You might recall from your past schooling the system of classifying organisms by Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species. Humans (as a species) share an "order" with other "Primates" (all monkeys, baboons, tarsiers, gibbons, lorises, and apes) and with the "family" called "Hominids" (the eight remaining species of 'great apes'). In this chapter, we will take a look at the evolutionary background of this order as well as taking a look at the currently living species which make up the rest of the primates.
This chart shows the 'nested' structure of classification from Domain to Species. Our own species (Homo sapiens) is provided as an example.
Biological anthropologists are particularly interested in evolution because they, like all anthropologists, are interested in what it means to 'be human' and, as science has learned more about the evolutionary past and about these related species, we have come to recognize that many of the features we thought made us 'special' are actually not quite that unique! Rather famously, anthropologists assumed humans were the only species to use tools until, in the 1960s, Jane Goodall discovered that chimpanzees would strip leaves off of sticks to go 'fishing' for termites. They even showed preference for certain kinds of implements over others and could be observed 'teaching' their techniques to others. When met with this discovery, Goodall's mentor Louis Leaky is quoted to have said: "Now we must redefine 'tool,' redefine '[human],' or accept chimpanzees as humans."
By looking at the evolutionary differences between other primates and/or other hominids and us, biological anthropologists can paint a better picture of not only what (maybe) sets us apart, but also what we might have in common. As for Leaky's quote, you can still find anthropologists today who happily carry on following any one of those three options.
Who are the Primates?
To begin with, I may have already showed my hand by asking "who" and not "what" in the heading for this section. Part of the reason anthropologists are interested in all primates is because the 'human' and even the 'cultural' line is increasingly blurry when we begin to look at our close cousins in evolutionary terms. In this section, we will look at what helps to define our primate order.
Humans are primates, but let's begin by dispelling perhaps the greatest misconception of human evolution: we (humans) did not evolve from monkeys. Rather, given a thorough understanding of evolution, all extant (currently present) primates (including monkeys as well as humans) evolved from a common ancestor. As a result, monkeys are not our ancestors but something more like very distant cousins. Similarly, we did not evolve from chimpanzees or another extant (currently living) ape species. Once again, we instead share a common ancestor. Humans, chimpanzees, and bonobos are a bit like first cousins. Humans, orangutans, and gorillas are more like second cousins.
Caution
It is wildly inaccurate to call chimpanzees or bonobos 'monkeys.' (Both species are actually far more closely related to us than to monkeys!) In terms of 'phylogenetic' ancestors, the relationship between a chimpanzee and a monkey is similar to that between a horse and a rhinoceros!
Humans have much in common with other animals, especially the other apes and the monkeys. We have in common tools, reasoning, complex sociality, reciprocity and even Culture, according to some definitions. We also share susceptibility to many of the same diseases like polio, measles, and Ebola. Because humans share so much in common socially and biologically with apes, monkeys and similar species, we are all classified in the same family called the Primates. Primatology is a sub-discipline of biological anthropology that focuses on primate behavior, biology, and conservation.
Defining Primates
We classify all animals according to the their similarities and differences. Carolus Linnaeus (1707-1778) was a Swedish botanist who set out to categorize all of Earth’s animal and plant species according to their physical similarities and differences called morphology. (Prior to Linnaeus many Europeans subscribed to the idea of the “Great Chain of Being” as a classification system of rocks, plants, animals, humans, and divine beings ranked according to their moral perfection with God at the top.) The system that Linnaeus initiated is not based on ranking species according to better or worse but on their physical similarities. More recently, genetic information has been used to paint an even clearer picture based on the evolutionary tree.
Four different primate skulls which are morphologically different. From left to right: Human, Chimpanzee, Orangutan, and Macaque. Skulls and jaw bones (mandibles) are especially telling for biological anthropologists because eyes and eye sockets, teeth, and brain cavities can be very revealing of perception, diet, and cognitive priorities or “intelligence.” [Source: PLoS Biology, Wikimedia Commons]
You are likely familiar with the class called Mammalia. Mammals are vertebrates which share characteristics such as nursing young, giving birth to live young, being warm blooded, and having fur.
