anthropology homework

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Let’s start with a short, but profound, observation. We are primates. We share many of the same characteristics as other primates, like a large brain, a strong parenting instinct, grasping hands, and generalized teeth. It is important too to recognize that our close resemblance to many primates is not a coincidence. Instead, we are so similar to them because we are related to them; we share a recent common ancestor with them. In particular, we share a very recent ancestor with the African apes, a more distance ancestor with the monkeys, and an even more distant ancestor with the prosimians. Do not make the mistake of thinking that we study primates as living time-machines, to somehow go back in time and see how our ancestors lived. Primates today have evolved too. But, they do provide us with examples of how a closely related animal has adapted to its environment, sometimes the same environment occupied by early human ancestors. In this chapter, we will dive into the field of primatology. Hopefully, you will be able to see why we study primates in the first place. We will examine what exactly a primate is, what features they share in common, and how one would identify a fossil primate. Finally, we will look at the different kinds of primates, from lemurs and baboons to chimpanzees and humans.

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Humans are primates. But, what are primates? Primates are a group of closely related mammals that live in the tropics. The map above shows the distribution of non-human primates, found along the equator in South America, Africa, and Asia. They are arboreally adapted mammals—possessing many behaviors and anatomies that help them live a life in the trees. Primates also display remarkable dietary plasticity—eating a wide variety of foods from grasses and leaves, compared to other animals. Finally, among the mammals, primates engage in the highest amount of parental investment, spending considerable time and energy in the few offspring they have. Let’s look in more detail at these characteristics, and the adaptations that help primates survive.

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This figure has a lot of information on it, so we’ll walk through it in detail. In the middle is a diorama showing the diversity of primates that can live in one forest in West Africa. The location of the forest is shown here in the map in the bottom right corner of the image. There are 13 different species of primate represented here, including humans. Notice that most of them live in the trees. Most live during the daytime, though others, like the small prosimians, come out at night. Primates have a generalized skeletal structure, giving them great flexibility when moving through the trees. Certainly, some species have evolved specializations, like the long arms of gibbons and the long legs of humans. Primates have dexterous hands, giving them grasping ability. This is enhanced by dermal ridges on their hands and feet, which make their sense of touch more sensitive. Primates also have forward-facing eyes, which gives them overlapping visual fields, a greater sense of depth, and thus enhanced vision. But, this comes at the expense of the sense of smell, which is reduced in most primates, and results in a shorter snout than what is found in other mammals. Importantly, primates have such a generalized tooth structure that they can eat a wide variety of foods. The teeth shown here are the upper and lower teeth of the red colobus monkey. Notice that the different shapes of the incisors, canines, premolars, and molars give primates the ability to chew up many different kinds of foods. Humans have the exact same number and kinds of teeth as other Old World monkeys and apes.

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Primates tend to have very mobile joints, which allow them to navigate through the trees, at times in acrobatic ways. They have mobile shoulders and elbows, and have quite dexterous hands and feet. An important primate characteristic is the opposable thumb, which means that the thumb can touch the tips of the other finders. This anatomy allows primates to grasp small objects, like thin branches, or insects. With our long, robust thumbs, humans have the greatest opposability, and this has allowed humans to possess both a power grip, like our ability to grasp and wield a hammer, and a precision grip, which allows us to pick up fine objects, like a pencil.

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Most primates, like the chimpanzee shown here on the left, also have a grasping big toe (hallux), which allows them to climb trees easily. Humans, and many human ancestors, however, lost this anatomy as bipedalism became the primary means of locomotion. Shown on the right is a primate vertebral column, which consists of five different kinds of vertebrae: cervical (neck), thoracic (chest), lumbar (lower back), sacral, and coccyx (tail). Apes in particular tend to have vertically oriented spines, which is a pre-adaptation to human bipedality. Notice the s-shaped curvature of the human spine, which positions the head and hips in the same plane and balances the body in an upright posture.

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As previously mentioned, primates also have an enhanced sense of touch compared to other mammals. There are sensitive dermal ridges on the tips of the fingers, which give us our unique fingerprints. This sensitivity provides important information about the texture of objects, which can be food or the branches many of our primate relatives walk on. In addition to this, primates are equipped mostly with fingernails, instead of claws. Claws can be beneficial to some mammals, but are a bit bulky and would get in the way of a fine precision grip. Many primates require fine gripping of branches or the insects they eat, and have evolved nails. Nails can be seen in this orangutan hand, and, of course, are present on human hands as well.

