The Classification System
In order to understand the exact place of humanity among the animals, it is helpful to describe the system used by biologists to classify living things. The basic system was devised by 18th-century Swedish naturalist Carl von Linné.
The purpose of the Linnean system was simply to create order in the great mass of confusing biological data that had accumulated by that time. Von Linné classified living things on the basis of overall similarities into small groups or species. On the basis of homologies, groups of like species are organized into larger, more inclusive groups, called genera.
Through careful comparison and analysis, von Linné and those who have come after him have been able to classify specific animals into a series of larger and more inclusive groups up to the largest and most inclusive of all, the animal kingdom.
The Primate Order
Primates are only one of several mammalian orders, such as rodents, carnivores, and ungulates.
As such, primates share a number of features with other mammals:
- mammals are intelligent animals
- in most species, the young are born live, the egg being retained within the womb of the female until it achieves an advanced state of growth
- once born, the young are nourished by their mothers
- mammals have a constant body temperature, an efficient respiratory system featuring a separation between the nasal and mouth cavities, an efficient four-chambered heart that prevents mixing of oxygenated and deoxygenated blood, among other characteristics
- the skeleton of most mammals is simplified compared to that of most reptiles, in that it has fewer bones. For example, the lower jaw consists of a single bone, rather than several.
In modern evolutionary biology, the term species is usually defined as a population or group of organisms that look more or less alike and that is potentially capable of interbreeding to produce fertile offspring. Practically speaking, individuals are usually assigned to a species based on their appearance, but it is their ability to interbreed that ultimately validates (or invalidates) the assignment. Thus, no matter how similar two populations may look, if they are incapable of interbreeding, they must be assigned to different species.
Populations within a species that are quite capable of interbreeding but may not regularly do so are called subspecies. Evolutionary theory suggests that species evolve from these populations or subspecies through the accumulation of differences in the gene pools of the separated groups.
Although living primates are a varied group of animals, they do have a number of features in common. These features are displayed in varying degrees by the different kinds of primates: in some they are barely detectable, while in others they are greatly elaborated.
All are useful in one way or another to arboreal (or tree-dwelling) animals, although they are not essential to life in trees.
Primate Sense Organs
The primates' adaptation to their way of life in the trees coincided with changes in the form and function of their sensory apparatus: the senses of sight and touch became highly developed, and the sense of smell declined.
Catching insects in trees, as the early primates did and as many still do, demands quickness of movement and the ability to land in the right place without falling. Thus, they had to be adept at judging depth, direction, distance and the relationship of objects in space.
Primates' sense of touch also became highly developed as a result of arboreal living. An effective feeling and grasping mechanism was useful to them in grabbing their insect prey, and by preventing them from falling and tumbling while moving through the trees.
The Primate Brain
By far the most outstanding characteristic in primate evolution has been the enlargement of the brain among members of the order. Primate brains tend to be large, heavy in proportion to body weight, and very complex.
Reasons for this important change in brain size are many:
- Prior to 65 Myrs ago, mammals seem to have been nocturnal in their habits; after 65 million years ago, primates began to carry out their activities in the daylight hours. As a consequence, the sense of vision took on greater importance, and so visual acuity was favored by natural selection.
- another hypothesis involves the use of the hand as a tactile organ to replace the teeth and jaws. The hands assumed some of the grasping, tearing and dividing functions of the snout, again requiring development of the brain centers for more complete coordination.
- The enlarged cortex not only provided the primates with a greater degree of efficiency in the daily struggle for survival but also gave them the basis for more sophisticated cerebration or thought. The ability to think probably played a decisive role in the evolution of the primates from which human beings emerged.
Although they have added other things than insects to their diets, primates have retained less specialized teeth than other mammals.
The evolutionary trend for primate dentition has generally been toward economy, with fewer, smaller, more efficient teeth doing more work.
- Our own 32 teeth are fewer in number than those of some, and more generalized than most, primates.
