What makes a Primate Human?

What are the implications of the shared characteristics between humans and the other primates?
Why do anthropologists study the social behavior of monkeys and apes?

Information about primate behavior and ecology plays an integral role in the story of human evolution.

Humans are primates, and the first members of the human species were probably more similar to living nonhuman primates than to any other animals on earth. Thus, by studying living primates we can learn something about the lives of our ancestors.
Humans are closely related to primates and similar to them in many ways. If we understand how evolution has shaped the behavior of animals so much like ourselves, we may have greater insights about the way evolution has shaped our own behavior and the behavior of our ancestors.

Primate social behavior

Over the past four decades, primatologists have made prolonged close-range observations of monkeys and apes in their natural habitats, and we are discovering much about social organization, learning ability, and communication among our closest relatives (chimpanzees, and gorillas) in the animal kingdom.

In particular, we are finding that a number of behavioral traits that we used to think of as distinctively human are found to one degree or another among other primates, reminding us that many of the differences between us and them are differences of degree, rather than kind.

The Group

Primates are social animals, living and travelling in groups that vary in size from species to species. In most species, females and their offspring constitute core of social system.

Among chimps, the largest organizational unit is the community, composed of 50 or more individuals. Rarely however are all these animals together at a single time. Instead they are usually ranging singly or in small subgroups consisting of adult males together, females with their young, or males and females together with their young. In the course of their travels, subgroups may join forces and forage together, but sooner or later these will break up into smaller units.

Dominance

Many primate societies are organized into dominance hierarchies, that impose some degree of order with groups by establishing parameters of individual behavior.

Although aggression is frequently a means of increasing one's status, dominance usually serves to reduce the amount of actual physical violence. Not only are lower-ranking animals unlikely to attack or even threaten a higher-ranking one, but dominant animals are also frequently able to exert control simply by making a threatening gesture.

Individual rank or status may be measured by access to resources, including food items and mating partners.

An individual's rank is not permanent and changes throughout life. It is influenced by many factors, including sex, age, level of aggression, amount of time spent in the group, intelligence, etc.

In species organized into groups containing a number of females associated with one or several adult males, the males are generally dominant to females. Within such groups, males and females have separate hierarchies, although very high ranking females can dominate the lowest-ranking males (particularly young ones).

Yet many exceptions to this pattern of male dominance:

Among many lemur species, females are the dominant sex
Among species that form monogamous pairs (e.g., indris, gibbons), males and females are codominant

Aggression

Within primate societies, there is an interplay between affiliative behaviors that promote group cohesion and aggressive behaviors that can lead to group disruption. Conflict within a group frequently develops out of competition for resources, including mating partners and food items. Instead of actual attacks or fighting, most intragroup aggression occurs in the form of various signals and displays, frequently within the context of dominance hierarchy. Majority of such situations are resolved through various submissive and appeasement behaviors.

But conflict is not always resolved peacefully.

High-ranking female macaques frequently intimidate, harass, and even attack lower-ranking females, particularly to restrict their access to food
Competition between males for mates frequently results in injury and occasionally in death
Aggressive encounters occur between groups as well as within groups
Aggression occurs in the defense of territories

Individual interaction

To minimize actual violence and to defuse potentially dangerous situations, there is an array of affiliative, or friendly, behaviors that serve to reinforce bonds between individuals and enhance group stability. Common affiliative behaviors include reconciliation, consolation, and simple interactions between friends and relatives.

Most such behaviors involve various forms of physical contact including touching, hand holding, hugging, and, among chimpanzees, kissing. In fact, physical contact is one of the most important factors in primate development and is crucial in promoting peaceful relationships in many primate social groups.

One of the most notable primate activities is grooming, the ritual cleaning of another animal's coat to remove parasites, shreds of grass or other matter. Among bonobos and chimps, grooming is a gesture of friendliness, submission, appeasement or closeness.

The mother-infant bond is the strongest and most long-lasting in the group. It may last for many years; commonly for the lifetime of the mother.

Play

Frequent play activity among primate infants and juveniles is a means of learning about the environment, testing strength, and generally learning how to behave as adults. For example, Chimpanzee infants mimic the food-getting activities of their mothers, "attack" dozing adults, and "harass" adolescents.

Communication

Primates, like many animals, vocalize. They have a great range of calls that are often used together with movements of the face or body to convey a message.

Observers have not yet established the meaning of all the sounds, but a good number have been distinguished, such as warning calls, threat calls, defense calls, and gathering calls. Much of the communication takes place by the use of specific gestures and postures.

Home range

Primates usually move about within circumscribed areas, or home ranges, which are of varying sizes, depending on the size of the group and on ecological factors, such as availability of food. Home ranges are often moved seasonally. The distance traveled by a group in a day varies, but may include many miles.

