Deciphering the early evolution of anthropoid primates
Deciphering the early evolution of anthropoid primates has been challenging paleontologists for decades. The Myanmar-French paleontological team, which has been tackling this problem in Myanmar for more than two decades, provides new insights into early anthropoid evolution in Nature Communications.
Anthropoids are a group of primates currently represented by New World and Old World monkeys, apes and humans (Fig. 1). They are distinguished from others groups of primates such as strepsirrhines (lemurs and lorises) by several dental and cranial characters (postorbital closure, shorter muzzle, increased vision due to more frontated orbits, fused frontal bones…). Deciphering the early evolution of anthropoid primates has been challenging paleontologists for decades over several continents. The recognition of fossil anthropoids is difficult because they display only a part of the features of extant anthropoids. Undoubted anthropoids have been traced back to the Eocene period and are as old as 45 Ma (ref. 1) but their early evolution is still poorly documented.
During several decades, anthropoids have been considered as originating from the Afro-Arabian continent where they have been recorded in the Eocene-Oligocene periods (39-32 Ma). However, successive discoveries in older localities (45-40 Ma) of more primitive anthropoids in China and Myanmar rather supports an Asian origin for this group. The Myanmar-French paleontological has been working in the Eocene of Myanmar for two decades in order to untangle the early evolution of the anthropoids in Asia. The work of excavation realized during the field campaigns 2015-2017 (Fig. 2) has led to the discovery of a new 40 million-year-old primate, Aseanpithecus myanmarensis, which further supports and refines the scenario of an early colonization of Africa by anthropoids originating from Asia (ref. 4; pdf: https://rdcu.be/bNcah).
Aseanpithecus does not belong to any known group of Asian anthropoids and thus may represent a new family, phylogenetically close to African crown anthropoids (all fossils more closely related to extant anthropoids) and their close fossil relatives. It exhibits a mosaic of primitive orbit features and derived dental characters several of which are found only among the African fossil crown anthropoids. This combination of characters delivers precious information about the sequence of appearance of modern characters among anthropoids by indicating that the dental features evolved earlier than those of the orbit, confirming former inferences from phylogenetic analyses (ref. 5). Additionally, the discovery suggests that the colonization of Africa by anthropoids involved not only primitive anthropoids like Afrasia/Afrotarsius (ref. 6) as previously envisaged (ref. 7) but also more advanced forms and may have implied several dispersal events.
Some other groups of mammals of Asian origin (hystricomorph and anomalurid rodents, anthracothere cetartiodactyls) also arrived diachronically in Africa during the Eocene and testify to multiple Asia to Africa dispersal events some of which involved anthropoids. Paleogeographic reconstructions indicate that a narrow Tethys ocean was separating the Eurasian and Afro-arabian continents during the Eocene, raising the problem of the dispersal path of these land mammals. Among the various envisaged scenarios of dispersal, the Iranian microplate may have served as path of dispersal across the Tethys. An alternative hypothesis is that of natural rafts of floating vegetation release by large rivers.
Future discoveries in Asia will refine the early evolution of anthropoids, which appear more and more as a group of primate having experienced rapid diversification and impressive geographic range extension during the Eocene.
Written by Olivier Chavasseau and Jean-Jacques Jaeger (University of Poitiers).
1 Beard, K. C., Qi, T., Dawson, M. R., Wang, B. & Li, C. A diverse new primate fauna from middle Eocene fissure-fillings in southeastern China. Nature 368, 604-609 (1994).
2 Williams, B. A., Kay, R. F. & Kirk, C. E. New perspectives on anthropoid origins. Proc. Natl. Acad. Sci. USA 107, 4797-4804 (2010).
3 Coster, P. et al. Uniquely derived upper molar morphology of Eocene Amphipithecidae (Primates: Anthropoidea): homology and phylogeny. J. Hum. Evol.65, 143-155 (2013).
4 Jaeger, J.-J. et al. New Eocene primate from Myanmar shares dental characters with African Eocene crown anthropoids. Nat. Commun. (In press).
5 Ross, C. F., Williams, B. A. & Kay, R. F. Phylogenetic analysis of anthropoid relationships. J. Hum. Evol., 221-306 (1998).
6 Chaimanee, Y. et al. Late Middle Eocene primate from Myanmar and the initial anthropoid colonization of Africa. Proc. Natl. Acad. Sci. USA 109, 10293-10297 (2012).
7 Marivaux, L. et al. A morphological intermediate between eosimiiform and simiiform primates from the late Middle Eocene of Tunisia: macroevolutionary and paleobiogeographic implications of early anthropoids. Am. J. Phys. Anthropol. 154, 387-401 (2014).