Friday, 31 May 2013

The origin of apes

The Rift Valley Kenya (Wikimedia Commons)

Trees based on molecular data often predict divergence points at odds with the fossil record. Thus the divergence between Old World monkeys (Cercopithecoidea) and apes (Hominoidea) has been dated to 25-30 Mya, implying a long ghost lineage for both superfamilies. New fossils from the East African Rift have now closed the gap (here). One (Nsungwepithecus gunnelli) is a cercopithecoid. The other (Rukwapithecus fleaglei) shares dental features with Miocene and extant hominoids that are not present in cercopithecoids.
Pregnant uterus of Hylobates agilis (rafflei) showing the decidua
capsularis (d.c.) reproduced by Hill (here) from Selenka

Implantation is superficial in Old World monkeys, whereas interstitial implantation occurs in all living apes including gibbons. This was shown more than a century ago by Emil Selenka. The illustration above is of Hylobates rafflei named in honour of Stamford Raffles (previous post) but now subsumed in H. agilis.

Trophoblast invasion by the interstitial route occurs neither in Old World monkeys nor in gibbons so post dates the origin of apes. This key feature of human placentation is shared by gorilla and chimpanzee (discussed here). No information is available for orang utan.

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