Human beings and their closest relatives in the animal kingdom belong to the order Primates. The name, which means “first ones,” is apt because early members of this group lived at the same time as the dinosaurs, although they were not the earliest eutherian mammals. The primates began to separate from the other mammalian orders about 80 million years ago, at which time they differed from them very little in appearance and behavior. However, they bore the evolutionary potential whose expression since that time has produced the diverse forms of their descendants today.
The present order is divided into two suborders. The Prosimii include tarsiers, lemurs, in-dris, aye-ayes, galagos, pottos, and lorises. Some zoologists classify the tree shrews as primates. The Anthropoidea consist of the New World monkeys (tamarins, marmosets, and cebid monkeys), the Old World monkeys (macaques, baboons, and coIobus monkeys), the lesser apes (gibbons and siamangs), and the great apes (orangutans, chimpanzees, and gorillas). Among the small primates is the pygmy marmoset, whose body length measures about 5.5 inches (14 centimeters). The largest primate is the gorilla, which stands about 6 feet (1.8 meters) high and weighs about 450 pounds (204 kilograms).
The prosimian group is dominated by the lemurs and their relatives, which are concentrated in Madagascar and the nearby Comoros Islands. About 50 million years ago, what is now the island of Madagascar separated from mainland Africa and lemurlike prosimians on Madagascar were isolated. There were few other types of mammals on Madagascar, and lemurs were able to diversify with little competition. Lemurs that remained on the mainland developed a nocturnal lifestyle in response to competition from the more versatile Old World monkeys and apes. The nocturnal prosimians of the mainland include the galagos—sometimes called bush babies—of tropi cal Africa and the lorises (Lorisidae) and tarsiers (Tarsiidae) of tropical Asia. Prosimians failed to reach Australia because it was isolated in the Pacific well before their evolution. Tarsiers and lemurs once existed in North America but are now extinct on that continent.
The success of the primates meant that some species increased in size and moved from the ancestral forest habitats. Today the largest primates spend little time in the trees. Primates, whatever their way of life, have remained essentially animals of the tropics, where there is an abundance of leafy vegetation and large fruits for them to eat.
The earliest primates are thought to have exploited an unoccupied environmental niche as fruit- and insect-eaters in the trees. This treedwelling tendency, and the adaptations that have resulted from it, have remained with the primates throughout their evolutionary history. The more distinctive developments among the primates include the evolution of hands and feet equipped with grasping fingers and toes. This involved the separation of the digits and the development of musculature that enabled them to fold their hands around branches or other objects. The first digit (thumb or big toe) became widely separated from the others but could be folded across the palm of the hand, providing a manipulative grasp. The tips of the digits became flattened, and in many species nails rather than claws developed on their dorsal surfaces. Friction pads of deeply folded skin developed to maintain a hold on smooth or slippery surfaces.
Another important development was the reduction in the size of the snout, which allowed the eyes to move forward and stereoscopic (depth-perceiving) vision to evolve. This was particularly important for judging distances when leaping among the trees. High activity levels and manipulative skills require enlarged brain centers, and the primates developed large brains in comparison with their body size. In general, larger brain-to-body ratio indicates a higher level of intelligence in animals, but this is not always the case. In addition, the development of the embryo in the womb is more complex in primates, and gestation is often longer than for other eutherian mammals of similar size. Parental care is also longer in most primates.
Most primates are highly social animals, whose life is characterizMonkey limbs, locomotion, and handsed by complex interactions among members of a group. With social safety and prolonged care, education of the young can develop. Strong links are forged between an infant and its mother in the early stages of life. Later, bonds are made with playmates and other members of the group. Such developmental trends have progressed to different points in different primates but they are generally less advanced in the prosimians than in the anthropoids.
Zoologists disagree on the classification of the predominantly ground-dwelling tree shrews of the family Tupaiidae. Some zoologists consider them a primitive member of the primate order. Others place them in the order Insec-tivora, and still others classify them in an order of their own, Scandentia.