Primates—including human beings—are characterized by a number of distinct physical features that distinguish them from other mammals. Some of these common tendencies are morphological and others revolve around life history, features having to do with the timing and duration of life events. Not all primates have every single trait listed and so these can be thought of as primates trends or tendencies. These include
opposable thumbs and (in nonhuman primates) opposable big toes;
the presence of five digits (fingers or toes) on the appendages;
flat nails instead of curved claws;
postorbital bars (bony rings that completely surround the eyes);
pads at the tips of the fingers made up of deposits of fat and nerves;
reduced reliance on sense of smell and a relatively small snout;
binocular vision (being able to see one three-dimensional image with both eyes);
depth perception;
a relatively slow reproductive rate and late sexual maturity;
a relatively long period of immaturity marked by learning from adults;
high degree of parental investment in offspring;
relatively large brain size; and
complex sociality.
Some of these traits enhance dexterity and enable primates to use their hands and feet differently from other mammals. Other traits on this list represent a shift in emphasis among the sense organs between primates and other mammals. The effects of these differences have all had an effect on the behavior of this related subset of mammal species.
The primate hand has five fingers and is capable of grasping. Some primates like apes and Old-World monkeys have true opposable thumbs, meaning the thumb can be oriented in opposition to the other digits. Grasping hands allows primates to manipulate their environment and for some, make and use tools. Primates have an expanded capacity for touch, especially by the hands, rather than smell. Some primates, especially those that are arboreal, living in trees, also have grasping feet.
The hands and feet of a Barbary macaque (Macaca sylvanus). [Source: Grez, Wikimedia Comons]
Primates tend to have relatively poor smell, but keen eyesight. The genes for smell in primates are either turned off by regulator genes or are deactivated by mutations. All primates have fields of vision that overlap, which allows for keener three-dimensional depth perception called stereoscopic vision or stereopsis. Because many primates are arboreal (live in trees), it has been suggested that keen eyesight is critical for judging distance and depth of tree branches. Others argue that this type of vision developed in relation to predation of insects. A binocular field of vision is common in predators who need to judge distances while non-overlapping fields of view are more common in prey animals, who need a wider field of vision to spot predators.
Primates tend to live in social groups. Social grooming is especially important for many primates. Baboons who groom each other regularly, are more likely to come to each other’s aid in a crisis. Thus social grooming is a form of reciprocity (exchanging favors). There are exceptions, for example, orangutans (especially their males) lead mostly solitary lives.
Primates also have larger-than-expected brains for their body size. This is called their encephalization quotient. Increasingly, however, we are coming to realize that brain functions have more to do with the folds and wrinkles that make up the brain than overall brain size. Brain areas related to memory, thought, and association are increased in primates. Primates proportionally devote more brain to the neocortex than any other animal—the portion of the brain related to cognition, memory, and abstraction. As a result, primates can solve complex problems. (In fact, chimpanzees have been observed to outcompete humans on some memory tasks.)
Primates use flexible thinking to forage for patchily distributed food and solve physical problems, but also to negotiate group dynamics. Primates often live in socially complex groups and must form and manage social ties and avoid conflict. Most primate relationships require skillful negotiation in some form. Studies with other primates (such as baboons) have revealed that the ‘stress’ we experience from these social relationships may be common across primates.
In addition, primate infants take a long time to develop and a strong mother-infant bond develops. Most primates give birth to a single offspring and offspring often receive extensive care called “parental investment” from the mother or, less commonly, from both parents. During this long period of dependency, the infant learns appropriate behavior and how to solve problems. Humans stand out in this arena with an especially long period of juvenile dependence relative to their lifespans.