A number of factors are responsible for the shape of the primate skull as compared with those of most other mammals: changes in dentition, changes in the sensory organs of sight and smell, and increase in brain size. As a result, primates have more a humanlike face than other mammals.
The upper body is shaped such as to allow greater maneuverability of the arms, permitting them to swing sideways and outward from the trunk of the body.
The structural characteristics of the primate foot and hand make grasping possible; the digits are extremely flexible, the big toe is fully opposable to the other digits in most species, and the thumb is opposable to the other digits to varying degrees. The flexible, unspecialized primate hand was to prove a valuable asset for future evolution of this group. It allowed early hominines to manufacture and utilize tools and thus embark on the new and unique evolutionary pathway that led to the revolutionary ability to adapt through culture.
Types of Living Primates
The most primitive of the primates are represented by the various prosimians, including the lemurs and the lorises, which are more similar anatomically to earlier mammalian ancestors than are other primates (monkeys, apes, humans). They tend to exhibit certain more ancestral features, such as a more pronounced reliance on olfaction (sense of smell). Their greater olfactory capabilities are reflected in the presence of a moist, fleshy pad at the end of the nose and in a relatively long snout.
Lemurs and lorises represent the same general adaptive level. Both groups exhibit good grasping and climbing abilities and a fairly well developed visual apparatus, although their vision is not completely stereoscopic, and color vision may not be as well developed as in anthropoids.
At present, lemurs are found only on the island of Madagascar and adjacent islands off the east coast of Africa.
As the only natural nonhuman primates on this island, they diversified into numerous and varied ecological niches without competition from monkeys and apes. Thus, the 52 surviving species on Madagascar represent an evolutionary pattern that has vanished elsewhere.
Lemurs range in size from 5 inches to a little over two feet. While the larger lemurs are diurnal and exploit a wide variety of dietary items (leaves, fruits, buds, bark), the smaller forms (mouse and dwarf lemurs) are nocturnal and insectivorous.
Lemurs display considerable variation regarding numerous other aspects of behavior. While many are primarily arboreal, others (e.g. ring-tailed lemur) are more terrestrial. Some arboreal species are quadrupeds, and others are vertical clingers and leapers.
Lorises are similar in appearance to lemurs, but were able to survive in mainland areas by adopting a nocturnal activity pattern at a time when most other prosimians became extinct. Thus, they were (and are still) able to avoid competition with more recently evolved primates (diurnal monkeys).
There are five loris species, all of which are found in tropical forest and woodland habitats of India, Sri Lanka, Southeast Asia and Africa.
Locomotion in lorises is a slow, cautious climbing form of quadrupedalism, and flexible hip joints permit suspension by hind limbs while the hands are used in feeding. Some lorises are almost entirely insectivorous; others supplement their diet with various combinations of fruits, leaves, gums, etc.
There are seven recognized species, all restricted to island areas in Southeast Asia. They inhabit a wide range of forest types, from tropical forest to backyard gardens.
They are nocturnal insectivores, leaping onto prey from lower branches and shrubs. They appear to form stable pair bonds, and the basic tarsier social unit is a mated pair and their young offspring.
Tarsiers present a complex blend of characteristics not seen in other primates. They are unique in that their enormous eyes, which dominate much of the face, are immobile within their sockets. To compensate for this inability to move the eyes, tarsiers are able to rotate their heads 180º, like owls.
Although there is much variation among simians (also called anthropoids), there are certain features that, when taken together, distinguish them as a group from prosimians (and other mammals)
- generally larger body size
- larger brain
- reduced reliance on the sense of smell
- increased reliance on vision, with forward-facing eyes placed at the front of the face
- greater degree of color vision
- back of eye socket formed by a bony plate
- blood supply to brain different from that of prosimians
- fusion of two sides of mandible at midline to form one bone
- less specialized dentition
- differences with regard to female internal reproductive anatomy
- longer gestation and maturation periods
- increased parental care
- more mutual grooming
Approximately 70 percent of all primates (about 240 species) are monkeys, although it is frequently impossible to give precise numbers of species because the taxonomic status of some primates remains in doubt and there are constantly new discoveries.
