3.2: Primate Classification

SYSTEMATICS: THE SCIENCE OF CLASSIFICATION

There are two means by which scientists classify organisms, classic taxonomy and cladistics. Paleoanthropologists are trained in evolutionary theory, and both biologists and paleontologists rely principally upon cladistics. There is definite utility in using a combination of both systems, that is, the binomial nomenclature (genus and species) of classic taxonomy combined with the cladistic arrangement of species in terms of shared characteristics. Classic taxonomy is based on the system begun by John Ray and elaborated by Carolus Linnaeus: kingdom, phylum, class, order, family, genus, species, etc. It classifies organisms based on descent from a common ancestor, using similarities in physical characteristics. We are Homo sapiens, as distinct from other members of our genus, such as Homo neanderthalensis (i.e., Neandertals). Note that since the “h” in neanderthalensis is silent, it is sometimes omitted from the common name. Cladistics refines classic taxonomy by linking organisms, based on the presence or absence of unique characteristics, into “grades.” For example, if three species share a suite of characteristics but only two of them have a particular trait that is not present in more distantly related species, those two are more closely related and would be depicted as a separate grade. Figure 2.1 depicts five primate grade.

“Cladogram of Primates” by Petter Bøckman, CC BY-SA 3.0.

The following terms are used to delineate characteristics in cladistics:

• Plesiomorphy—a primitive trait that is present in the ancestor as well as descendent species, for example, pentadactyly (five digits) in primates is an ancient trait seen in amphibians and reptiles.
• Apomorphy—a derived trait that is not found in the ancestor but is present in descendent species, for example, nails in primates.
• Autapomorphy—a unique derived trait present in member species of a particular grade, for example, the lack of a tail in apes.
• Synapomorphy—a trait inherited by members of two or more grades from their common ancestor, wherein the trait was an apomorphy, for example, bipedalismin the various grades within our tribe, Hominini.
• Homoplasy—a trait in more than one grade that evolved independently, for example, brachiation (swinging by one’s arms) in some New World monkeys and apes.

PRIMATE CLASSIFICATION

We are primates, that is, members of the order Primates (prī-mā’-tēz). The pie chart in Figure 2.2 shows the various orders of animals within the class Mammalia. We are most closely related to tree shrews (order: Scandentia) and colugos (order: Dermoptera, also known as flying lemurs). Primates are distinguished by a suite of characteristics known as evolutionary trends (see table below). However, we do not exhibit all of them to the same degree, and some are absent in certain species or lineages. For example, prosimians retain a claw on the second digit of their feet, whereas anthropoids do not (more about the two primate groups later). These trends were first proposed by Napier and Napier (1967) and Le Gros Clark (1959), and more recently primatologists have refined and added to the list.

Orders within the class Mammalia. “Mammal species pie chart” by Aranae is in the public domain.

PRIMATE EVOLUTIONARY TRENDS

• Generalized, unspecialized skeleton:
  • No loss of limb bones from the ancestral condition.
  • Presence of a clavicle that allows greater mobility.
  • Capable of varied movement and locomotion.
• Large, complex brain (relative to body size), especially cerebral cortex.
• Decreased reliance on olfaction:
  • Reduction of snout and olfactory bulb in frontal cortex.
• Increased reliance on vision:
  • Enlarged visual cortex, greater visual acuity, and color vision.
  • Forward-oriented, overlapping fields (binocular) of vision, and excellent depth perception.
• Prehensile (grasping) hands and feet and opposable thumb and big toe.
  • Nails instead of claws.
• Long pre- and post-natal life periods with greater reliance on learning.
• Tendency toward diurnality.

Taxonomic charts of the living primates can be found below. The primates are divided into two major taxonomic groups: strepsirrhines, which retain primitivecharacteristics, such as the lemurs of Madagascar and the bushbabies of Africa, and the more derived haplorrhines, that is, the tarsier, monkeys, and apes. The older terms for the suborders that are still in popular use are Prosimii (see figure 2.3) and Anthropoidea. However, tarsiers (small, nocturnal prosimian from the islands of the Southeast Asian archipelago) have characteristics of both groups. The strepsirrhine primates have more typical mammalian noses or rhinaria (see Figure 2.4) that are moist and more complex. They have a larger olfactory bulb in the frontal cortex of their brains and scent glands in various locations on their bodies. They use those glands to communicate to other members of their species. We haplorhines have simpler, dry noses and do not smell as good!

Prosimian classification.

Prosimian noses (A through D) and the nose of a New World monkey (E). “Prosimian noses” by Reginald Innes Pocock is in the public domain.