Zoologists agree, however, that tree shrews are related to both primates and insectivores and combine characteristics of both. Tree shrews have claws on each of their toes, unlike primates, which have at least one nail on each foot. Like insectivores, the teeth of tree shrews are adapted to a diet of insects, fruit, and worms. While primates have forwardfacing eyes, the eyes of tree shrews are situated on each side of the head, and as a result they lack stereoscopic vision. However, their moderately large eyes and bone-encased eye sockets resemble those of primates. Like primates, tree shrews also have bony ridges called postorbital bars surrounding the eye sockets, rounded ears with folds, and relatively large brains.
Tree shrews inhabit the forests of tropical eastern and southeastern Asia. The name of their genus, Tupaia, is derived from the Malay word for a squirrel, which tree shrews resemble in size and, to some extent, in appearance. Tree shrews are small, often bushy-tailed creatures. They are nocturnal, and their eyes are highly sensitive to light but not to color. They have prominent snouts and depend greatly on their good sense of smell. Tree shrews generally produce twins. Each of the twins develops in one of the two long branches of the bicor-nuate (two-pronged) uterus common to the primitive placental mammals. Gestation lasts about six weeks and parental care is minimal.
The status of the tarsiers as primates is certainly less controversial than that of the tree shrews. These arboreal animals inhabit the forests of the East Indies and the Philippines, and one of their most notable features is the greatly flattened tips of their digits. These digits enable the tarsiers to jump and support themselves even on smooth, vertical surfaces. The ability to grasp and climb among smooth branches, which is where most trees produce their most succulent buds and leaves, enabled the early primates to live in parts of the forest in which no other mammals lived.
Travel between high fruit-bearing branches was made possible by the development of leaping as a means of locomotion. The tarsier gets its name from its means of leaping—its greatly elongated ankle bone, or tarsal bone, allows it to jump at least 6 feet (2 meters).
The tarsier has a relatively flat face and forward-facing eyes with stereoscopic vision, which aids it when jumping. Its enormous, fixed, bulging eyes are its most remarkable feature. Unable to move them, it can swivel its head a full 180 degrees to the left or right. The size of its eyes may contribute to its good night vision, but a more important factor is the abundance of rods in the retina, which operate well at low light intensities, and absence of color-sensitive cones. This arrangement is typical in nocturnal vertebrates because color vision and fine resolving power are less useful in the dark. Its eyes lie in eye sockets completely encased by bony ridges.
The largest number of prosimians are found among the lemurs. These monkeylike, treedwelling animals have forward-facing eyes with less well developed binocular vision than other primates. However, they have better night vision as a result of a reflector at the back of the retina, called a tapetum. With the exception of the second toe, lemurs resemble tarsiers in that they have nails rather than claws on their digits, which allow them to grip more easily. Unlike the tarsier, however, the thumbs of lemurs are somewhat opposable in that they can be used to grasp objects. Their nose is foxlike in shape, although not as pointed as that of the tree shrews. Scent, an important feature in the life of these animals, is secreted from glands to convey a complex language of signals. It may be secreted to convey aggression or to mark territory, for example. The lower incisor teeth are modified to form a comb, which is used for cleaning the fur, as is the claw on the second toe.
Related to the true lemurs and also found in Madagascar are the indris and the aye-aye. The aye-aye was once thought to be a rodent, mainly because its curved, chisellike incisors continue to grow throughout its life. But if they did not, the teeth would be worn away because the aye-aye tears at rotten wood and gnaws at tree branches for hidden grubs detected by their sensitive ears. It then dislodges the prey with its extraordinarily long, thin, hookea middle finger. The finger is twice as long as the others and has a sharp claw.
