Chapter 15
Primates originated in Asia and became recognisable in Africa around 55 Ma, quite early in the Eocene, although still not very distinct from other mammals of that time.1 During the Oligocene, a split developed between the colobine subfamily of monkeys, which are primarily leaf-eaters, and the more omnivorous cercopithecines. Colobine monkeys have sacculated stomachs, facilitating the fermentation of leaves along with unripe fruits but less complex than those of ruminants. Strangely, colobus monkeys have their thumbs reduced to stubs. The cercopithecine monkeys, or guenons, have simpler stomachs and consume a variable mix of fruits and young leaves, augmented by insects. They characteristically possess cheek pouches where food can be stuffed and chewed later in more secure places. Apes split off from the cercopithecine monkeys around 15 Ma during the Miocene, when forests were becoming more open.1,2,3 Besides lacking tails, they possess shoulder adaptations for brachiating through treetops. A putative stem to the hominids is a monkey named Proconsul, found both in Africa and Asia during the early Miocene.2 Another candidate for ape ancestor is Kenyapithecus, which consumed hard fruits and nuts gathered from forest floors in seasonally dry woodlands in Africa.4 Both showed adaptations for walking bipedally. Partly terrestrial monkeys, like the vervet (Chlorocebus pygerythrus), were also around at that time. The emergence of ground-dwelling primates was associated with the opening of the forest cover taking place in parts of equatorial Africa during the early portion of the mid-Miocene.
The great apes are placed in a separate family, the Hominidae. Very little is known about their ancestry because the wet forests that they inhabit are not conducive to fossil formation. Rather large teeth recovered from the Afar formation in north-eastern Ethiopia, dated to 8 Ma when forests were still present there, have been claimed to represent an ancestor of the gorilla.5 For chimpanzees, the only fossils known are a few teeth discovered in the rift valley in northern Kenya, along with hominin fossils, in deposits dated to the mid-Pleistocene. The split of the hominin lineage from its shared ancestor with the great apes evidently occurred during the obscure period in the late Miocene between 10 and 6 Ma.
By the start of the Pliocene 5 Ma, largely terrestrial baboons with their eclectic food habitats had emerged from monkey-like ancestors, first in southern Africa then spreading northward.1 In eastern Africa, ground-dwelling geladas made their appearance early in the Pliocene. One giant form (T. oswaldi) became primarily a grazer, like the modern gelada found in Ethiopia. It remained abundant in fossil faunas throughout Africa, as far south as the Cape, until quite late in the Pleistocene. Baboons became common in eastern Africa only after the demise of the giant gelada, suggesting competition for food between these two terrestrial primates.
Primates currently rival the ungulates in diversity of species, but remain associated primarily with tropical evergreen forests (Figure 15.1). They include generalist guenons, leaf-eating colobines, seed-crunching mangabeys, ground-dwelling baboons, baboon-like mandrills and the modern gelada. Mandrills (Mandrillus sphinx) are largely terrestrial scavengers moving in large herds, seeking fruits and other plant parts plus invertebrates on rainforest floors within central Africa. They are more closely related to mangabeys than to baboons. The modern gelada is narrowly restricted to high-elevation grasslands in Ethiopia, where it lives in large aggregations. Vervet monkeys are represented by various local species or subspecies throughout Africa from the Cape to Sudan and western Africa. They inhabit savanna woodlands and even strips of riverine thicket within karoo shrublands. The long-limbed, largely terrestrial patas monkey (Erythrocebus patas) has a restricted distribution in open grassy savanna in equatorial regions. Among the forest-inhabiting guenons, the most widely distributed is the blue monkey (Cercopithecus mitis), present in forest patches in subtropical South Africa in the form of the local subspecies known as the samango.
Figure 15.1
A miscellany of monkeys. (A) Blue monkey (Cercopithecus mitis); (B) red-tailed monkey (Cercopithecus ascanius); (C) red colobus monkey (Procolobus badius); (D) black-and-white colobus monkey (Colobus guereza); (E) Patas monkey (Erythrocebus patas); (F) Chacma baboon (Papio ursinus).
Baboons (Papio spp.) are widely distributed throughout Africa. Their broad habitat tolerance stretches from savanna woodlands to high-altitude grasslands in the Drakensberg foothills, semi-arid shrublands in the Cape Province, and river corridors through the Namib Desert. Geographically distinct populations connecting the chacma baboon of South Africa with the hamadryas baboon in Ethiopia have been assigned to distinct species, but interbreed and produce fertile offspring.6 Their lineage diverged in southern Africa ~2 Ma and has since shown repeated range shifts, fragmentation, isolation and reconnection of populations.
All three of Africa’s great apes are restricted to equatorial forests or woodlands (Figure 15.2). Chimpanzees are distributed from the forests of western Africa through the Congo Basin into Uganda, while bonobos (or pygmy chimpanzees) are found in tropical deciduous forest in a restricted region south of an upper tributary of the Congo River. Gorillas are represented by three distinct subspecies. The shaggy mountain gorilla is found in the region near the Virunga volcanoes stretching between Rwanda, Uganda and Congo DRC. It is replaced by eastern lowland gorillas in eastern regions of Congo DRC, and by western lowland gorillas more widely from Gabon through adjoining countries in west-central Africa.
Figure 15.2
The great apes. (A) Chimpanzee brachiating in Kigale Forest, Uganda; (B) mountain gorilla on forest floor in Bwindi Forest, Uganda.
Primates are numerous and diverse in tropical forests and woodlands in other continents. In Asia, various macaques and langurs spend much time on the ground, while apes, represented by the orang-utan and various gibbons, remain arboreal forest inhabitants. The monkeys occupying South and Central America have retained mostly a treetop existence in forests, although capuchins do spend time on the ground cracking nuts. None of them inhabits the grassy cerrado savannas prevalent there.
SUGGESTED FURTHER READING
Jablonski, N; Frost, S. (2010) Cercopithecoidea. In Werdelin, L; Sanders, WJ (eds) Cenozoic Mammals of Africa. University of California Press, Berkeley, pp. 393–428.
REFERENCES
1.Jablonski, NG; Frost, S. (2010) Cercopithecoidea. In Werdelin, L; Sanders, WJ (eds) Cenozoic Mammals of Africa. University of California Press, Berkeley, pp. 393–428.
2.Harrison, T. (2010) Dendropithecoidea, proconsuloidea, and hominoidea. In Werdelin, L; Sanders, WJ (eds) Cenozoic Mammals of Africa. University of California Press, Berkeley, pp. 429–469.
3.MacLatchy, LM. (2010) Hominini. In Werdelin, L; Sanders, WJ (eds) Cenozoic Mammals of Africa. University of California Press, Berkeley, pp. 471–540.
4.Benefit, BR. (1999) Biogeography, dietary specialization, and the diversification of African Plio–Pleistocene monkeys. In Bromage, TG; Schrenk, F (eds) African Biogeography, Climate Change, and Human Evolution. Oxford University Press, Oxford, pp. 172–188.
5.Suwa, G, et al. (2007) A new species of great ape from the late Miocene epoch in Ethiopia. Nature 448:921–924.
6.Fischer, J, et al. (2019) The natural history of model organisms: insights into the evolution of social systems and species from baboon studies. Elife 8:e50989.