Primate Evolution
Studying nonhuman primates in their environment is key to understanding variations in behavior. For example, the behavior (such as tool use and hunting patterns) of chimpanzees that live in the tropical regions of Africa is quite different from the behavior of chimpanzees that live in the savanna at Fongoli in Senegal, in West Africa. The two environments also show differences in gender roles with both males and females in the Fongoli savannah group involved in hunting while only male chimpanzees hunt in the rainforests.
An important question that primatologists and biological anthropologists seek to answer is: do nonhuman primates have culture? Whenever we see an exchange of ideas where one individual is involved in teaching another and when that knowledge is passed on to others in a group is according to anthropologists, a form of culture. We see this happen in chimpanzee groups where older chimpanzees teach the young how to use sticks to termite-fish, the process of extracting termites from a termite mound using a stick.
Primatologists are still working to address why primates evolved as they did and how they filled and exploited the range of ecological niches they have filled through today (if we include ourselves, it is virtually all of them). Over the last century, various hypotheses have been raised to account for the evolution of primates and their unusual anatomical characteristics.
Hypotheses for Primate Evolution
For many groups of mammals, there is a key feature that led to their success. A good example is powered flight in bats. Primates lack a feature like this. Instead, if there is something unique about primates, it is probably a group of features rather than one single thing. Because of this, anthropologists and paleontologists struggle to describe a single ecological scenario that could explain the rise and success of our own order. Three major hypotheses have been advanced to consider the origin of primates and to explain what makes our order distinct among mammals.
The three major hypotheses for primate evolution are Left: the arboreal hypothesis, Center: the visual predation hypothesis, and Right: the angiosperm-primate coevolution hypothesis. [Source: Explorations: An Open Invitation to Biological Anthropology (2nd ed.) by Mary Nelson under a CC BY-NC 4.0 License]
The arboreal hypothesis proposes that primates evolved the traits they did as an adaptation to life in the trees. Specifically, primates evolved thumbs and big toes that are perpendicular to the other digits to help them grasp onto branches. Leaping limb to limb helps to explain the need for binocular vision and depth perception. And movement into the air (versus against the ground) may have contributed to our loss of smell—perhaps evolved to better taste as they began to consume more fruits.
We might note, however, that forward-facing eyes are characteristic not only of primates but also of predators such as cats and owls that prey on small animals. Thus, forward-facing eyes, grasping hands and feet, and the presence of nails instead of claws may instead be adaptations that helped early primates succeed as predators. According to the visual predation hypothesis, primate features are adaptations first for hunting insects and other small prey in the shrubby forest undergrowth and the lowest tiers of the forest canopy.
The visual predation hypothesis was unpopular with some anthropologists. One reason for this is that many primates today are not especially predatory. Another is that, whereas primates do seem well adapted to moving around in the smallest, terminal branches of trees, insects are not necessarily easier to find there. A counterargument to the visual predation hypothesis is the angiosperm-primate coevolution hypothesis.
This coevolution hypothesis states that the basic primate traits developed in coevolution with the rise of flowering plants, also known as angiosperms. Flowering plants provide numerous resources, including nectar, seeds, and fruits, and their appearance and diversification were accompanied by the appearance of ancestral forms of major groups of modern birds and mammals. Some argue that visual predation is not common among modern primates and that forward-facing eyes and grasping extremities may have arisen in response to the need for fine visual and tactile discrimination to feed on small food items, such as fruits, berries, and seeds, found among the branches and stems of flowering plants—particularly at their extremities.
Further support for this hypothesis came in 1990 when D. Tab Rasmussen noted several parallel traits in primates and the South American woolly opossum known for similarly foraging on both fruits and insects. Noting that insects are also attracted to fruit (and flowers) and provide a convenient opportunity for a primarily fruit-eating primate to gather protein, Rasmussen proposed something of a compromise between the visual predation hypothesis and the angiosperm-primate coevolution hypothesis.
