Ape and monkey

*For the purposes of this note the word ape means all animals closer to or within the clade containing the gibbon and man than to the clade containing the langur and the macaque. Monkey means all animals closer to or within the clade containing the langur and the macaque than to the clade containing the gibbon and man. Any animal closer to the monkeys and apes to the exclusion of the New World platyrrhines, but not a monkey or an ape as defined above, is termed a stem catarrhine.

New fossils from the Rukwa Rift in the Tanzanian part of Rift Valley described by Stevens et al are of interest with respect to the timing of the split between apes and monkeys. One of the most basal catarrhines, Catopithecus, was found in Fayum, Egypt, dating to about 34 Mya, i.e., the beginning of the Oligocene. Molecular evidence strongly suggests that the divergence between apes and monkeys happened sometime between 30-25 Mya. However, between 34 Mya, which is the age of the one of the earliest stem catarrhines, down to the earliest fossil monkey, Prohylobates (18 Mya), and the extremely archaic ape, Kalepithecus (~20 Mya), there are no fossils of clear cut apes or monkeys. Thus, we have lacked fossil from the critical period around the time when the molecular evidence suggests they diverged. This does not mean that there are no catarrhine fossils from the 34-20 Mya time window. We have basal catarrhines from the Fayum beds in Egypt such as Aegyptopithecus from around 30 Mya. We also have an enigmatic radiation of basal catarrhines that appears to have begun around 22 Mya, the pliopithecoids, and persisted till at least 7.5 Mya. The earliest member of this clade is Lomorupithecus from Uganda and the latest is Laccopithecus from Lufeng, China dating to the late Miocene. This suggests that the pliopithecoid radiation of catarrhines migrated to Asia and persisted alongside other more derived catarrhines including apes and monkeys. Closer to the crown group comprised of apes and monkeys are two fossils namely Saadanius, from what is today the hellhole of Saudi Arabia (~28 Mya), and Kamoyapithecus from Kenya (~25 Mya). Then there are two clades that cannot be specifically placed as being closer to or earlier branching than either monkeys or apes. These are Limnopithecus evansi and the dendropithecid clade comprised of Dendropithecus, Micropithecus and Simiolus and Limnopithecus legetet. Better fossils would probably be required to tell whether they are apes or monkeys or fall just outside of both of them. While they preserve anatomical features that might have been close in certain respects to the common ancestor of the apes and monkeys, all of these are from around 20-17 Mya, so they are not temporally very close to the branch point of the crown clades.

If these proposed affinities of Nsungwepithecus and Rukwapithecus are true then it would mean that the ape-monkey split happened before 25.2 Mya, pushing the paleontological evidence for this split within the range estimated from the molecular evidence. In our opinion Stevens et al convincing demonstrate anatomical similarity between Nsungwepithecus and the basal monkeys of the victoriapithecid clade (which includes forms like Victoriapithecus and Zaltanpithecus). Of these Victoriapithecus is known from well-preserved remains from around 15 Mya and it a mid-sized monkey. Nsungwepithecus is clearly larger than Victoriapithecus and suggests that such a lineage of monkeys had persisted for more than 10 Myr. Likewise there are distinct similarities between Rukwapithecus and Rangwapithecus thus pulling it into the Nyanzapithecine/Mabokopithecine clade. Given that Rangwapithecus is from about 20 Mya, the discovery of Rukwapithecus suggests that a such a clade of basal apes was more than 5 Myr older. Given that the latest member of this clade Nyanzapithecus harrisoni is from around 13 Mya, this clade was rather long-lived (more than 12 Myr). Thus, at their upper end they overlapped temporally with more derived apes such as Morotopithecus and Afropithecus which are closer to the crown formed by extant lesser and great apes.

Continued…