In Africa, the prosimians are mainly represented by the galagos. These animals are fastmoving, with tremendous jumping power, and are nocturnally active on the ground or in the shrub layer of the forest. Their eyes are large and forward pointing, and the snout is short, giving them good binocular vision. They have the curious habit of urinating on their hands and feet, which probably helps them spread their scent for purposes of territorial claim. Galagos are unlike the lorises and pottos, which although similar in appearance, travel differently. They exert a viselike grip on a branch with their hands and feet while they move slowly upside down along the branch.
New World monkeys
Most of the primate group inhabiting the New World are considered to be more primitive than the other anthropoids. They include two families—the Callitrichidae, which consists of the 21 species of marmosets and tamarins, and the Cebidae, which contains 26 species, including howlers and capuchins. They are confined to tropical forest areas and are, in general, diurnal animals. Only the douroucouli, or owl monkey, is nocturnal.
New World monkeys are termed platyrhine, because they have flat noses with widely spaced nostrils. They have better stereoscopic vision than the prosimians and also are more dexterous. These two complementary developments may partly explain why they displaced prosimians in the New World.
The callitrichids are the smallest of the New World monkeys, measuring 10 inches (25 centimeters) or less. They have curved, clawlike nails on all their digits except for the big toe, which has a flattened nail. These claws help them grip branches as they travel along on all fours. They cannot make use of the thumb to grasp and so they grasp objects between the fingers and the palm of the hand. Like the tree shrews, callitrichids usually have twins that are carried by their father, who hands them over to the mother for suckling.
Cebids have better manipulation than the callitrichids. Larger and heavier than the marmosets and tamarins, they have to hold onto branches rather than merely balance on them. Like the callitrichids, some cebid monkeys live in family groups. Others, however, are found in large, multi-male troops. The females generally tend the young entirely, but in one or two species, the males carry and care for them.
The prehensile tail
In addition to their manual dexterity, some of the South American monkeys are further supported by a prehensile tail. Only the howlers, the spider monkeys, woolly monkeys, and the woolly spider monkey have a prehensile tail. Howler and woolly monkeys use their tail as an extra hand. The tail is most fully developed in the spider monkey, which can use it to support its entire weight. The end of the underside of the tail is naked and has ridged skin, which provides a better grip. It is also very sensitive and can pick up even small objects.
New World monkey anatomy and senses
The spider monkeys are typical of New World monkeys in that they move through the trees by jumping from branch to branch as well as swinging by their arms. This latter method of locomotion is called brachiation, the supreme exponents of which are the Old World gibbons. Like many brachiators, spider monkeys have hooklike hands with small thumbs, and arms that are much longer than the legs.
The sense of smell is important to these monkeys. Marmosets and tamarins mark their territory and points of reference with scent released from glands in the scrotal and anal regions. Capuchins rub their chest glands on branches for the same purpose and also urinate into a hand and spread the urine onto a foot before moving along their territory.
Color vision and visual acuity are also significant to the New World monkeys, with the exception of the nocturnal douroucouli. The bald uakaris, for example, have brilliant red faces set off by their white or red-brown fur. As well as species recognition, color also plays a role in food recognition those with color vision can evaluate the ripeness of fruit or the freshness of foliage by looking at it.
The brains of the New World monkeys are much larger and more complex than those of the prosimians. The greater size of the brain is largely attributable to the expansion of the cortex, which coordinates sensory and motor functions and controls memory and intelligence. This overall trend in the dominance of the cortex (corticalization) is more advanced in the cebid than in the callitrichid monkeys.
Males are, in most cases, larger than the females. Like most other animals, they are territorial. The male howlers are most notable for the method in which they proclaim their dominance over their territory. They have an enormous hyoid bone that makes a cup-shaped voice box in the throat. This air sac is inflated to resonate sounds that carry a distance of about 2 miles (3.2 kilometers).
Few New World monkeys show evidence of menstrual cycles. Actual menstruation is rare and minimal, and swelling of the female external genitals is minor. Gestation lasts about 18 to 32 weeks, and their infantile, juvenile, and adult phases are longer than those of the prosimians. Prolonged parental care is attend further developed in the higher primates as it is critical to learning and to the evolution of complex, flexible social behavior.