But we now know that evolution is a complex process continually modifying populations in light of their environments. It is possible primates evolved as a result of some further combination of elements from each of these hypotheses. It is also possible none of them hit the mark. They are simply some of the best guesses put forward and so far supported by fossil and genetic evidence.
Given the length of time the primate order is thought to have been around, it is no surprise that there are many more specific classifications within it. While it is not strictly necessary to know them all for most anthropologists, having a sense of how broad the order is can help us to make sense of our common traits. And again, those common traits are what make all of what makes us ‘human’ possible. Let’s take a brief look at the primates still alive today.
The senegal bushbaby (Galago senegalensis) is an example of an arboreal primate thought to closely resemble some of our earliest primate ancestors. Their behavior and lifestyle seem to fit with many elements of each of the three main hypotheses proposed for primate evolution. [Source: Wikimedia Commons]
Primates in Deep Time
Over the last half century or so, scientists have developed a number of reliable techniques for determining the age of some very old things—including fossils. They have gotten very good at being able to reliably put fossils in order producing what is known as a ‘chronology.’ This chronology is broken into nested spans of time known as eons, eras, periods, epochs, and ages. At the far end of the scale (eons), the spans of time are extremely large. Four eons span the formation of the earth as a plant to the present time. Eons, apart from the current one at 539 million, are measured in billions of years. At the near end (age), the spans of time are still large but much more fathomable (thousands or millions of years).
In evolutionary terms, you will often hear about periods and epochs spanning hundreds of thousands to tens of millions of years. (One famous epoch you may have heard of is the Pleistocene. This can be a particularly confusing epoch because its everyday name is the Ice Age.) Regardless of the time scale, these spans of time constitute the convention of ‘geologic time’ used by scientists (including paleoanthropologists) to describe Earth’s history. It is called geologic time in part because the main way of defining a given span is by stratigraphy—the relative position of layers (strata) of rock and other materials over time with older layers lower than newer ones.
A butte in Palo Duro Canyon in Randall County, Texas, United States shows clear layers (strata) of sediment deposited, eroded, and moved about over millions of years. [Source: Larry D. Moore, CC BY 4.0, Wikimedia Commons]
Coupled with absolute dating techniques paleontologists, geologists, ecologists, and other scientists can produce an increasingly accurate picture of Earth’s past. In contrast to relative dating which can only say x is younger/older than y (a relative comparison), absolute dating is called ‘absolute’ because it can say x is n years old (an absolute value). Techniques of absolute dating rely on known values and rates of change to determine the numerical age of a material. One common absolute dating technique is radiometric dating which uses calculations based on known and constant rates of radioactive decay in materials. An example is ‘carbon-14’ / ‘radiocarbon’ dating. Another technique is thermoluminescence (TL) dating which uses lab-controlled heat exposure to measure stored energy caused by natural background radiation over time. Cosmogenic surface exposure dating measures rare isotopes created when cosmic rays interact with the Earth's surface. Dendrochronology is a form of dating which uses a combination of absolute and relative techniques. This specialty relies on the fact that trees in seasonal climates produce distinct annual growth rings, with ring width reflecting the environmental conditions during that year. Dendrochronology involves counting tree rings (including fossilized ones) based on known patterns to determine the age of the wood.
All this talk of ‘deep time’ is to help put the earliest primates—and how scientists know anything at all about them—into context. The techniques described above along with the use of the “molecular clock”—which can estimate the time since two species diverged from a common ancestor based the principle that certain DNA mutations accumulate over time at a relatively constant rate—help to produce an increasingly accurate picture of our earliest primate ancestors.
An artist rendition of Purgatorius, a squirrel-like species from the North American continent thought to be one of the first primates. [Source: PNAS, Wikimedia Commons]
The Paleocene epoch began approximately 65 million years ago (MYA) and ended about 54 MYA. It is the most poorly understood epoch of the Cenozoic era, as it is the time period with the fewest fossils to represent it. However, this epoch is considered important to primate evolution because it offers the first unequivocal record of the earliest primates. Evidence of the most primitive primate yet identified was found in the U.S. state of Montana, in a geological deposit that was dated to the earliest part of the Paleocene. This creature is known as Purgatorius. Purgatorius is similar to extinct and living primates and distinct from other mammals based on its teeth.
