Primates

The primates are an order of mammals that includes monkeys, apes, lemurs, tarsiers, and humans. The other primates are thus the nearest relatives to human beings. Anatomically, primates have many similarities to the earliest placental mammals. Primitive features retained by primates, but lost in many other mammals, include five-fingered hands and feet with individually mobile fingers and toes, a collarbone (clavicle), a relatively simple cusp pattern in the molar teeth, and the freedom of movement within the forearm that allows the wrist to rotate without moving the elbow. Many primates live in trees; those that do not, have anatomical features showing that their ancestors were tree-dwellers. These include features directly concerned with arboreal locomotion, those concerned with vision and intelligence, and those concerned with reproduction. Arboreal locomotion, the ability to climb trees, has many direct consequences in primate anatomy. Most primates, for example, possess long, agile arms and legs. Grasping hands and feet give them the ability to hold objects and to climb by wrapping the fingers around branches. This is different fromthe many other arboreal animals that dig their claws into the bark. The primate grasp is aided by the development of opposable thumbs and big toes. If a primate grasps an object, the fingerprint surface of the thumb faces the corresponding surfaces of the other fingers. This is possible because the bone supporting the thumb can rotate out of the plane of the other fingers. Most primate thumbs and big toes are opposable; human feet are unusual. Individual mobility of all fingers and toes is a primitive mammalian ability that primates have kept but which many other mammals have lost through evolution. Combined with the grasping ability of hands and feet, this characteristic allows primates to manipulate objects of all sizes rather skillfully. Primates also have hairless friction-skin on their palms and soles, which are supplied with a series of parallel ridges in complex patterns (fingerprints). These ridges provide a high-friction surface, which helps in grasping objects-or in holding branches without slipping. Claws, which could get in the way and cause injury, have generally been reduced to fingernails that rub against things and thus stay short. The clavicle (collarbone), which strengthens the shoulder region, is present in primitive mammals. Many modern mammals have lost this bone, but in primates, it has been retained and often strengthened.

Primate Vision
Primates are primarily visual animals. Whereas many other animals rely strongly on smell, primates rely on sight, and particularly on color vision. One reason for this visual orientation of primates is that the exact position of the next branch can best be judged visually, and good depth perception is essential for avoiding a fall. Primates often hunt by visual predation, stalking an insect or other small prey, judging its exact position and distance, and then pouncing on it. Vision in depth (stereoscopic vision) requires two eyes with overlapping visual fields (binocular vision). Each eye sees the same objects from slightly different angles; the brain combines the two images into a single three-dimensional image. Binocular vision is made possible by the forward position of the eyes. The eyes need protection in this position, and it is furnished in most primates by a bony structure called the postorbital bar. A complex organization of the brain is required for binocular vision. The cerebral cortex of the brain is therefore expanded, especially the part known as the visual cortex. The surface of the brain develops a large number of folds-primate brains are more heavily folded than any others. One characteristic fold is called the calcarine fissure, a characteristic feature of all primates. Primates can pick up unfamiliar objects with the fingers and bring them in front of the face for closer visual inspection. This behavior requires coordination of eye, brain, and hand in a very complex interaction. It is through this behavior that young primates learn to manipulate and understand their environments and to investigate them with a characteristic curiosity. Primates rely heavily on learned behavior, including exploratory play. Primates are the most intelligent animals, and the great expansion of the cerebral cortex reflects this. Most primates grow up in social groups, where they interact in complex ways with other individuals. They have a curiosity about the world, especially about other primates. Primate behavior is complex, and most of it is learned. The price paid for reliance on learned behavior is youthful inexperience. Infant primates have much to learn, and they have not yet had much time to learn it; they therefore depend significantly on their parents. A long and extended period of intensive parental care of offspring characterizes nearly all primates. This would not be possible if primates were born in large litters, because their mothers would be too busy caring for so many babies at once. Furthermore, infant primates are carried frequently, placing a burden on their mothers, some of whom need both arms free for climbing. Accordingly, primates are usually born one at a time.Twins and other multiple births are uncommon. There are several related features in primate reproductive systems. Female primates need only one uterus to carry their young and one pair of nipples to nourish them after birth. The right and left uteri, paired in other mammals, are fused into a single uterus called a uterus simplex. Unlike female dogs or pigs, which have many nipples in parallel rows, female primates have only a single pair, located high in the chest region.

Primate Classification
There are many different types of primates and several ways of classifying them. The classification followed here divides primates into the suborders Paromomyiformes, Haplorhini, and Strepsirhini. The suborder Paromomyiformes contains the earliest primates, all now extinct. They lived from about fifty to eighty million years ago (from the Cretaceous period to the Eocene epoch). They are known mostly from fossilized teeth, but the genus Plesiadapis is known from nearly complete skeletons. All these primates were small, from roughly the size of a mouse to that of a cat. Their limbs were long and agile, with hands and feet showing the grasping ability typical of arboreal primates. The teeth, which differed among the three families of Paromomyiformes, were modified for a diet of mostly fruits but also some insects. A postorbital bar was not present; the eyes had not yet shifted forward. The visual fields overlapped much less than they do in modern primates, showing that these animals were not efficient visual predators. Some paleontologists have suggested excluding them from the order Primates for this reason.