Within this home range is a portion known as the core area, which contains the highest concentration of predictable resources (water, food) and where the group is most frequently found (with resting places and sleeping trees).

The core area can also be said to be a group's territory, and it is this portion of the home range that is usually defended against intrusion by others:

Gorillas do not defend their home ranges against incursions of others of their kind
Chimps, by contrast, have been observed patrolling their territories to ward off potential trespassers

Among primates in general, the clearest territoriality appears in forest species, rather than in those that are terrestrial in their habits.

Tool use

A tool may be defined as an object used to facilitate some task or activity. A distinction must be made between simple tool use and tool making, which involves deliberate modification of some material for its intended use.

In the wild, gorillas do not make or use tools in any significant way, but chimpanzees do. Chimps modify objects to make them suitable for particular purposes. They can also pick up and even prepare objects for future use at some other location, and they can use objects as tools to solve new and novel problems.

Examples:

use of stalks of grass to collect termites
use of leaves as wipes or sponges to get water out of a hollow to drink
use of rocks as hammers and anvils to open palm nuts and hard fruits

Primates and human evolution

Studies of monkeys and apes living today [especially those most closely related to humans: gorillas, bonobos and chimpanzees] provide essential clues in the reconstruction of adaptations and behavior patterns involved in the emergence of our earliest ancestors.

These practices have several implications:

Chimpanzees can be engaged in activities that prepare them for a future (not immediate) task at a somewhat distant location. These actions imply planning and forethought
Attention to the shape and size of the raw material indicates that chimpanzee toolmakers have a preconceived idea of what the finished product needs to be in order to be useful

To produce a tool, even a simple tool, based on a concept is an extremely complex behavior. Scientists previously believed that such behavior was the exclusive domain of humans, but now we must question this very basic assumption.

At the same time, we must be careful about how we reconstruct this development. Primates have changed in various ways from earlier times, and undoubtedly certain forms of behavior that they now exhibit were not found among their ancestors.

Also it is important to remember that present-day primate behavior shows considerable variation, not just from one species to another, but also from one population to another within a single species.

Primate fossils

The study of early primate fossils tells us something we can use to interpret the evolution of the entire primate line, including ourselves. It gives us a better understanding of the physical forces that caused these primitive creatures to evolve into today's primates.

Ultimately, the study of these ancient ancestors gives us a fuller knowledge of the processes through which insect-eating, small-brained animals evolved into a toolmaker and thinker that is recognizably human.

Rise of the primates

For animals that have often lived where conditions for fossilization are generally poor, we do have a surprisingly large number of primate fossils. Some are relatively complete skeletons, while most are teeth and jaw fragments.

Primates arose as part of a great adaptive radiation that began more than 100 million years after the appearance of the first mammals. The reason for this late diversification of mammals was that most ecological niches that they have since occupied were either preempted by reptiles or were nonexistent until the flowering plants became widespread beginning about 65 million years ago.

By 65 million years ago, primates were probably beginning to diverge from other mammalian lineages (such as those which later led to rodents, bats and carnivores). For the period between 65-55 Myrs ago (Paleocene), it is extremely difficult to identify the earliest members of the primate order:

available fossil material is scarce
first primates were not easily distinguished from other early (generalized) mammals

Eocene primates

First fossil forms that are clearly identifiable as primates appeared during Eocene (55-34 million years ago). From this period have been recovered a wide variety of primates, which can all be called prosimians. Lemur-like adapids were common in the Eocene, as were species of tarsier-like primates.

These first primates were insect eaters and their characteristics developed as an adaptation to the initial tree-dwelling environment:

larger, rounder braincases
nails instead of claws
eyes that are rotated forward, allowing overlapping fields of perception and thus binocular vision
presence of opposable large toe

This time period exhibited the widest geographical distribution and broadest adaptive radiation ever displayed by prosimians. In recent years, numerous finds of Late Eocene (36-34 Myrs ago) suggest that members of the adapid family were the most likely candidates as ancestors of early anthropoids.

Oligocene primates

The center of action for primate evolution after Eocene is confined largely to Old World. Only on the continents of Africa and Eurasia can we trace the evolutionary development of apes and hominids due to crucial geological events; particularly continental drift.

During the Oligocene (34-23 Myrs ago), a great deal of diversification among primates occurred. The vast majority of Old World primate fossils for this period comes from just one locality: the Fayum area of Egypt. From Fayum, 21 different species have been identified.