The species density of modern non-human primates thought to all descend from Purgatorius or a creature like it. Darker red indicates greater density. Note the lack of primates in the region in which Purgatorius itself originated and the similarities of climate in the shaded regions. [Source: Wikimedia Commons]
The Eocene epoch, which began approximately 54 MYA and ended about 34 MYA, is marked by the disappearance of Purgatorius and the first appearance of primates that more closely resemble modern-day primates, especially in their bony eye sockets. This contrasts with other mammals whose postorbital bars are part bone and part cartilage. Some fossil specimens also possess a toothcomb and/or a grooming claw, characteristics that are exclusively found in lemurs, lorises, and “bushbaby” primates today. Other anatomical characteristics that are significant would be the ankle bones which researchers believe played a key role in the evolutionary success of primates. The evolution of primates during the Eocene was tremendous. Fossil primates in Eocene deposits are common in North America and Europe and are becoming known in Asia and Africa. However, there are currently no known fossil primates from the Eocene in South America or Antarctica.
The Oligocene epoch, which began approximately 34 MYA and ended about 22 MYA, marks the appearance of the first fossil monkeys. The earliest unambiguous such fossils were found about 60 miles from Cairo, Egypt. Again largely based on their teeth, these “Fayum primates” are divided into two main groups believed to be the earliest New World and Old World monkeys, respectively.
The Miocene epoch contains fossil evidence of some of the earliest apes. The earliest Miocene ape, found in Africa, is Proconsul which lived from about 23 MYA to about 14 MYA. Unlike modern apes, the Proconsul lacked long, curved digits, suggesting that they were able to hang from branches but more often moved about on all four of their limbs. Proconsul also lacked a tail, which is why they are considered apes and not monkeys. Another well-known ape from the Miocene is Sivapithecus. Sivapithecus fossils are very common throughout Asia, with a particularly large number having been found in Turkey. Like modern-day humans, they exhibit very thick dental enamel, suggesting that these apes regularly ate very hard foods. The most intriguing aspect of Sivapithecus morphology is that the skulls show a tremendous resemblance to the living orangutan in features such as its tall nasal openings and high eye sockets.
Proconsul africanus. Left: A reconstructed skeleton (lighter white material) created based on fossil evidence (darker brown portions visible in the limbs, spine, and skull) [Source: Wikimedia Commons]. Right: An artist rendition [Source: Mauricio Antón, Wikimedia Commons].
The Middle Miocene was also a time of great ape diversification, or "adaptive radiation," in Africa, though many species would later go extinct as the climate changed. One example is Kenyapithecus africanus an early African ape discovered in modern-day Kenya, known for its relatively large, triangular molars and robust jaw, suggesting adaptations for hard object feeding and possibly knuckle-walking locomotion. Discovered by Louis Leakey and initially considered a potential human ancestor due to its more derived teeth, Kenyapithecus is now viewed as an early member of the great ape and human clade, diverging after Proconsul and but before the common ancestor of modern great apes and humans.
Modern Primate Classifications
As paleontologists work to reconstruct the primate family tree from ‘deep time’ up to the species alive today, they have established a number of groupings within the Primate order. There are three main branches: monkeys, apes, and prosimians. Humans are also primates and can be classified as apes. The ancestry of primates can be charted by ‘branching.’ The main branches of primates are: prosimians, monkeys, and apes. The classification of primates is quite complex, but dividing the primates into these three simple categories provides a baseline for understanding the more nuanced aspects of primate classification.