Suborder Haplorhini
The majority of primates belong to the suborder Haplorhini and possess relatively large brains and relatively short faces. The noses of all Haplorhini are dry and have no external connection to the mouth; the upper lip goes straight across the mouth and is not divided by a groove, as it is in the Strepsirhini. The structures of the ear region, the placenta, and certain other anatomical features show that the Haplorhini are closer to the ancestral mammals than are the Strepsirhini. The geologic record of haplorhines extends from the Paleocene to the Recent (or Holocene) epoch. There are three subgroups of Haplorhini: Tarsioidea, Platyrrhini, and Catarrhini. Of these, the Tarsioidea are the oldest and most primitive. The tarsioids are small primates with rounded heads. The eyes are rotated forward and protected from behind by a postorbital bar; visual fields overlap significantly. Tarsioids flourished during the Eocene epoch, about forty to fifty million years ago. They lived across Europe, Asia, and North America, all of which then had subtropical climates. Tarsioids disappeared from Europe and North America as climates became colder, but they survived in Asia. The living genus Tarsius now inhabits the East Indies from Borneo to Celebes. Tarsius is a big-eyed, nocturnal primate with an elongated ankle. It has a peculiar form of locomotion called vertical clinging and leaping. It clings to vertical bamboo stalks much of the time, then uses its hind legs to jump to the next perch. At least one extinct tarsioid had similar adaptations. The Platyrrhini include the South and Central American primates. They are characterized by nostrils that open directly forward, three pairs of premolar teeth, and tails that are strong and sometimes assist in locomotion. Platyrrhines probably evolved from tarsioid primates that reached South America prior to Miocene times. Included in the Platyrrhini are the small forest primates called marmosets (Callithricidae). Male marmosets often have striking white or yellowish facial markings (such as tufted ears, eyebrows, and mustaches) which may be useful in mate recognition. The other Platyrrhini are the New World monkeys (Cebidae). Familiar ones include squirrel monkeys (Saimiri), capuchin monkeys (Cebus), and howler monkeys (Alouatta). All these monkeys are arboreal, and several of them are skilled acrobats. A few species, including the squirrel monkeys, can hang by their tails. The Catarrhini include the Old World monkeys, apes, and humans. All Catarrhini are characterized by noses that protrude from the face with the nostrils opening downward, as human noses do. There are only two pairs of premolars. The tails are weak or absent and are useless in locomotion. Two very early catarrhine species occurred in Burma, but they are poorly known and of uncertain relationships. The oldest undoubted Catarrhini occur as fossils in the Fayum deposits of Egypt, which are Oligocene in age-about thirtyfive million years old. Fayum primates are thought to be of tarsioid ancestry. One type, Apidium, has definite tarsioid resemblances. Other Fayum primates include several small apes, such as an early gibbon, a possible ancestor of Old World monkeys, and Aegyptopithecus. Living Catarrhini include the OldWorld monkeys, or cercopithecoids, along with apes and humans. Familiar cercopithecoids include baboons (Papio), guenons and vervet monkeys (Cercopithecus), macaques (Macaca), langurs (Presbytis and Pygathrix), and colobus monkeys (Colobus); there are many others. The large and complex social groups of macaques and baboons have been studied repeatedly. There are dominance relationships within these primate societies, but they are usually expressed by gestures and displays instead of by fighting. The family Pongidae includes the apes. The smaller, more lightly built apes are called gibbons (Hylobates and Symphalangus). These skillful acrobats are often placed in a family by themselves, the Hylobatidae. The more typical "great" apes include the orangutans (Pongo) of Asia, the gorillas and chimpanzees (Pan) of Africa, and several fossil apes, such as Dryopithecus. Modern apes have long arms and shorter legs. They exhibit a variety of locomotor patterns: arm-swinging (brachiation) in gibbons, knuckle-walking in gorillas and chimpanzees, and a four-handed type of clambering in the orangutan. The family Hominidae includes humans, who walk upright and communicate using language. All living humans belong to the single species Homo sapiens.

Suborder Strepsirhini
The Strepsirhini, or Lemuroidea, include lemurs and their relatives. Once thought to be "lower" on the evolutionary scale, the lemuroids are now known to be separately specialized in many ways. Their placentas, for example, are of a peculiar type not found among other primates or among primitive mammals. The ear regions of their skulls show certain anatomical peculiarities; the same is true of the brain and the facial muscles. All true lemurs live on the island of Madagascar, off the eastern coast of Africa. Most lemurs have the lower front teeth modified to form a comblike structure used in cleaning the fur. Other lemuroids include the small, agile galagos of mainland Africa and four types of slower-moving lorises in Africa and southern Asia. Many fossil lemuroids are known from the Eocene epoch.

