Primate

Primates Fossil range: Paleocene - Recent
An "Old World" monkey - Papio anubis. This male baboon at Ngorongoro Crater, Tanzania is holding an infant.
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Mammalia Infraclass: Eutheria Superorder: Euarchontoglires Order: Primates Linnaeus, 1758

Primates are an order of mammals which in living diversity include, prosimians (galagos, lorises, lemurs and tarsiers), platyrrhines (New World monkeys), cercopithecids (Old World monkeys) and hominoids (apes and humans). They have their origins in some type of an insectivorous mammal that lived in the late Cretaceous.

Description

All primates have a primitive dental plan, five fingers, nails instead of claws, a thumb and a generalized body plan. All primates share a similar eye orbit morphology and have a post-orbital bar. All primates have forward facing eyes.

Geographical distribution

Due to the recent spread of humans in the last 12,000 years, primates are a probably the most geographically distributed mammal, occupying every continent. Non-human primates are indigenous to South and Central America, Africa, Asia and the the islands of Indonesia.

Prosimians

Prosimian primates are the most primitive of living primates and share many morphological characteristics with other mammals such as tree shrews and bats. The greatest diversity of prosimian primates is found on the island of Madagascar, but all species of living prosimian species are found only in the Old World.

Lemurs

Lemurs exist only on the island of Madagascar and the neighbouring islands of the Comores. They are among the most primitive of primates. They have moist noses and reflective eyes. They range in body size from the 30 gram Pygmy lemur to the 10kg Indri. Due to their geographically restricted range and destruction of habitat by humans, all lemur species are endangered.

Lorises

Lorises are arboreal prosimian primates who live in India and southeast Asia. They live in tropical areas, typically high in the canopy and rarely come to the ground. They are known to be very slow moving.

Galagos

Galagos, also known as bushbabies, are small, arboreal nocturnal primates with large eyes. They are native to Africa. Bushbabies are extremely adept leapers.

Tarsiers

Tarsiers are a nocturnal, arboreal primate restricted to several islands in southeast Asia. They are primarily insectiverous and are agile leapers. Unlike most nocturnal primates, they have non-reflective eyes.

Monkeys

There are two basic types of monkeys - platyrrhines and catarrhines. South American monkeys are known as platyrrhines and differ considerably from the catarrhines monkeys, having evolved largely in isolation.

Platyrrhine monkeys

The name platyrrhine derives from the broad, flat shape of their external nostrils. All platyrrhines are small, the largest is only around 10kg. They also exhibit many primitive dental and skeletal features including having three pre-molars.

All platyrrhines have a tail of some sort and five genera have prehensile tails, the most well known of these probably being the spider monkey.

Catarrhine monkeys

Catarrhines monkeys are "Old World Monkeys" and all fall under the Superfamily Cercopithecidae. They have narrow noses, eight pre-molars and none have prehensile tails. There are two Subfamilies within the Superfamily - Cercopithecinae and Colobinae. There are approximately sixty species of cercopithicus monkey and about forty species of Colobus monkey.

Apes and humans

The living apes are generally considered to be primates who are tailess, of relatively large body size and all live in the old world. Typically included in the grouping "apes" are gibbons and siamangs from southeast Asia, Orangutans from Borneo and Sumatra, Mountain and Lowland Gorillas, Chimpanzees and Bonobos and humans. Under the modern genetic classification scheme (see Hominini for more on this), apes are in the superfamily Hominoidea. Underneath this hominoid umbrella falls orang-utans, gorillas, chimps and humans in the Family Hominidae. In recognition of their genetic divergence some 11-13 million years ago, the orangutans are placed in the sub-family Ponginae and the African apes, including humans, are lumped together in the Subfamily Homininae. The bipedal apes, namely all of the fossil species as well as living humans, fall into the Tribe Hominini. Some evolutionary biologists include humans and chimpanzees within the same genus, the genus Homo. The more traditional Linnaean clasification of primates may be found in the "List of Primate Species" at the end of this article.