Old World monkeys
Old World monkeys are found in Africa and the warmer parts of Asia. They live in a wide range of habitats and are far more varied in their ways of life than are the cebids or marmosets. Some Old World monkeys inhabit semiarid areas beyond forest boundaries, where they are forced to hunt on the ground for any food they can find. These monkeys are all grouped in one family, the Cercopithecidae, which is divided into two subfamilies: the Cer-copithecinae, which includes the macaques, the African baboons, and the guenons; and the Colobinae, which includes the langurs, the CoIobus monkeys, and the proboscis monkey. The subfamily divisions reflect different eating habits. Most coIobine monkeys are leaf-eaters, whereas the cercopithecine monkeys, most of which are ground-dwelling, generally eat whatever foods are available. Most species of cercopithecine monkeys have cheek pouches, in which they may carry food that they do not eat immediately.
The most obvious difference between Old World and New World monkeys is that the nostrils of the former are closer together and point downward rather than sideways. They are therefore known as narrow-nosed, or ca-tarrhine, monkeys. Another distinguishing feature of all catarrhine monkeys is the presence of bare patches of hardened skin on the rump, called ischial callosities. These patches have neither nerves nor a blood supply and allow the monkeys to remain in a sitting position for long periods without serious discomfort.
Monkey limbs, locomotion, and hands
Monkeys usually walk and run on all fours, either on tree branches or on the ground, but many can stand and even run on their legs for short periods. They usually stand or run on their hind legs when carrying food, peering over high grass, or threatening enemies or other members of their group. The legs of most species are slightly longer than the arms.
The wooded savannas and open dry grasslands of sub-Saharan Africa are the home of primates that tend to move on all fours. The patas monkeys have exceptionally long limbs relative to their trunk, enabling them to sprint through the grass with long, bounding strides, faster than any other primate.
Many Old World monkeys have fine manipulative skills that are associated with true opposability of the thumb that is, its tip can be placed opposite any of the other fingers on the same hand. True opposability depends on the movement of the carpo-metacarpal joint and development of the thenar muscles at the base of the thumb. New World monkeys, in contrast, cannot move their thumbs at the base, but only at the second joint, or metacarpo-phalangeal joint. Thus, they do not have true opposability. The predominantly
ground-dwelling baboons and mandrills have relatively long, highly opposable thumbs and use them to pull up grass and other plants. Colobus monkeys, on the other hand, grip between their fingers and the palms of their hands because, like the spider monkeys of South America, their thumbs are very short or entirely absent. This feature is the source of their name, which means “maimed.” Also like the spider monkeys, colobus monkeys brachiate (swing by their arms) through the trees.
The feet of most monkeys are larger and more powerful than their hands. AH monkeys have five toes on each foot. The big toes look and function much like thumbs, giving the monkey an extra pair of grasping “hands.”
Monkey anatomy and behavior
The males and females of cercopithecine monkeys are very different in appearance a phenomenon called sexual dimorphism. A female, for example, is only one-third to one-half the weight of a male.
Male physical prowess is important both inside and outside the group in determining the social pecking order and in defense of the troop. Many open-country monkeys are at risk from predators, and their social organization helps them to minimize the danger.
The powerfully built male baboons, geladas, and mandrills possess fearsome canine teeth, whose display, rather than actual use, is often enough to discipline troop members or to frighten off predators or strangers. Their teeth are housed in prognathous jaws that is, jaws that extend beyond the upper part of the face but the monkeys still have good binocular vision.
Higher primates have a well developed visual center in their brains. This is necessary because they communicate by color. Mandrills have brilliant blue and red facial and genital markings. The colors become even more intense to express arousal or excitement.