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Compared to many other mammals, primates also have an increased reliance on vision. Primates have forward-facing eyes, as can be seen in this image of an orangutan. Forward-facing, or convergent, eyes result in the fields of vision produced by each eye overlapping, giving primates depth perception. This makes certain primates excellent insect hunters. Additionally, this depth perception is critical in an arboreal environment: Imagine leaping from one branch to another without knowing the distance between them! In many primates, the eye orbit is also fully encased in bone, protecting this important sense organ. Additionally, in many primates, color vision evolved, which was an important adaptation for identifying ripe fruit or insects in diurnal primates, active during the day. The drawings on the right show a anthropoid primate (gibbon) at the top, a prosimian (lemur) in the middle, and a non-primate (raccoon) at the bottom. Notice that raccoons do not have bone surrounding their eyes at all. Lemurs have a postorbital bar, which encases the eye orbit. Anthropoid primates, like the gibbon, or humans, have an eye orbit fully encased in bone. Of course, there are always trade-offs in biology, and the enhanced sense of vision in primates has come at the expense of our reliance on both smell and hearing. While prosimians retain long snouts and a rhinarium (the wet nose that most mammals, like dogs, have), anthropoid primates have short snouts and a dry nose separated from our mouths by an upper lip.

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Many mammal species have evolved quite specialized teeth. Primates, however, have retained the primitive mouth—with four different kinds of teeth: incisors, canines, premolars, and molars. This allows primates to eat a wide range of different foods. Not only do primates have these different kinds of teeth, but they appear in quite consistent patterns. All Old World anthropoids, for instance, (monkeys and apes) have a 2/1/2/3 dental formula. That means that in each quadrant of their mouth, they have 2 incisors, 1 canine, 2 premolars, and 3 molars. Check your own mouth: this is your dental formula. New World monkeys and many prosimians have an extra premolar (2/1/3/3 dental formula). Tarsiers are like this too, but they have lost a bottom incisor for a dental formula of 1/1/3/3 in the lower dentition. This image on the right shows the teeth from the side (left) and front (right) of five primates from top to bottom: human, chimpanzee, gorilla, orangutan, and baboon. Notice that although humans have differently shaped canines, they have exactly the same number of each tooth type, and the same 2/1/2/3 dental formula, as all of the other primates shown here.

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We will spend some more time on teeth. The reason for this detailed look at teeth is twofold. First, teeth are very effective at distinguishing different groups of animals, and this includes the different primates. Second, teeth are some of the hardest objects in the body, and they therefore preserve very well in the fossil record. Much of what we know about fossil primates is from their preserved teeth. An example of how teeth differ between different primates is shown at the top. As we just saw, Old World monkeys and apes have the exact same number of teeth (they have the same dental formula: 2/1/2/3). That means monkeys and apes both have three cheek teeth (molars) in each quadrant of their mouth. So, if you find a fossil molar, how can you tell if it is from a large monkey or a small ape? One way is to look at the shape of the tooth. Monkeys have four high ridges on their teeth—two in the front and two in the back—producing what is called a bilophodont pattern. But, apes do not. Apes have five low cusps that form a pattern that looks like the letter Y. This is called a Y-5 pattern and helps us identify ape teeth. Similarly, while most monkeys and apes have flat, vertically oriented incisors, many prosimians have combined their lower incisors and canines into a scraper-like structure called a tooth comb, shown here at the bottom. This helps these little primates extract sap from tree bark.

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One final anatomy of the primate dentition we’ll look at is the canine. We’ve already mentioned that the canine tooth is much larger in the male compared to the female in primates that engage in intense male-male competition for access to mating opportunities. Small canines, or canines that are not sexually dimorphic, tend to be found more in primates that form long-term pair bonds. These large upper canines, shown for instance on this yawning male baboon, serve to warn competitors and even predators. But, how do these primates keep the canines sharp as they get older? Through something called the canine/premolar honing complex. When a primate with a large upper canine closes its mouth, the upper canine slides into a space between the lower canine and premolar called a diastema. This is shown on the lower jaw to the left. Notice, the human jaw on the right does not have a diastema (or a large upper canine for that matter). When the canine slides into the diastema, the back side of the canine rubs against the front side of the sharp, or sectorial, premolar, keeping that upper canine nice and sharp. Humans, in contrast, wear the very tip of the canine when they close their mouths, something called apical wearing. This dulls the tooth over time, which would not be beneficial if we used our canines to ward off competitors.