Left: One of the most well-known prosimians, the ring-tailed lemur (Lemur catta) endemic to the island of Madagascar [Source: Sannse, Wikimedia Commons]. Right: A largely nocturnal prosimian, the Sunda slow loris (Nycticebus coucang) is native to Indonesia, West Malaysia, southern Thailand and Singapore [Source: David Haring, Wikimedia Commons].
Prosimians are thought to have branched off from the primate line earliest and are therefore different in many respects from other primates. That is, they retain features of earlier fossil primates. They tend to be smaller, more often nocturnal, better smellers, less social and more insectivorous (insect eaters) than other primates. The broad groups of prosimians are lorises, galagos, pottos, and lemurs. Lemurs only live on the island of Madagascar (where there are no monkeys) and have diversified into more than 30 species, all of which are endangered. Most lemurs are arboreal, or tree-dwelling, but others are terrestrial, living on the ground. Arboreal lemurs move about mainly by clinging and leaping. Because their bodies are adapted for leaping, lemur legs are long in comparison to their arms. While ideal for moving among branches, moving on the ground results in an odd balletic leaping movement.
The tarsier presents something of an evolutionary puzzle for primatologists. [Source: Jasper Greek Golangco, Creative Commons, Wikimedia Commons]
It is worth mentioning the tarsier. It belongs to the family Tarsiidae and has both prosimian and anthropoid characteristics making it difficult for scientists to classify. Tarsiers were once thought to be in the prosimian group, but through genetic research are now known to be closer to monkeys and apes. All tarsiers live in Southeast Asia and are arboreal, nocturnal, insectivorous and very small. One of the characteristics that tarsiers share with other humans is the inability to manufacture their own Vitamin C. At the same time, they are the smallest known primate and with extremely large nocturnal-adapted eyes that take up much of the space in their skull. Tarsiers are also the only primate carnivore, eating largely flying insects and sometimes small animals like bats and lizards. Tarsiers do not do well in captivity. They are extremely sensitive to noise and become easily stressed.
Consider the puzzle tarsiers present anthropologists. When thinking about the evolution of traits which make primates more human, features like hands and the inability to manufacture their own Vitamin C seem important. At the same time, the oddity of being carnivorous and nocturnal complicates things. This puzzle remains for primatologists because it still seems to be an important piece of the picture of primate evolutionary history.
The other infraorder to share a suborder with tarsiers is the ‘Simiiformes.’ Simiiformes are further divided into Platyrrhini (“New World Monkeys” of the Americas) and Catarrhini (“Old World Monkeys” and the apes of Africa and Asia).
A capuchin monkey (Cebus capucinus). A species of New World (South and Central American) Monkey. [Source: Steven G. Johnson, Wikimedia Commons]
The name Platyrrhini is derived from the rounded shape of the external nostrils, which open off to the sides in New World monkeys. They are also distinguishable by their prehensile tails that serve as an extra limb for extra support when moving in the trees.
Olive baboons, seen here grooming, are Old World Monkeys. [Source: Muhammad Mahdi Karim, Wikimedia Commons]
The Catarrhini differ from the New World primates in that they possess narrow nostrils that face downward. The Catarrhini contain two superfamilies, Cercopithecoidea (Old World Monkeys) and Hominoidea (the apes; and finally a more familiar-sounding Latin term). The most distinguishing feature of theses “hominoids” is that they do not have tails and are largely terrestrial (ground-dwelling versus arboreal). Examples of Hominoidea include gibbons, chimpanzees, bonobos, gorillas, orangutans… and humans.
The approximately 75 species of Old World monkeys (Cercopithecoidea) live in Africa and South Asia and are typically larger than their New World counterparts. Next to the apes, these are our closest living relatives. Notably, all New World monkey species live in social groups. Some species (like the proboscis monkey) are also sexually dimorphic—displaying different “secondary sexual characteristics” like size, weight, coloration, and behavior between males and females. Old world monkeys are also more terrestrial than their arboreal New World cousins.
