December 23. Primates

The end of the Paleocene and start of the Eocene was apparently a stimulating time, in terms of evolutionary diversity for the mammals. The oldest known primate-like animal dates to this time, from lake beds in France and from North America, about 56 million years ago. Plesiadapis was a small lemur-like animal adapted to climbing trees and eating plants, although it may have been an opportunistic omnivore. Its ancestors are uncertain, but it probably evolved in North America and migrated to Europe.

Mesopithecus (Gaudry, 1867)

By Oligocene time, the lineages of both the Old World and New World monkeys had been established. Many fossils have come from the East African rift valleys, also the source of many hominid fossils. The lake deposits there preserve these rare remains relatively well. 

The Miocene, around 12 to 18 million years ago, saw the divergence of groups such as the great apes, orangutans, and gibbons. By very late Miocene, 5 to 7 million years ago, many monkeys were probably quite recognizably modern in appearance. There’s an excellently preserved example called Mesopithecus, found in Greece, that lived about that time and likely lived on fruits and leaves. It was about 16 inches long.

The primitive primates of Madagascar, lemurs and their kin, present a challenge to plate tectonics. Madagascar was already separated from the other continental masses by Paleocene, when these animals developed. So how did they get there? In fact there are only five orders of land mammal on Madagascar today. The favored idea has usually been rafting – drifting of land animals on floating mats of water-borne debris, though that idea relies on a lot of luck. The luck depended on a relatively short window of geologic time, between 60 and 20 million years ago, when the distances and currents would make the rafting idea plausible. The best estimate for the colonization of Madagascar by primates is probably early to middle Eocene time, 40 to 52 million years ago. 

Alternatives to the rafting idea include the presence of an ancestral primate on Madagascar, but none have been found, and “island hopping,” crossing the distance in short spurts to intervening islands – but the likelihood of such islands is low. I don’t think there is unanimous agreement as to how primates got to Madagascar, but for now I think the rafting idea is most likely, even if it was actually pretty unlikely. Just more likely than the other possibilities. Once they got there, the lemurs and other primates probably survived because they lacked the competition from the more advanced monkeys that developed in Africa by Oligocene time.

—Richard I. Gibson

Lemur origins 

Reconstruction of Mesopithecus from A. Gaudry, Animaux Fossiles et Géologie de l’Attique. Recherches faites en 1855–1856, 1860 (1867), public domain.