Apidium

Most abundant of all Oligocene forms
This animal was about size of squirrel
Teeth suggest diet composed of fruits and probably seeds
Preserved remains of the limbs indicate that this creature was a small arboreal quadruped, adept at leaping and springing

Propliopithecus

Morphologically, quite primitive
Not showing particular derived tendencies in any direction
Small to medium in size
Likely fruit eaters

Aegyptopithecus

Largest of Fayum anthropoids, roughly the size of modern howler (13 to 18 pounds)
Primitive skull, which is small and resembles the one of a monkey in some details
From analysis of limb proportions and muscle insertions, Aegyptopithecus was a short-limbed, heavily muscled, slow-moving arboreal quadruped
This form of primates important because, better than any other fossil primate, it bridges the gap between the Eocene prosimians and the Miocene hominoids.

Miocene Primates

A great abundance of hominoid fossil material has been found in the Old World from the Miocene (23-7 million years ago).

Based on size, these fossils can be divided into two major subgroupings: small-bodied and large-bodied hominoids.

Small-bodied varieties comprise gibbon and siamang
Large-bodied hominoids are Pongo (orangutan), Gorilla, Pan (chimpanzees and bonobos) and Homo. These four forms can then be subdivided into two major subgroups: Asian large-bodied (orangutans) and African large-bodied (gorillas, chimpanzees and humans).

The remarkable evolutionary success represented by the adaptive radiation of large-bodied hominoids is shown in its geographical range from Africa to Eurasia. Large-bodied hominoids first evolved in Africa around 23 million years ago. Then they migrated into Eurasia, dispersed rapidly and diversified into a variety of species. After 14 million years ago, we have evidence of widely distributed hominoids in many parts of Asia and Europe. The separation of the Asian large-bodied hominoid line from the African stock (leading ultimately to gorillas, chimps and humans) thus would have occurred at about that time.

African Forms

A wealth of early hominoid fossils has come from deep and rich stratigraphic layers of Kenya and Uganda
These diverse forms are presently classified into at least 23 species of hominoids
Best samples and thus best known forms are those of genus Proconsul
Environmental niches were quite varied: some species were apparently confined to dense rain forests, others potentially exploited more open woodlands
Considerable diversity of locomotor behaviors: some were at least partially terrestrial - on the ground, some may have even occasionally adopted a bipedal position
Most forms were probably fruit eaters, some may also have included considerable amounts of leaves in their diet

European Forms

Very few fossils have been discovered
Most researchers would place these forms into the genus Dryopithecus
Evolutionary relationship with other hominoids is both difficult and controversial at this point.

South/Southwest Asian Forms

Remains have been found in Turkey, India and Pakistan
Attributed to genus Sivapithecus
Probably good-sized hominoid (70-150 pounds), that inhabited a mostly arboreal niche
Facial remains have concave profiles and projecting incisors, which bear striking similarities with the orangutan
Other traits are distinctively unlike an orangutan. For example, the forelimb suggests a unique mixture of traits, indicating probably some mode of arboreal quadrupedalism but with no suspensory component.

Miocene apes and Human Origin

The large-bodied African hominoids appeared by 16 million years ago and were widespread even as recently as 8 million years ago.

Based on fossils of teeth and jaws, it was easy to postulate some sort of relationship between them and humans. A number of features: position of incisors, reduced canines, thick enamel of the molars, and shape of tooth row, seemed to point in a somewhat human direction.

Although the African hominoids display a number of features from which hominine characteristics may be derived, and some may occasionally have walked bipedally, they were much too apelike to be considered hominines.

Nevertheless, existing evidence allows the hypothesis that apes and humans separated from a common evolutionary line sometime during the Late Miocene and some fossils, particularly the African hominoids, do possess traits associated with humans.

Not all African apes evolved into hominines. Those that remained in the forests and woodlands continued to develop as arboreal apes, although ultimately some of them took up a more terrestrial life. These are the bonobos, chimpanzees and gorillas, who have changed far more from the ancestral condition than have the still arboreal orangutans.

Summary

When did the first primates appear, and what were they like?

The earliest primates had developed by 60 million years ago and were small, arboreal insect eaters. Their initial adaptation to life in trees set the stage for the subsequent appearance of other primate models.

When did the first monkeys and apes appear, and what were they like?

By the Late Eocene (about 37 Myrs ago), monkeys and apes about the size of modern house cats were living in Africa. By about 20 million years ago, they had proliferated and soon spread over many parts of the Old World. Some forms remained relatively small, while others became quite large, some even larger than present-day gorillas.

When did group of primates give rise to the human line of evolution?

Present evidence suggests that our own ancestors are to be found among the African large-bodied hominoids, which were widespread between approximately 17 and 8 million years ago. Some of these ape-like primates lived in situations in which the right kind of selective pressure existed to transform them into primitive hominines.