Primates

Primates are one group (order) of mammals that evolved about 65 million years ago (mya). The early primates were probably small tree-dwelling (arboreal) animals that hunted for insects at night. Today we recognize at least 167 living species in the order primates. We can classify these species (and the primate fossils we find) into two major groups (suborders): prosimians (at least 38 species) and anthropoids (at least 129 species). Prosimians include lemurs (Lemuriformes), galagos and lorises (Lorisiformes) and tarsiers (Tarsiiformes). Anthropoids include New World monkeys (Platyrrhini; from South America), Old World monkeys (from Africa and Asia), apes and humans (Catarrhini). However, some scientists feel that tarsiers should be classified as anthropoids rather than as prosimians.

The first primates to evolve were prosimians. These early primates split into several subgroups about 55 mya. One lineage (see graph) gave rise to Lorisiformes and Lemuriformes, and the other lineage gave rise to Tarsiiformes and the ancestors of all other primate groups (anthropoids). The first New World monkeys probably evolved about 45 mya, followed by Old world monkeys about 40 mya. The first apes appeared about 25 mya, human ancestors (e.g. Australopithecus) about 4 mya, and humans (Homo) about 2 mya.

Many modern prosimians still resemble the early primates in appearance and life style. For example galagos, lorises, tarsiers and some lemurs (e.g. from the dwarf lemur family) are all small arboreal animals that are active at night (nocturnal), hunting for insects or feeding on gum, tree sap, fruit, flowers or leaves. Nocturnal prosimians range in size from 60 g (mouse lemur) to about 2800 g (aye-aye). Only the larger lemurs (about 2-10 kg) have made the transition to being active during the day (diurnal). We assume that their increased size allowed them to make this transition because larger animals are less susceptible to being caught by diurnal predators. Diurnal lemurs mostly feed on fruits, flowers or leaves, and some of them may even come down from the trees to travel on the ground (e.g. ring-tailed lemurs and sifakas). The largest prosimians that ever lived were gorilla-sized lemurs on the island of Madagascar. However, when humans colonized the island about 1000 years ago these large lemurs were hunted to extinction. Lemuriformes occur exclusively on the island of Madagascar. Lorisiformes can be found all over Africa and Asia, and Tarsiiformes are restricted to Asia.

All anthropoids (monkeys, apes and humans) are diurnal with one exception: the owl monkey from South America. Monkeys occur in the Old and New World, but apes are restricted to Africa and Asia. Humans probably originated in Africa, but are now distributed worldwide. Monkeys and apes range in size from about 150g (pigmy marmosets) to about 180 kg (gorillas). They have variable diets including insects, fruits, leaves, meat and grains. Among the Old-World monkeys and apes we find many terrestrial species that have left the trees for the ground. One advantage of terrestrial life is that these primates can now use their hands more freely to manipulate objects rather than holding on to branches, and that they can gather in larger social groups.

The social life of primates is much more complex than that of other mammals. Among primate species we find a wide range of gregariousness from the small nocturnal primates who spend most of their time looking for food alone, only occasionally meeting others (e.g. mouse lemurs), to primates living in small family groups (e.g. tamarins), and to those that live in large social groups (e.g. baboons) with many individuals of different ages, sex, dominance ranks and relatedness. Individuals within such large groups may compete with each other for food and mating partners, they may collaborate together against others, and may even form friendships.

Many of the primate characteristics that we see in primates today (e.g. opposable thumbs, nails instead of claws, forward facing eyes) probably evolved as an adaptation to life in trees. For example primate hands have opposable thumbs that allows them to grasp tree branches, but also allows the handling of tools in apes and humans. The replacement of claws by nails went along with an increased sensitivity of the hands and fingertips and an improved ability to manipulate objects. During primate evolution we also seea trend from having eyes on the side of the face, as most lemurs do, to having both eyes up front as in monkeys, apes and humans. Moving the eyes up front reduces the visual field on the side of the head, but it increases the overlap of the area seen by both eyes simultaneously, allowing for improved depth perception. Now distances (e.g. between branches) can be better estimated, and vision overall is improved. As the eyes are moved up front we also see a shortening of the snout and a reduction of the sense of smell. As a consequence visual signals are now replacing chemical signals as the main form of social communication.

There are still many open questions about primate evolution and how primates became dispersed to the localities where we find them today. For example we do not know how lemurs reached Madagascar. When prosimians evolved in Africa about 65 mya, Madagascar had already separated from the mainland. One possibility that is being discussed is that a few prosimians may have reached Madagascar by rafting on large trees floating in the ocean, and upon arrival, gave rise to all the lemur species. As an alternative our estimated timeline of primate evolution may be incorrect and prosimians may have evolved much earlier. However, this is highly unlikely and not supported by fossil evidence. A similar problem is posed by the exclusive presence of New World monkeys on the South American continent, which was isolated from Africa and North America when anthropoids evolved. Rafting from North America or from Africa is being discussed, but morphological evidence and the direction of past ocean currents seems to favor Africa as the origin of New World monkeys. And finally, the discovery of new anthropoid fossils from Asia in the 1990s suggests that the first anthropoids may have evolved in Asia rather than in Africa.

In addition to the dispersal questions there are still many puzzling questions about the evolution of the diverse primate social systems. For example researchers are studying which factors may determine whether primates live alone or in groups, or which circumstances may shape the nature of the interactions between individual primates.

Anthropologists are interested in studying non-human primates in the hope to gain insight into the evolution of the complex human social behavior. At present, 50 percent of all non-human primate species are threatened by extinction, and unless their habitats are protected from the ever-expanding human population in the near future, many primates will be lost forever.

Fleagle, John G. Primate Adaptation and Evolution. Academic Press, 1988.

Smuts, Barbara B., Dorothy L. Cheney, Robert M. Seyfarth, Richard W. Wrangham, and Thomas, T. Struhsaker, eds. Primate Societies. Chicago: The University of Chicago Press, 1987.

Beard, Chris. "Vertebrate Paleontology." Carnegie Museum of Natural History. <http://www.primate.wisc.edu/pin /factsheets/links.html>.

Primate Ancestors in China. <http://www.cruzio.com/~cscp/bea rd.html>.

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