Our relationship to other kinds of primates is in part reflected by the pattern of similarities and differences we share with them. This pattern of similarities and differences is also used to classify different primate species into groups. There are six major branches of primates, classified as superfamilies. These include:
- including the lemurs of Madagascar
- including lorises and galagos
- with the tarsiers
- or New World monkeys
- or Old World monkeys
- including apes and humans
The last three of these, hominoids, cercopithecoids and ceboids, share a common ancestor that lived sometime before 55 million years ago. These three superfamilies form a single branch, or clade, on the primate evolutionary tree.
Scientists have often grouped these superfamilies into two major categories, called grades, which express the broad adaptations shared by different groups. Grades are not necessarily evolutionary lineages, but are meant to express the ways that different lineages share common sets of adaptations. One of these grades, the prosimians, includes lemurs, lorises, galagos, and tarsiers. Prosimians share a basic set of adaptations with other primates, including binocular vision, a slower rate of reproduction than many mammal groups, nails on the toes and fingers instead of claws, and other adaptations to life in the trees.
Other primates include the monkeys, apes, and humans, who are grouped as anthropoids. Unlike the prosimians, the anthropoids form a single evolutionary lineage, or clade, because monkeys, apes, and humans are more closely related to each other than to any other living group. The evolutionary relationship of the anthropoids leads them to share many derived features, including
The lemurs are living and fossil primates of Madagascar. Lemurs share several derived features with lorises, including a set of closely-spaced and projecting lower incisors and canines, called a tooth comb, and a claw rather than a nail on the second toe. Both these features are used for grooming, and are so specialized relative to other primates, that lemurs and lorises may be classified as sister groups in a single clade, called the Strepsirrhini, a relationship supported by the close genetic relationship of the two superfamilies. Lemurs and lorises also share general features that were most likely present in the earliest primates, including a single postorbital bar instead of a bony enclosure for the eye orbits, and an external nose membrane connected to the upper lip.
Today, the lemurs include five distinct families, all limited to Madagascar. These range in size from the mouse lemurs, which are the smallest living primates at only 60 grams, to the indri, approaching 10 kg. The most divergent living lemuroid is the aye-aye, a nocturnal creature with long bony clawed fingers and perpetually growing incisors, both supporting an adaptation to finding and eating grubs inside of wooden branches. The other lemuroids show a broad range of dietary and locomotor adaptations. Some species, like the sifaka, primarily leap using long hindlimbs and cling to vertical branches. Others are arboreal quadrupeds, or spend substantial time on the ground.
In the recent past, a greater diversity of lemurs existed on Madagascar than remain today. Large extinct lemurs such as Megaladapis reached over a hundred kilograms at their largest, exceeding the size of female gorillas. Some large extinct lemurs appear to have had a sloth-like adaptation for below-branch suspension and feeding, and most of the larger forms primarily ate leaves. These lemur species existed within the past fifteen hundred years, and were likely driven to extinction by humans, who reached the island within that time period. The discovery of lemur skeletal remains in association with human archaeological sites confirms this extinction hypothesis.
The living lorisoids include galagos and lorises. Galagos, or bushbabies, are small prosimians weighing for the most part a fraction of a kilogram. Nocturnal creatures with large eyes and ears, and long tails, galagos are found across West Africa and into the central and southern portions of the continent. Galagos eat mainly fruits, insects, and gums, and some species are mostly quadrupedal, while others are adapted to leaping and a vertical posture. Lorises share a similar diet and a nocturnal activity pattern with the galagos, but differ in their relatively slow and deliberate style of foraging. Several species of lorises are distributed across Africa, South Asia, and Southeast Asia.
Tarsiers are small primates from the islands of Southeast Asia: Java, Borneo, Sulawesi, and the Philippines. Averaging slightly greater than 100 grams, tarsiers have several distinctive skeletal adaptations, including very long legs and ankles, immense eyes, and large hands and feet. These features support the adaptation of tarsiers of clinging to vertical branches and leaping between them, as well as their nocturnal activity pattern. Tarsiers eat insects, lizards, and other small vertebrates.