New World anthropoid classification.

Old World monkey classification.

Ape classification.

As we learn more about biochemical and evolutionary relationships among the various groups of primates, primate taxonomy is changing. The New World monkeys (see Figure 2.5) have changed substantially in recent years, with the creation of multiple families that were formerly grouped into two or three.

The Old World anthropoids (monkeys and apes) and New World monkeys are also distinguished by our noses. Old World anthropoids have more ovoid, downward-facing nostrils, whereas New World monkeys’ nostrils are round and forward-facing.

The Old World monkeys (see figure 2.6) are divided into the cercopithecines, with their cheek pouches and more generalized diet, and the leaf-eating colobines, with their complex guts. Think baboon (Africa) or snow monkey (Japan) for the former and black-and-white colobus (Africa) or Hanuman langur (primarily Indian subcontinent) for the latter. When asked, most people are more familiar with the cercopithecines, but if they see a picture of a black-and-white colobus (also known as a guereza) leaping through the air with its white mantle of fur and tail flying (or not…see Figure 2.8! I couldn’t find an action shot!) or a Hanuman langur sitting on the steps of a temple (OK, a fort . . . see Figure 2.9!) in India, they usually recognize them.

Black-and-white colobus monkey. “Colobus guereza Mantelaffen” by Yoky is licensed under CC BY-SA 3.0.

Hanuman langur. “Langur-Amber Fort” by McKay Savage is licensed under CC BY 2.0.

The taxonomy of the apes (see Figure 2.7) has finally been updated. Until recently, humans were separated from the other great apes at the “family” level. All great apes are too closely related to be separated into different families. The lesser apes, i.e. the gibbons and siamangs of Southeast Asia, are still separated into their own family, the Hylobatidae. All of the great apes are now in the family Hominidae, formerly our exclusive domain. The orangutans come out at the subfamily level, leaving the African great apes in the subfamily Homininae. The gorillas have their own tribe, Gorillini (using the genus Gorilla to form the name) and if the chimps (genus Pan) are taken out of our tribe (Hominini), they are assigned the tribe Panini! I did not make that up! Some experts suggest that chimps and humans should be included in the same genus.

APE CHARACTERISTICS

“Why are sleeves always too short for me?”

We apes share a suite of characteristics (in varying degrees), and we humans have radically changed as we abandoned a more typical ape habitat and adapted to a more open terrestrial landscape. The table below lists great ape characteristics.

Great Ape Characteristics

• Relatively large brains.
• Y-5 molar—apes have a characteristic pattern of cusps and fissures on one or more mandibular molars.
• Honing complex consisting of large canines that are sharpened (honed) on the first lower premolar, termed a sectorial premolar.
• Upright trunk posture.
• Short, shallow, wide rib cage.
• High degree of mobility in joints of shoulders and wrists, termed the “suspensory hanging adaptation.”
• Long arms and short legs.
• Long, curved hand and foot bones.
• Variable degree of sexual dimorphism (i.e., differences between male and female morphology) in body size.
  • Low in humans, moderate in chimps, high in gorillas and orangutans.
  • Pronounced male prognathism (jutting jaws or muzzle) and large canines, depending on species.
• Long life stages, especially the juvenile dependency period.
• Build nests.
• Capable of learning and using symbols.
• Tool use with some modification (e.g. chimps and orangutans preparing a stick for probing).

The ancestor of the great apes was an arboreal climber. At some point, apes deviated from the more quadrupedal monkey-like morphology, in favor of (1) a more upright, shorter, broader, shallower trunk (just think of our thorax versus a dog’s); (2) elongated upper limbs; and (3) more mobile shoulder and wrist joints. While we cannot swing by our arms as well as the lesser apes (brachiation is the technical term), we great apes retain the suspensory hanging adaptation and can swing to varying degrees, as long as the tree will support us. Adult male gorillas do not swing because they are too massive!

As hominins, or bipedal apes, came to rely less on an arboreal environment, the bones of our ancestors’ hands and some foot bones became shorter and straighter and our legs became longer and more efficient for covering long distances on African landscapes. Beginning with the emergence of our own genus: Homo (~2 mya), we became increasingly encephalized (i.e. an increase in brain size relative to body size), leaving our fellow great apes behind. While all great apes are sexually dimorphic in terms of body size (i.e., males are larger than females), humans are less so and the trend began even prior to our own genus. Depending on which fossil hominins we include in our lineage, male canines were either monomorphic (i.e., male and female canines were the same size) or became less dimorphic over time. This is significant because male canine dimorphism is associated with competition for females, i.e. males bite one another! If we accept, for example, that we are descended from the ardipiths (see Chapter 8) that lived over 4 mya, ancestral males did not have large canines. However, there is better evidence that we are descended from the australopiths and within that lineage, male canines were initially larger than females’ and became smaller over time.

