Oligocene around 34 million years ago, it was a swampy forest with a great density of ancient primates. Aegyptopithecus zeuxis was a small primate, around 6 kg, with essentially apelike teeth, including broad flat incisors, low molars with somewhat bulbous cusps, and sexually dimorphic canines. These dental features are more similar to living apes than to Old World monkeys, but because the distinctive shearing molars of cercopithecoids evolved later, Aegyptopithecus probably represents the ancestral condition for all catarrhines. Unlike living apes, the molar teeth had a broad extra ridge, called a cingulum, surrounding the main cusps, which increased the grinding area of the teeth.
The postcranial skeleton of Aegyptopithecus was basically monkey-like, with short, non-suspensory forelimbs and a tail. The skull had many features found in later hominoids, including strong temporalis muscle attachments, forming a low sagittal crest in most individuals. The two living catarrhine superfamilies, the hominoids and the cercopithecoides, may have diverged before Aegyptopithecus existed or after. Since Aegyptopithecus shows no derived similarities to either group, it may be very similar to the primitive catarrhine lineage that gave rise to both living groups, even if it represents an early hominoid or cercopithecoid.