The tarsiers share several features with anthropoid primates that may indicate a close phylogenetic relationship between the two. Unlike other prosimians, tarsiers lack a moist external nose, and they have a wide postorbital plate instead of a narrow bar. These and several more subtle cranial features link tarsiers with monkeys and apes. Many paleontologists believe that anthropoids may have originated from an Eocene group, called the omomyids. This group shares many features with living tarsiers and which may represent the common ancestor of both groups of living primates. If true, then the tarsiers are the closest primate relatives to the anthropoids, and the two groups form a clade called the Haplorrhini.
Ceboidea includes the American, or New World, monkeys. Much of the earliest record of primate evolution, dating to greater than 40 million years ago during the Paleocene and Eocene, is from North America. These lineages apparently became extinct in North America by Oligocene times, and the New World monkeys that appear in the Oligocene of South America derive from an early Old World anthropoid lineage. The earliest anthropoid primates now known are from East Asia, which may have been the original source location for the anthropoids. Present-day New World monkeys possess three upper and lower premolars on each side, like most prosimians and the earliest anthropoids. This primitive dental formula distinguishes the New World monkeys from both the Old World monkeys and apes, which have only two premolars rather than three. Because of their round, forward-facing nostrils, the New World monkeys are called Platyrrhini, or flat-nosed, in contrast to the more narrow noses of the cercopithecoids and hominoids, which together are called Catarrhini, or downward-nosed. These differences imply that the platyrrhines existed as a lineage apart from catarrhines at least as early as 35 million years ago, when the first fossil catarrhines lived.
South America was an island continent until around 5 million years ago, when the connection to North America arose. The first fossil New World monkeys date to the Late Oligocene, some 30 million years ago, meaning that these primates reached South America by an ocean crossing, probably an accidental journey on rafting vegetation from Africa. Once they reached South America, these monkeys underwent an impressive diversification. There are five living subfamilies of New World monkeys, with a total of sixteen genera. These vary from the relatively tiny callitrichines, including the marmosets and tamarins, to the relatively large atelines, including spider monkeys and howler monkeys, but most species range from one to five kilograms in mass. Several New World monkeys, including spider monkeys, howlers, and capuchin monkeys, have prehensile tails.
The cercopithecoids, or Old World monkeys, include two major groups. The cercopithecines, including macaques, baboons, guenons, and mangabeys, have a broad diet based on fruits, leaves, seeds, nuts, and insects. The colobines, including African colobus monkeys and Asian langurs and proboscis monkeys, tend to specialize to a greater degree on leaves in their diets. Old World monkey species have a diversity of anatomical adaptations to support these general patterns. The dietary diversity of cercopithecines is supported by broader incisors and low-crowned molars, typical of fruit eaters, and pouch-like cheeks for stashing food. Colobines have high-crowned molars for shearing leaves, and a large specialized gut for digesting leafy matter.
A key adaptation shared by all living cercopithecoids is the distinctive shape of their molars. These teeth have cusps aligned into two high ridges extending across the tooth from side to side, a pattern called bilophodont. The ridges on the top and the bottom teeth interlock in opposing sawtooth patterns, creating a strong shearing action, ideal for reducing leaves and other fibrous plant matter, such as fruit rinds. This dental pattern is first found in fossil monkeys from the Early to Middle Miocene of East Africa. The living varieties of cercopithecoids arose later and underwent a major adaptive radiation during the Pliocene. Today the cercopithecoids include some of the most successful varieties of primates because of their geographic extent, the number of their species, and their dense populations.
The hominoids include the living apes and humans, and their fossil relatives. The living great apes belong to four species, including orangutans, gorillas, chimpanzees, and bonobos. The gibbons and siamangs are in the hylobatid family, and include several different species sometimes called lesser apes. Humans and their extinct relatives are the hominins.