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Compared to many other mammals, primate mothers are very dedicated. They give birth to few offspring, and spread out their births so they can invest time and energy in each of their babies. For instance, orangutans will have a baby and then nurse and care for it for up to seven years before having another child. Chimpanzee mothers, like the one shown here, will go an average of four years between offspring. These behaviors are beneficial because of the important role that social learning has in improving a primate’s chance of survival. Shown here as well is the growth and development of six different primates: lemurs, macaques, gibbons, orangutans, chimpanzees, and humans. Notice that apes tend to have an elongated life history: They have longer infancy periods, longer childhoods, and longer overall lives than monkeys or prosimians. Humans are the most extreme, having a very long childhood and long lifespan. Related to this stretched-out juvenile period is the opportunity to learn, and this requires a high level of intelligence.

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Elevated primate intelligence can be seen in the organ of intelligence—the brain. Compared to other mammals, primates have large brains. The graph on the right compares the sizes of the brain (on the y-axis) in mammals of different body mass (plotted on the x-axis). The average mammal is represented by the straight line. Notice that primates in general fall above the line, meaning they have larger brains than expected for a mammal of their size. Humans in particular have much larger brains than expected, and we can trace this increase in brain size (called encephalization) through the human fossil record. On the left are cartoon drawings of three different primates: a prosimian, monkey, and ape (a chimpanzee). Notice that the brain of the chimpanzee is more folded and complex than the brain of the prosimian. Also notice that the olfactory bulb, which processes smell, is reduced in monkeys and apes, compared to prosimians. We have already discussed how the reliance on the sense of smell is reduced in anthropoid primates, and even the brain reflects this. Notice, however, that the occipital lobe is increased in monkeys and apes: This is where visual signals are processed, and the enlarged size reflects a greater reliance on the sense of sight in monkeys and apes.

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Humans are one of over 200 different species of primates. But, we are not equally related to all of them. As we discussed in chapter 2, primates, like all other animals, can be clustered into closely related nested hierarchies. This is shown in this diagram here, a family tree known as a phylogeny. By understanding the phylogeny of living primates today, researchers can determine which anatomies unite particular groups. This is important because if researchers find a fossil with a particular anatomy, they can determine which primate group it most likely is related to, and also infer when certain features evolved. We’ll get into some of these features, like upright walking and non-honing canine teeth, in just a moment. For now, let’s look at the family tree. The order primates can be split into two suborders, prosimians and anthropoids. Prosimians are often called lesser primates because they retain more primitive features than the more derived anthropoids. But, do not make the mistake of thinking that living prosimians represent what the earliest primates looked like. Prosimians have evolved too, and some features of their teeth are highly specialized and derived. Interestingly, there are many features that unite one group of prosimians (tarsiers) with the anthropoids. DNA evidence supports this grouping as well. Therefore, some researchers will subdivide primates into a group that includes the anthropoids AND tarsiers, called haplorhines, while grouping all prosimians like lemurs and lorises (but not the tarsiers) as stepsirrhines. Your text uses the prosimian/anthropoid classification. Anthropoids consist of the monkeys and the apes. But not all monkeys are equally related. The New World monkeys in South America, the platyrrhines, are less closely related than the Old World monkeys (cercopithecoids) and apes (hominoids). These latter two have the collective grouping of catarrhine. Within the hominoids are the hylobatids, or gibbons, the pongids (great apes—gorilla, chimpanzee, bonobo, and orangutan), and hominids (humans). Let’s look at how the hominoids are grouped together and named.