In both Old World and New World monkeys, female reproductive behavior is seasonal. Many cercopithecine females, however particularly mandrills, some of the macaques, and colobus monkeys advertise their peak of fertility to the males by pink, swollen genitalia. Old World primates also menstruate more heavily than the New World primates and have regular menstrual cycles. Gestation and the postnatal phases are longer in the Old World monkeys than in the New World monkeys.
The last group of the primates, the superfamily Hominoidea, includes the great apes. The main characteristic that distinguishes apes from monkeys is their lack of an external tail, although as with humans, there are small, internal tail bones. Like the brachiating monkeys, those apes that brachiate have very long arms, which are longer than their legs.
The gibbons and the siamangs of Asia are often referred to as the lesser apes because they are smaller than the others. The gibbons populate the tropical forests of southeastern Asia, Malaysia, and Indonesia, whereas the siamang is confined to those of Sumatra and mainland Malaysia. The other Asian ape is the orangutan of Borneo and Sumatra. Together with the gorilla, the chimpanzee, and the pygmy chimpanzee of the African tropical forests, they constitute the great apes.
Ape locomotion and anatomy
The Asian apes are the most accomplished brachiators of all the primates. The siamang, which may weigh up to about 28 pounds (13 kilograms), and the still lighter gibbons are tremendously agile in the trees and rely almost exclusively on brachiation to move about. Effective brachiation requires the smooth rhythmical transfer of the body weight from one arm to the other, as the hands alternately curl around and release a branch. But this method of locomotion is not without its dangers. Many animals fall at some time during their life, and many skeletons of brachiating apes that have been studied show evidence of broken bones. Unlike the spider and coIobus monkeys, the brachiating apes have retained their thumbs, though they are short in relation to their fingers and the rest of the hand. During brachiation, the hand assumes the shape and function of a hook. The thumb plays no role at all and is folded across the palm. On the ground, gibbons and siamangs can run and walk bipedally for short distances. Because their arms are so long, they hold them out at their sides for balance and to avoid dragging them on the ground.
In contrast to the lesser apes, the great apes do not rely heavily on brachiation as a mode of transport The orangutan walks on branches while holding on to branches above with its hands. Gorillas and chimpanzees walk on the knuckles of their hands and on the curled toes of their feet. Gorillas have better opposability and make greater manipulative use of their thumbs than do the other apes, but it is the chimpanzees that make and use tools.
Whereas monkeys use color for purposes of display and threat, the apes use their size. In addition, the orangutan uses the fatty deposits around its face, called blinkers for display. Blinkers contain reserves of fat that can be drawn upon in times of food shortage.
Vertebral column and skull of apes
In contrast to monkeys, which carry their slender bodies horizontally on four limbs, apes walk erect or semiupright Apes also have fewer vertebrae in their trunks, which are wider than they are deep, whereas monkeys’ chests are deeper front to back than they are across. Apes thus demonstrate the evolutionary trend toward the reduction of trunk length in the more recently evolved primates. Instead of a flattening of the face, the great apes have developed prognathic (protruding) jaws. Such heavy jaws and high skull capacity are associated with increased brain size and result in a heavy skull. The forward position and the weight of the skull make the head fall toward the chest This feature and an enlarged crest, which runs from front to back across the top of the head, is characteristic of many great apes. Males twitch their crest in displays of aggression, but its height also serves to attract females.
The female chimpanzee alone among the apes exhibits external genital swelling as an indication of estrus. Female ape reproductive behavior is characterized by light menstrual bleeding, and the sexual cycles last about a month. Gestation lasts about 30 weeks in gibbons compared to 27 weeks in the much larger baboons and up to 36 weeks in gorillas and orangutans.
Evidence from fossils has convinced most scientists that human beings evolved over millions of years from ancestors that were not completely human. The fossil record does not, however, provide enough information to trace human evolution in detail. As a result, not all experts agree on how humans evolved. Scientists, however, classify human beings as primates, and a survey of the living primates reveals that humans are most closely related to the apes and resemble them in a number of ways. Those trends that are preeminent in the evolution ofthe primates, such as the development of grasping hands, good stereoscopic eyesight, enhanced development ofthe cerebral cortex and other parts of the brain, and longevity, are far advanced in the modern apes but have progressed furthest in human beings. Humans, however, are vastly different from the apes in that they walk on two feet rather than on their hands and their brain is more than twice the size of that of any other living primate.