Monkeys are divided into two groups (New World and Old World) separated by geographical area as well as by several million years of separate evolutionary history.
New World monkeys exhibit a wide range of size, diet, and ecological adaptation. In size, they vary from tiny marmosets and tamarins to the 20-pound howler monkey. Almost all are exclusively arboreal; most are diurnal. Although confined to trees, New World monkeys can be found in a wide range of arboreal environments throughout most forested areas in Southern Mexico and Central and South America. One of the characteristics distinguishing New World monkeys from Old World is the shape of their nose: they have broad noses with outward-facing nostrils.
Old World monkeys display much more morphological and behavioral diversity than New World monkeys. Except for humans, they are the most widely distributed of all living primates. They are found throughout sub-Saharan Africa and Southern Asia, ranging from tropical jungle habitats to semiarid desert and even to seasonally snow-covered areas in northern Japan. Most are quadrupedal and primarily arboreal.
Apes and humans
This group is made up of several families:
- Hylobatidae (gibbons and siamangs)
- Pongidae (orangutans)
- Hominidae (humans, gorillas, common chimpanzees, bonobos)
They differ from monkeys in numerous ways:
- generally larger body size, except for gibbons and siamangs
- absence of a tail
- shortened trunk
- differences in position and musculature of the shoulder joint (adapted for suspensory locomotion)
- more complex behavior
- more complex brain and enhanced cognitive abilities
- increased period of infant development and dependency.
Found today only in heavily forested areas on the Indonesian islands of Borneo and Sumatra, orangutans are slow, cautious climbers whose locomotor behavior can best be described as "four-handed", a tendency to use all four limbs for grasping and support. Although they are almost completely arboreal, they do sometimes travel quadrupedally on ground. They are very large animals with pronounced sexual dimorphism: males weigh over 200 pounds while females are usually less than 100 pounds.
The largest of all living primates, gorillas are today confined to forested areas of western and equatorial Africa. There are four generally recognized subspecies: Western Lowland Gorilla, Cross River Gorilla, Eastern Lowland Gorilla, and Mountain Gorilla. Gorillas exhibit strong sexual dimorphism. Because of their weight, adult gorillas, especially males, are primarily terrestrial and adopt a semiquadrupedal (knuckle-walking) posture on the ground. All gorillas are almost exclusively vegetarian.
The best-known of all nonhuman primates, Common Chimpanzees are found in equatorial Africa. In many ways, they are structurally similar to gorillas, with corresponding limb proportions and upper body shape, because of their similar locomotion when on the ground (quadrupedal knuckle-walking). However, chimps spend more time in trees; when on the ground, they frequently walk bipedally for short distances when carrying food or other objects.
They are highly excitable, active and noisy. Common Chimpanzee social behavior is complex, and individuals form lifelong attachments with friends and relatives. They live in large, fluid communities of as many as 50 individuals or more. At the core of a community is a group of bonded males. They act as a group to defend their territory and are highly intolerant of unfamiliar chimps, especially nongroup males.
Found only in an area south of the Zaire River in the Democratic Republic of Congo, Bonobos (also called Pygmy Chimpanzees) have a strong resemblance to Common Chimpanzees, but are somewhat smaller. Yet they exhibit several anatomical and behavioral differences. Physically, they have a more linear body build, longer legs relative to the arms, a relatively smaller head, and a dark face from birth. Bonobos are more arboreal than Common Chimpanzees, and they appear to be less excitable and aggressive.
Like Common Chimpanzees, Bonobos live in geographically based, fluid communities, and they exploit many of the same foods, including occasional meat derived from killing small mammals. But they are not centered around a group of closely bonded males. Instead, male-female bonding is more important than in Common Chimpanzees.