Our Closest Living Cousins: The Apes
All nonhuman apes, more closely related to Old World monkeys than New World monkeys, live in Africa and southern Asia. Humans, of course, live on every continent. Compared to monkeys, apes are large-bodied (the largest of the primates), large-brained, and most are terrestrially adapted. Also, unlike monkeys, apes do not have tails. The ape shoulder has greater rotation than monkeys allowing them to hang and swing from branches. All apes are diurnal, active during the day. Apes are divided into the lesser apes or (gibbons) and the great apes (gorillas, chimpanzees, bonobos, orangutans, and humans).
Gorillas, chimpanzees and bonobos live in tropical Africa, while the orangutan lives in southeast Asia on only two islands of Indonesia. Most are primarily herbivorous eating leaves or fruits, and to a lesser extent in some species insects and meat. The African great apes live in complex social groups, while the orangutan is mainly solitary. All great apes except humans are endangered.
Drawing comparing ape skeletons. [Source: Public Domain, Wikimedia Commons]
The lesser apes (gibbons) live in southeast Asia and are smaller than other apes with smaller brains. Lesser apes are mainly arboreal and have a particular type of locomotion called brachiation which involves swinging from branch to branch by the arms. Lesser apes resemble monkeys, but they lack tails. They often live in socially monogamous pairs, but males do not typically provide much in the way of parental investment.
A 'lesser ape.' Hylobates lar entelloides or central lar gibbon, a subspecies of white-handed gibbon vulnerable to extinction and endemic to Malaysia, Myanmar, and Thailand. [Source: JJ Harrison, Wikimedia Commons]
There are two general varieties of gorillas, the western and the eastern, all of which are critically endangered. Mountain gorillas are a well-known eastern gorilla living in central Africa in the Congo, Rwanda, and Uganda. Gorillas are the largest of the apes and live exclusively in Africa in social groups called troops consisting of 10-20 gorillas including the silverback male, adult females, and their children. The silverback is the only breeding male in the group and he protects his access to females. As in humans, this is referred to as polygyny. Gorillas have a mainly vegetarian diet of leaves sometimes supplemented with insects and spend most of their waking hours eating. They move about by knuckle-walking rather than on their palms. Gorillas are mostly terrestrial, typically building nests on the ground for sleeping. Infants are helpless and require a high degree of parental investment from the mothers. Newborn gorillas nurse at least once per hour. Silverbacks will protect offspring aggressively and socialize juveniles. The lifespan of a gorilla in the wild is between 35 and 40 years.
A male mountain gorilla looking contemplative with his head in his hand. [Source: Charles J. Sharp, Wikimedia Commons]
Orangutans lives exclusively in Indonesia on Sumatra and Borneo (the only great ape to live outside Africa). Orangutans differ from other great apes in that they are mainly solitary and live high in the rainforest canopy. Orangutans live mainly on fruits and leaves, and like all great apes build nests. They’ve even been known to fashion leaf umbrellas to protect themselves from rain. Orangutans are sexually dimorphic. Orangutans are critically endangered due to the pet trade, logging, and palm oil production. Like other apes, orangutan infants require a great deal of maternal care, staying with their mothers for six years and are particularly susceptible to extinction because they reproduce only every seven or eight years—among longest ‘interbirth intervals’ (IBI) among mammals.
An orangutan female sharing a piece of fruit with her baby. [Source: Bernard DUPONT, Wikimedia Commons]
There are two species of chimpanzees, Pan troglodytes (chimpanzees) and Pan paniscus (bonobos). Chimpanzees live in tropical Africa as well as a savanna environment. Like gorillas, chimpanzees are knuckle-walkers. Chimpanzees spend time on the ground and in trees. They prefer fruits and occasional protein such as mammals, birds, or eggs. Chimpanzees are less sexually dimorphic than gorillas, though males are larger. Chimpanzees form multi-male and multi-female troops of up to 60, but typically travel in smaller parties. Next to humans, they have the highest incidence of tool use among the primates, using termite and ant sticks, rocks for nut cracking, leaf sponges, and even a kind of spear. Female chimpanzees use tools most often.
