During the early Miocene (~25–17 million years ago), in the warm, humid forests of East Africa, a quiet but decisive shift unfolded in primate anatomy. A branch separated from the tailed Old-World monkeys (Cercopithecoidea): primates without a tail, with broad shoulders, thickened wrists, and long arms — features adapted to brachiation (arm-swinging through branches) and suspensory locomotion. This branch is the superfamily Hominoidea, and it is the root of the lineages that would later become orangutans, gorillas, chimpanzees, and humans.

The best-documented early representative of this transformation is the genus Proconsul. Skeletons recovered from Rusinga Island and Songhor in Kenya show a body plan more robust than that of a modern ape: limbs not yet sharply differentiated, walking quadrupedally along branches but with no tail, ranging in body size from monkey-like to gibbon-sized across the group. Genera like Proconsul, Afropithecus, Morotopithecus, and Ekembo trace this early radiation.

The early-Miocene Hominoidea was not a single lineage; African forests of that period carried far more ape diversity than today. By 18–14 million years ago, with the connection between Africa and Eurasia open, apes spread into Europe and Asia (Sivapithecus, Dryopithecus). When mid-Miocene cooling contracted the forests, ape diversity collapsed — but a thread persisted in Africa, eventually splitting from the chimpanzee–bonobo lineage about 7 million years ago to give the Homininae (the lineage that includes humans).