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Based on anatomical similarities and differences, researchers have historically grouped humans separately from the other apes. Gibbons had their own grouping—the hylobatids—or lesser apes. All of the great apes were thought to be closely related to one another in the pongid family, and humans had their own family—hominid. This all changed with the use of DNA to determine which animals are most closely related to humans. It turns out, the great apes are NOT all equally related to one another. In fact, chimpanzees and bonobos are more closely related to humans than they are to the other apes. This new genetic classification groups humans and members of the genus Pan (chimpanzees and bonobos) together in the subfamily hominine. All of the great apes, including humans, are in the hominid family and together with gibbons comprise the hominoid superfamily. So, what do we call just the humans and extinct human ancestors? In the first scenario, we called them hominids. But, in the second genetic classification scheme, hominids includes all of the great apes. So, the word we use for just the human ancestors is “hominin.” So, which classification scheme is better? Most researchers have adopted the genetic classification scheme since it portrays the actual relationships between these organisms. But, the first classification scheme allows us to examine the adaptive shift humans took relative to the other great apes, and thus we’ll use the anatomical classification in these lectures. But, let’s get back to those other primates, starting first with the prosimians.

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The least related primates to humans are the prosimians. These are the lemurs, lorises, galagos, and a small primate found in Asia called a tarsier. The tarsier is shown in this slide. They have very large eyes to gather what little light there is at night (they are nocturnal), and the long bones in their feet provide excellent leverage for leaping through the trees. Prosimian primates tend to have smaller brains for their body size than anthropoids have. They rely heavily on their sense of smell for finding food and therefore tend to have longer snouts than monkeys and apes. The fossil record indicates that the very first primates are similar in many ways to modern prosimians. Prosimians are found today in Asia, Africa, and on the island of Madagascar. In fact, the island of Madagascar was once home to one of the largest primates: a gorilla-sized lemur called Megaladapis. The image on the bottom of the slide shows how big these lemurs used to be. Identified in the image on the right are skulls of a mouse-lemur and an indri, the smallest and largest lemurs on Madagascar today. The other skulls are from extinct lemurs, many of which went extinct when humans arrived in Madagascar over 1,000 years ago.

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Anthropoids and prosimians diverged from one another 40 million years ago. This divergence was, in part, because each primate lineage evolved slightly different adaptations that allowed them to survive. Many of these adaptations persist today in the living anthropoids. For instance, anthropoids tend to have larger brains and are more sexually dimorphic then prosimians. That means that the males are larger and have larger canines than the females. Anthropoids also have an eye orbit that is fully encased in bone, and can see in color. But, their enhanced vision has evolved at the expense of smell, which is more dominant in the prosimians. Anthropoids also have a generalized diet, and have fewer teeth than most prosimians. Let’s look at the anthropoids in more detail.

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There are two infraorders of the anthropoids: platyrrhines and catarrhines. We’ll start with platyrrhines. These are the New World monkeys that live in South America. They are named platyrrhine because their noses tend to be quite broad (platyrrhine means broad-nosed), with round nostrils and a wide septum between them. In contrast catarrhines (which means hook-nosed) tend to have nostrils that are close together and downwardly pointing as is illustrated here. Platyrrhines can be identified in two distinct ways. First, they are the only primates in South America. Second, they have a very distinctive dental formula: 2/1/3/3. Catarrhines, as we’ll see, have one less premolar in each quadrant (a dental formula of 2/1/2/3). There is a group of platyrrhines called ceboids. These are the cebid monkeys and the ateline monkeys. One type of ateline, called the woolly spider monkey, is shown here. As you can see in this image, these monkeys are more suspensory than most other monkeys, using their arms to support their bodies in a manner similar to how many apes move through the trees. They also make use of a fifth appendage—their tails. These monkeys have a grasping, or prehensile, tail. Though many have the misconception that all monkeys can hang by their tails, it is actually quite rare and possible in just a few species of platyrrhine.

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Catarrhines are monkeys and apes that live in the Old World—Africa and Asia. They first appear in the fossil record around 30 million years ago. Catarrhines diverged into the two groups—monkeys and apes—about 25 million years ago. Old World monkeys, or cercopithecoids, are quite numerous and successful. They live in a wide range of habitats, have a diverse diet, and live both on the ground and in trees. Most cercopithecoids are quite sexually dimorphic. There are two different subfamilies of cercopithecoid: colobines and cercopithecines. Images a, b, and c are colobine monkeys—black and white colobus, gray langur, and the Asian proboscis monkey. The others are cercopithecines: mandrill, DeBrazza’s monkey, and the olive baboon. Colobines tend to live in the trees and eat mostly leaves. They have sharp molars to slice into leaves and often have a multi-chambered stomach to help with digestion of plant material. Cercopithecines have a more diverse diet, consisting of more fruit, and thus have more rounded molars and lack the multi-chambered stomach. Many cercopithecine monkeys are well studied—these include baboons, geladas, macaques, and vervet monkeys. Many of these cercopithecines live in savanna environments in East Africa and provide an interesting model for understanding the evolution of our own lineage, which occupied a similar habitat. By understanding how these monkeys adapt to their grassland environment, researchers can gain some insight into how our own ancestors did the same millions of years ago.