Left: A vocalizing mother chimpanzee and her baby [Source: H. Zell, Wikimedia Commons]. Right: A chimpanzee seen using a stick as a tool to obtain and eat ants from a tree branch [Source: Colorado State University Libraries, Wikimedia Commons].
Males tend to be dominant among chimpanzees. Females move out of their group upon sexual maturity while males remain. This allows males to form coalitions—critically important social networks based on friendships cemented through grooming and other interactions. For this reason, chimpanzees are often described as “political”. Dominant males will have preferential access to food and sexual partners but must also ‘keep to peace’ to maintain their position. Males also perform dramatic displays—hooting, jumping, dragging objects, and thumping—designed to intimidate other males.
Males will also hunt on occasion and eat more meat the females. Chimpanzees and bonobos are the only adult primates besides humans to share food. They share food for many of the same reasons humans do—to support close relatives (mother to offspring), to support friendships (reciprocation), and “tolerated theft” (when protecting food is more costly than sharing).
Among chimpanzees, females, rather than males, move out of their natal troop into a new troop. Males remain in their natal troop and form reciprocal bonds and strong male-male relationships. They also defend their territory and perform silent boundary patrols of their territory. Jane Goodall reported one troop of chimpanzees systematically killing all the males in a neighboring troop that had splintered off from the first. Female chimpanzees undergo an obvious ovulation cycle called estrus. Males are only interested in copulation during estrus. Male chimpanzees also prefer to mate with older females. It is thought this may be because they are more competent mothers than younger female chimpanzees.
Chimpanzees use a wide range of vocalizations from grunts to pant-hoots. They also use gestures. The outstretched hand is used to request an item (usually food). Chimps do not ‘point’ in the wild but can be taught to do so in captive environments.
Pan paniscus (bonobos) are closely related to chimpanzees but are more matriarchal than their 'common chimpanzee' (Pan troglodytes) cousins. [Source: Pierre Fidenci, Wikimedia Commons]
Bonobos have been isolated from other chimpanzees for about a million years. While bonobos look very similar, they are socially very different. Among bonobos, females tend to be more dominant and females display by dragging objects. Males bonobos, who stay in their natal group, get their status from their high-ranking mothers. Overall, bonobos are less aggressive than chimpanzees and resolve social tension through sexual behavior. Bonobos engage in non-reproductive sexual behavior to alleviate social stresses, especially among females, and resolve disputes.
Looking at Primates to Understand Ourselves
By studying other primates, anthropologists aim to gain insight into both our own evolutionary history and the traits that connect us to other members of this order. Are there, for example, behaviors we might label “cultural” among our Old World monkey cousins? Among even more distantly related New World monkeys?
When we look at primate features from morphology and sensory adaptations to complex social behaviors, we can see that primates display a wide range of features that have allowed them to thrive in diverse ecological niches over millions of years. And yet, we cannot seem to shake the similarities. These characteristics—such as opposable thumbs, stereoscopic vision, and extended juvenile dependence—are not only central to primate survival but also lay the groundwork for the traits we recognize today as “distinctly” human.
The study of primate evolution is equally important. Recalling what we now know about evolution and its mechanisms, an understanding of the emergence primates through a combination of environmental pressures and adaptive responses has helped set the stage for the emergence of some other, more familiar, great apes in the next chapter—hominins including the genera Australopithecus and Homo (including us). Fossil evidence, genetic research, and behavioral studies all contribute to our understanding of primates from the perspective of biological anthropology. By examining both our closest relatives and more distant primate cousins, we can begin to see that the qualities we value in human societies—such as culture, cooperation, and “intelligence”—are perhaps more ‘primate’ than special. Primatology not only enriches our knowledge of the natural world in general but should also deepen our understanding of what it means to be human in particular.
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