Apes live a relatively long time and consequently all of our life stages are prolonged as well, especially our juvenile dependency period. Of the nonhuman primates, orangutans win. Female orangutans have an interbirth interval (i.e. a period of time in between births) of eight years and juveniles do not even start wandering off until they are seven. That is not good news from a conservation perspective!

All great apes build nests and scientists speculate that our ancestors likely did as well, at least until they left the trees. We also use tools. Chimps are the champs when it comes to tool use, e.g. nut cracking, ant fishing, etc. Orangutans are very adept at tool use as well, but their thumbs are short and more distant from their fingers, so that their opposability is poor. Consequently, they use their mouths to trim sticks and manipulate them for the desired task. In captivity, they also use their mouths to draw and paint. While gorilla tool use had been known from observations of gorillas in captivity, the use of a tool in the wild was finally recorded in 2005 when a female used a stick to test water depth. Of course, we humans are on a whole different level and we can trace the development of hominin technology in the archaeological record to over 3 mya.

We have two language centers in our brain, Broca’s and Wernicke’s areas (see Figure 2.10). Broca’s area is found in all Old World monkeys and apes; it plays a role in the motor control involved with speech production. As we well know from language studies, all nonhuman great apes are capable of learning and using symbols and have even made up a few. Washoe, the famous chimp that was taught American Sign Language, strung together the signs for “water” and “bird,” when she saw a duck for the first time. Where they fall short is in using syntax; they are poor at correctly stringing symbols together into meaningful sentences. However, Kanzi, the super bonobo at Sue Savage Rumbaugh’s lab, does pretty well. Check him out in the video below.

It is pretty convincing that he understands some syntax with following her commands, such as putting the pickles inside the vacuum cleaner versus the vacuum cleaner inside the pickles! Okay, I made that one up but the things she has him do, in order to convince us that he understands, are amusing and amazing! We and our ancestors, beginning with the genus Homo, also have Wernicke’s area. While a homologous area is present in monkeys and apes, ours has much broader interconnections and is uniquely involved with speech comprehension. At what point our ancestors began to speak is a highly contested topic but the archaeological record provides some clues (see Chapters 23 and 28).

Broca’s and Wernicke’s areas (left side of brain). “BrocasAreaSmall” by the National Institutes of Health is in the public domain.

Great ape social organization varies by species. The chimps and bonobos (genus: Pan) are most like the majority of human traditional societies in that they live in male philopatric, multi-male/multi-female communities. Philopatry refers to the sex that remains in their natal group, i.e. the group into which they were born. This type of social organization is seen in some ripe fruit specialists, such as the chimps and bonobos of Africa and the New World spider monkeys. Since fruit is an ephemeral resource, females cannot cooperatively defend it. Groups of related males cooperatively defend a home range against outsider males. Females emigrate from their natal community and join a different community when they reach sexual maturity. Group members come together intermittently into larger aggregations wherein they may interact. This grouping pattern is termed fission-fusion. Since males patrol and protect the area and females are relatively large and powerful and can climb, the danger of predation is relatively low and they therefore have less of a need to congregate. The mating pattern is termed polygynandry, meaning that males and females are promiscuous and may have multiple sexual partners. Males may attempt to monopolize and coerce females to mate but females are adept at getting around bullying males and may even mate with males outside of their group.

Orangutans are considered to be solitary foragers and their prominent mating pattern is polygyny (males have multiple mates), wherein females in a given area usually mate with the resident, large, dominant male. Smaller males who are sexually mature but lacking the pronounced secondary sexual characteristics, such as facial pads (termed flanges) and an enlarged throat sac for loud calls, may try to forcibly copulate with females. Females are thought to monitor the dominant male’s location so that they can stay within calling distance if they are being harassed by subordinate males.

Gorillas live in one-male groups, except for the mountain gorillas, where two males may reside, an older dominant and younger subordinate. Both sexes tend to leave their natal group. Females join a male who may or may not already have other female mates. Thus the mating system is also polygynous. Males defend their females and offspring from outsider males who may be infanticidal. In mountain gorillas, females are thought to prefer groups with two males as they provide better protection for her offspring.

It can thus be seen that there are characteristics of the human sexes in all three great ape genera (plural of genus): male defense and mate-guarding, female choice for dominant males and good genes, male philopatry with females maximizing resources for themselves and offspring in chimps and bonobos, etc.