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Though cercopithecoids are a model for early human behavior, we are not cercopithecoids. We are hominoids. Hominoids, or apes, are large-bodied, large-brained, tailless primates with broad faces, and a Y-5 pattern on the lower molars. Shown here are, across the top, chimpanzees, bonobos, and orangutans, and across the bottom, gorillas and gibbons. Keep in mind, however, that humans belong in this grouping too. Chimpanzees, bonobos, and gorillas live in Africa, while orangutans and the lesser apes (gibbons and siamangs) live in the forests of Southeast Asia.

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As has already been discussed, apes have a Y-5 pattern to their lower molars, quite distinct from the bilophodont pattern seen on Old World monkey teeth. In addition, the large male apes have huge chewing muscles that are attached to the top of the skull via a large ridge of bone called a sagittal crest. This feature appears in some human ancestors too, so it is useful to understand it now. If you put your hands on the sides of your jaw and clench your teeth, you’ll feel the masseter muscle, an important chewing muscle. Likewise, if you put your hands on the sides of your skull and bite down, you’ll feel the temporalis muscle contract and bulge. Imagine if the muscle was not only on the side of your head, but was large enough to continue to the very top of your skull. That is the situation in large male apes, and in some human ancestors. Therefore, by finding a skull with a sagittal crest, researchers can infer that these creatures had large chewing muscles and were eating leaves and plant stems, like modern gorillas eat.

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Though there are clearly differences in the skull between apes and other primates, many of the differences actually reside in the postcranial skeleton, or the bones below the neck. These bones are important in locomotion. The non-human apes all have very long arms and long curved fingers to help them navigate through the trees, as is shown in the drawing of the gibbon skeleton on the right. These anatomies allow apes to engage in suspensory, arm-hanging, postures in which the body is positioned in an upright, or orthograde, manner. Gibbons are very suspensory, and these animals can fling themselves from tree limb to tree limb in a form of locomotion known as brachiation. The larger apes, like chimpanzees and gorillas, will often come to the ground and move from food patch to food patch along the ground (terrestrially). They tuck those long arms and fingers under their bodies in a quadrupedal form of locomotion called knuckle-walking. This is shown in the image on the left, a gorilla bearing most of its weight on its knuckles.

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What is important to recognize is that the human body plan generally resembles that of the African great apes. However, there are some differences, and these differences allow us to walk on two legs more often and more efficiently than any of the great apes do. For instance, as can be seen in the drawing on the left, the position of the hole in the base of the skull for the spinal cord, the foramen magnum, is toward the back of the skull in quadrupedal monkeys and apes, but at the bottom of the skull in humans. Keep in mind why this is important. If we find a skull, we can tell how it may have moved around its world by seeing whether the foramen magnum is under the skull or toward the back. The hip joint is also quite different between humans and apes, and this results in small anatomical differences in both the pelvis and the femur. Muscles that extend the leg in apes are repositioned in humans so that they help balance the body when we stand on one leg. In fact, the next time you are walking around, put your hands on your hips and you will feel these muscles contracting to keep the body from falling over when you walk (they contract on the same side as the leg you are standing on). These muscles are crucial for balancing our bodies during two-legged walking, and they are able to do so because the pelvis and the femur are shaped a bit differently in humans compared to apes. If we find a fossil femur and it is shaped more like a human, we can infer that these muscles served the same role, and that the creature walked on two legs, like we do. Notice too in these drawings that humans have slightly different proportions of our limb bones; we have longer legs and shorter arms, while other apes have longer arms and shorter legs. We’ll discuss these differences, and how we can detect the evolution of these features using fossils, in later chapters.

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