This paper deals with some of the implications of the discovery of four ape-like teeth from the Middle Miocene (12,5 Ma) and Late Miocene (6–5,9 Ma) of Kenya. An unworn, isolated lower molar from Member B of the Ngorora Formation (12,5 Ma), Tugen Hills, Kenya, differs markedly from lower molars of Middle and Early Miocene large hominoids but is closer in morphology to chimpanzee molars (peripheralised cusps, buccolingually compressed lingual cusps, thin enamel, large and deep occlusal basin, reduced buccal cingulum). If the tooth is part of the chimpanzee clade then it is important for estimating the timing of the dichotomy between chimpanzees and hominids and suggests that this event would have occurred several million years earlier than is currently estimated by most researchers.
An incomplete, unworn isolated upper molar, an upper central incisor and a lower molar from the Lukeino Formation (6–5,9 Ma), Tugen Hills, Kenya, are morphologically closer to those of Gorilla gorilla than to any other fossil or extant hominoid with which they were compared. The upper molar is a large tooth (mesio-distal length 14 mm) with peripheralised cusps, bucco-lingually wide distal fovea, fairly voluminous trigon basin and high dentine penetrance, all features which suggest affinities with gorillas. Its enamel thickness (1,6–1,7 mm on the hypocone) is similar to that of gorilla molars. It differs markedly from molars of the early hominid, Orrorin tugenensis, which occurs at the same site, which are smaller, have more centralised cusps, smaller trigon basin, reduced distal fovea and low dentine penetrance. The Kapsomin molar differs from teeth of australopithecines for much the same reasons, even if its dimensions overlap with those of Australopithecus antiquus and Praeanthropus africanus. It is highly divergent from chimpanzee teeth, not only in its dimensions, but also in its morphology. An upper central incisor from Kapsomin is large and wedge-shaped in lateral view without the lingual fossa that characterises teeth of hominids and chimpanzees. It is close in size and morphology to those of gorillas.
If the species from which the Kapsomin and Cheboit teeth came is part of the gorilla clade, then there are important implications for the timing of events in gorilline evolution, and they make it less likely that European genera such as Ouranopithecus are ancestral to African apes or hominids.