Hominids are members ofthe group of species that includes human beings and our close prehuman ancestors. Hominid fossils, recovered from paleontological sites throughout the world, have been classified into several genera. New finds are increasingly frequent, and scientists are constantly revising their understanding of human evolution.
The earliest known species of hominid, the australopithecines of southern and eastern Africa, lived from more than 4 million to about 1 million years ago. Australopithecus was slender and about the same size as the modern male chimpanzee with a similar cranial capacity of 24 cubic inches (400 cubic centimeters). In australopithecines, the hole in the skull through which the spinal cord runs up to the brain (the foramen magnum) is relatively far forward and vertical. This suggests that the spinal cord must have entered the brain perpendicularly and the skull on the vertebral column would have faced forward rather than slightly downward, in contrast to the great apes. The australopithecine pelvis is bowlshaped with short, stout hipbones that extend backward; their hip surface area permitted the attachment of powerful gluteal (buttock) muscles, and their pelvic bones allowed the anchoring of strong abdominal and back muscles. In these and other respects, many of their skeletal elements are similar to those of modern humans. The australopithecines were undoubtedly bipedal and walked upright.
Other hominids shared parts of Africa with the australopithecines. But whereas the australopithecines may have used very simple tools—usually stone fragments, which they found but did not make—Homo habilis, or “handy human being,” definitely made them. With a large cranial capacity of about 30 to 50 cubic inches (500 to 800 cubic centimeters) and less massive jaws, Homo habilis resembled modern humans more than did the australopithecines.
Homo erectus, the immediate predecessor of modern humans, emerged about 1.5 million years ago. These early bipedal hominids were neither very large nor particularly strong, nor could they run fast It is very likely then that cooperation based on communication and sign language in food-gathering and group defense would have been vital for the survival of Homo erectus.
Except for Homo sapiens, or the modern human, Homo erectus had the largest brain of all the primates, accommodated in a cranium with a capacity of 42 to 76 cubic inches (700 to 1,250 cubic centimeters). Despite the evolutionary tendency within the primates for the skull to become domelike, H. erecfusstill had a sloping forehead with heavy, bony brow ridges and jutting jaws without a chin. But despite its more primitive appearance, the brain of H. erectus was complex and efficient.
Herectus ushered in Acheulian stone technology, which featured a wide range of carefully manufactured tools for the killing or butchering of game and the preparation of food. This species was perhaps a good hunter. Paleontologists believe that deformations observed in fossil H. erectus bones were caused by excessive vitamin A intake as a result of eating too much raw meat Fossil remains of these hominids in East Africa, Europe, China, and Java point to their northward and eastward expansion out of their native Africa.
Modern Homo sapiens
According to scientific theory, the modern human being, or Homo sapiens, became a recognizable species about 450,000 to 100,000 years ago. Earlier fossil remains of H. sapiens have been found across Europe into Asia. They gradually spread from western and eastern Eurasia to America and Australia, walking over land bridges created by lower water levels. during ice ages. Human beings, at this stage, basically resembled H. erectus, but had a larger brain and smaller jaws and teeth. As time passed, H. sapiens began to look like today’s human beings.
Although their facial muscles enable human beings to form a great many expressions, humans nevertheless rely primarily on symbolic spoken language to express and communicate most aspects of its culture. Of all animals, this characteristic is unique to human beings. Certain animals, including apes and monkeys, communicate by making a wide variety of sounds. Although these sounds express emotion and may communicate simple messages, they apparently do not symbolize any object or idea, as does the language of humans. Language therefore distinguishes human culture from all forms of animal culture.