Primate evolution

The earliest possible primate discovered so far comes from North America and is about 60 million years old. Named Purgatorius, this tiny insectivore is only loosely morphologically allied with later primates. There are possible older Purgatorius specimens going back into the latest Cretaceous, but most are from dubious context. It is only in the late Paleocene (about 55 million years ago) that we see more numerous remains of primate-like animals appearing in the fossil record.

Known as the Plesiadapiforms these near-primates appear to have evolved in North America and Europe. The Plesiadapiforms radiated into many different niches, but in the early Eocene (about 45 to 50 million years ago), it seems that the rise of rodents caused a rapid decline in the number and diversity of Plesiadapiforms. But Plesiadapiforms are probably not the ancestors of living primates, as they possessed too many specializations to have given rise to the first Prosimian primates that would appear a few million years later. So at this time, the only primate-like mammal that is a firm candidate as ancestor of all higher primates is the tiny Purgatorius.

The first true prosimian primates appeared, and exploded in diversity, in the early Eocene Epoch (between 54 and 38 million years ago). Eocene aged Prosimian primates are commonly found in North America and Europe and more rarely in Asia and Africa. No early primates have ever been found in South America or Antarctica as the former was an island continent, while we have as of yet found fossil deposits of this age in the latter. In these earliest primates the bony ring around the orbit was complete like in modern primates, nails replaced claws and larger brains were evolved. These early Eocene primates were clearly true primates and took two distinct forms: lemur-like adapids and galago- (bushbaby) like omomyids. Although the living forms are different species, these animals descendants can clearly be seen in the lemurs of Madagascar and the galagos, lorises and tarsiers of Africa and Asia.

As the Eocene drew to a close and the Oligocene epoch began about 37 million years ago, the continents were approaching their modern form and position, with the exception that there was no land bridge between South America and North America. The world was however, in a state of geographical transition. India was colliding with the continent of Asia, lifting the great Himalayas. South America and Australia had pulled away from Antarctica and formed independent island continents. Deep water currents could thus circulate around Antarctica, bringing cold waters northward and subsequently cooling the oceans of the world. At the same time, the rise of the Himalayas blocked the northward curve of the jet stream, changing the climate south of this great mountain range.

Primates in Europe suddenly go extinct while in North America their fossils become increasingly rare and there is a general decline in mammalian diversity. Until recently, the global climatic changes of the early Oligocene have been blamed almost wholly on the mammalian extinctions that occurred at this time, but in November of 2001, scientists from the United States Geological Service announced that what was previously thought to be a relatively small extraterrestrial impact in the Chesapeake Bay area was in fact quite large (around 137 kilometers in diameter), and struck at approximately 35 million years ago, right at the point of extinction of many of the North American primates and at a point of general loss of mammalian diversity.

A primate puzzle

Prior to about 30 million years ago there are no primates or even primate-like animals in South America. Around 25 to 30 million years ago a wide variety of new forms of mammal suddenly appear in South America but it is unknown where they come from. Among the new forms are rare fossil primates which look very much like existing Platyrrhines. As the only major difference in continental position between that of today and then was that no land bridge existed between North and South America, any introduction of primates into South America would require some form of open water crossing, possibly rafting on large fallen tree trunks or large mats of vegetation. Because of relatively shallow water in the South Atlantic during lower sea level periods, there were almost certainly many islands exposed between Africa and South America, effectively bringing the two continents much closer together and making potential rafting hops shorter. In fact, most geophysicists suggest that at the time the open water distance between North America and South America was probably greater than that between the latter and Africa.

Predications of current direction also tentatively support a West to East crossing rather than a North to South rafting event. The fossil record also supports an “Out of Africa theory by raft” for the origin of Platyrrhines. At this time, there are simply no known primates advanced enough in North America to be suitable ancestral candidates of the early Platyrrhines, but Africa has a host of possible ancestral Platyrrhine forms. Further tantalizing evidence of an African connection is the fact that the closest living relatives of South American rodents are the African Hystricids, more commonly known as porcupines.

Another possible source for Platyrrhine origins would be Antarctica, but we know nothing about the later fossil record, if it exists at all, of Antarctica at this time