Genetic Comparisons
For many years, the classification of primates was based largely on anatomy, physiology, and paleontology. Now, however, the molecular structure of proteins and other types of biochemical evidence are used to make additional comparisons. Closely related primates have proteins with similar or identical sequences. For the most part, family trees based on protein sequences confirm the traditional classifications, with some exceptions; the separateness of gibbons from other apes is based largely on such biochemical evidence. Geneticists have compared the chromosome sequences of humans and apes. They have found that the sequences of banding patterns in chimpanzee and gorilla chromosomes are nearly identical and that fewer than twenty chromosomal inversions (end-to-end changes) separate chimpanzees and humans.

Types of Primate Study
Primates are studied by different specialists, each using different methodologies. Primate anatomists and functional morphologists dissect primates and compare their structures to those of other species. Morphologists may also take numerous measurements and analyze the results statistically, often with the aid of a computer. Primate ethologists and sociobiologists study the behavior of living primates, both in the field and in the laboratory. Field studies have been conducted among the chimpanzees in the Gombe Stream Reserve in Tanzania, the rhesus macaques of India, the Japanese macaques of Japan, the howler monkeys of Barro Colorado Island in the Panama Canal Zone, and the baboons of the East African savannas. Studies have confirmed that these primates have diverse locomotor patterns, varied diets, and complex and flexible social organizations that help them find food, avoid predators, and survive under difficult circumstances. Molecular biologists and geneticists analyze the structures of important proteins and of deoxyribonucleic acid (DNA). This information provides important evidence of the relatedness of one group of primates to another. It was through such studies that molecular biologists were able to show that chimpanzees and gorillas are nearly identical, that humans are very closely related to these African apes, and that the gibbons of Asia are much more distantly related. In general, the findings of molecular biology and genetics have tended to confirm the earlier findings based on comparative anatomy and paleontology, but with occasional exceptions. Observations on living species are supplemented by studies of fossil primates. Paleontologists are always trying to connect living and extinct species into common family trees. Some molecular biologists have made estimates, based on biochemical differences, of the time of evolutionary divergence between apes (Pongidae) and humans (Hominidae). These estimates are somewhat controversial because they place the hominid-pongid split at less than five million years ago-in contrast to earlier estimates of ten million years or more made by paleontologists in the 1960's. Reinterpretations of the fossil primate Ramapithecus and other fossils from Asia support the biochemists' view that the hominid-pongid split took place less than five million years ago. Many scientists with other fields of interest use nonhuman primates as experimental subjects for medical or biochemical research, including cancer research, space exploration, and the testing ofnew drugs. These scientists also make important additions to knowledge of how similar these primates are to humans. In particular, monkeys and apes are subject to nearly all the same diseases as are humans. Many drugs and surgical procedures designed to aid in the fight against these diseases are first tested in monkeys or apes because the physiological responses of these primates to the drugs or other medical procedures are nearly always the same as they are in humans. One case of particular importance is acquired immunodeficiency syndrome (AIDS). This disease, which attacks the immune system, does not occur in rats and mice, but it does occur in monkeys and apes. For these reasons, monkeys and apes are used extensively in tests on the AIDS virus and on possible treatments or cures for this disease.

Humans and Other Primates
Since the nonhuman primates are Homo sapiensпїЅ? closest relatives, most studies of primates involve inevitable comparisons with humans. Most of the knowledge that is gained in studying the primates helps scientists understand the human species better. Primates, especially macaques, are commonly used in medical and behavioral research. New drugs, for example, are often tested in monkeys because the physiological reactions of these primates are likely to be more similar to those of humans than would be the case in rats or mice. Chimpanzees are used when an even closer approximation to human anatomy or human intelligence is important to the experiment. From the use of these and other primates in medical research, knowledge of human health and human diseases has been greatly enhanced. Scientific understanding of human behavior and other aspects of human biology is similarly enhanced by studies of other primates. Studies of sign language among apes, for example, have greatly increased the understanding of human language and the ways in which it is learned. Both gorillas and chimpanzees have successfully been taught American Sign Language (ASL), a gesturelanguage commonly used by deaf people. Many researchers who have worked with these apes believe that they show true language skills in their use of ASL, although some linguists disagree. Apes who use sign language can converse about past, present, and future events, faraway places, hypothetical ("what if?") situations, pictures in books, and individual preferences. These apes apparently can use language to lie, to play games or make puns, or to create definitions, such as "a banana is a long yellow fruit that tastes better than grapes." Studies on the social organization of baboons, langurs, and other nonhuman primates have greatly increased understanding of how human social organization might have originated. Most nonhuman primates have social groups based on friendships and dominance relationships. Larger and stronger individuals tend to get their way more often, but usually by gesturing or threatening rather than by actually fighting. Even this observation, however, must be qualified, because encounters involving three or more individuals are generally very complex, and less dominant individuals can often manipulate these complex situations to their advantage.