Taxonomy
Primates belong to the order primates. Members of this order include prosimians, monkeys, apes, and humans. The primates are divided into two suborders, Prosimii and Anthropoidea. Prosimians are the more primitive members of our order (i.e., they more closely resemble the earliest primates), whereas members of Anthropoidea (i.e., monkeys [New and Old World], apes, and humans) are more derived, that is, they exhibit evolved characteristics not present in the ancestral primates.
Evolutionary Trends
Primates are distinguished from other mammals by a variety of physical and life history characteristics, termed evolutionary trends. The term “trend” is used because they are either 1) not present in all species (for example,the lack of an opposable thumb in prosimians and New World monkeys) or 2) vary in the degree to which they are present (for example, prosimians retain a claw on the second digit of their feet and have smaller brains and shorter developmental periods as compared with higher primates).
Primate Evolutionary Trends
- Relatively large complex brain, especially cerebral cortex.
- Decreased reliance on olfaction and corresponding reduction of snout and brain area associated with smell.
- Increased reliance on vision and corresponding enlarged visual cortex and adaptations that increase visual acuity and color and depth perception.
- Generalized diet and dentition.
- Tendency toward upright posture.
- Prehensile (grasping) hands and feet with opposability (big toe and thumb can touch other digits of foot and hand, respectively).
- Nails instead of claws.
- Increase in efficiency of placental nourishment to developing fetus.
- Long pre- and postnatal periods with greater reliance on learning.
- Tendency toward diurnality.
Primate Groups
Prosimians vs. Anthropoid Primates
Prosimians exhibit skeletal characteristics that distinguish them from anthropoid primates. Two joints of the craniofacial region differ between prosimian and anthropoid primates. When growth ceases in anthropoids, the mandibular symphysis (midline lower jaw joint) and the frontal suture (midline forehead joint) fuse. In prosimians, these joints remain open throughout life. The prosimian dental formula, that is, the numbers of the various types of teeth (for example, incisors, canines, premolars, molars) in each half of each of the jaws differs from all but some of the New World monkeys, in that they have an extra premolar. Thus, they have 36 rather than 32 teeth. They exhibit procumbent, or forward-oriented, lower incisors. They use this “dental comb” to comb through and groom their fur. The retention of a claw on the second digit of the feet of most species also serves a grooming function. Anthropoid primates are distinguished by the presence of extra bone in the back of the eye sockets/orbits so that their eyes are encased in bone. Prosimians lack this postorbital closure.
Prosimian Characteristics
Unfused metopic suture and mandibular symphysis.
Dental formula 2-1-3-3. Dental comb. Grooming claw. Lack of postorbital closure.
Nocturnality with corresponding visual adaptations: enlarged eyes with rite mirabile.
Greater dependence on hearing and mobile external ear flaps.
Greater reliance on olfaction: relatively long muzzle with moist rhinarium.
Upper lip is split and fused to the upper jaw.
Lack of facial musculature result in inexpressive face.
Digits act together (powerful grip).
Tendency toward vertical clinging and leaping as loco-motor pattern.
Multiple births and corresponding pairs of teats.
Most of the extant prosimians are nocturnal and, with the exception of the tarsier, which is intermediate in many physical characteristics between the prosimians and anthropoids, they possess an adaptation for increased visual acuity in low light conditions, termed a tapetum lucidum. They have relatively more acute auditory abilities as well, and have mobile external ear flaps, as seen in other mammals. They also rely on olfaction more than most anthropoid primates and, thus, retain the primitive mammalian characteristic of a moist fleshy pad (rhinarium) at the tip of their relatively elongated muzzle. They practice scent marking and possess scent glands on various parts of their body. Their upper lip is split and fused and they lack the facial musculature seen in the higher primates, resulting in a lack of facial expression capabilities.
Prosimians, as well as New World monkeys, lack opposable thumbs and, therefore, lack the degree of manual dexterity seen in the Old World anthropoid primates. Prosimian digits instead act together, giving them a powerful grip. They have a tendency to locomote by a pattern termed vertical clinging and leaping (see “Locomotor Categories”).
There is a tendency among the prosimians for multiple births per birthing event, and thus most species have multiple pairs of teats. Within the anthropoid primates, the norm is for single births (except for the New World tamarin and marmoset monkeys) and two teats.
New World Monkeys
New World monkeys are characterized by a suite of characteristics. Of interest is the fact that tail prehensility evolved independently in the atelids and the capuchins. Squirrel monkeys, which are closely related to capuchin monkeys, have prehensile tails only in infancy.
New World Monkey Characteristics
All species:
- Arboreal.
- Platyrrhine nasal morphology (i.e., rounded and laterally placed nostrils vs. Catarrhine nasal morphology, which consists of narrow, downward-oriented nostrils as seen in the Old World monkeys and apes).
- Long tails in most species.
Callitrichines: tamarins and marmosets:
- Dental formula 2-1-3-2 (all other species have 2-1-33 formula).
- Modified claw-like nails on all digits except big toe (which has a nail).
- Twins are the norm.
Some atelids (for example,spider monkeys, howler monkeys, muriquis, and wooly monkeys) as well as capuchins have prehensile tails.
One species, the owl monkey, is secondarily nocturnal, meaning that it is derived from a diurnal ancestor.
Old World Monkeys
The Old World monkeys also have several characteristics that distinguish them from the other anthropoid primates. There are important differences between the two sub-families of Old World monkeys. The members of the subfamily, cercopithecinae, are commonly referred to as the cheek-pouch monkeys. These monkeys have a cavity inside their cheeks wherein they temporarily store food. It is hypothesized that once they encounter a food source, they can quickly stuff their cheek pouches and mouths and move to a safe location to chew and eat. This could be potentially important in the case of semiterrestrial species encountering food on the ground whereby they might avoid predators and/or conspecifics (i.e., members of their own species). Enzymatic digestion also begins in these pouches. The colobinae are also distinguished by a dietary-related adaptation. Colobines have complex sacculated stomachs and benefit from foregut fermentation processes. Stomach microflora separate the structural components of plant material, particularly leaf cell walls. The process yields volatile fatty acids that provide energy to the animals. Thus, colobine monkeys can ingest comparatively large amounts of leaf matter relative to the other anthropoid primates. In addition, they have higher capabilities for breaking down the secondary compounds plants produce as deterrents to animals and microorganisms (for example, fungi). Additional differences between the two subfamilies can be seen in the following list.
Old World Monkey Characteristics
Tendency for arboreality but a relatively large number of species are semi-terrestrial.
Larger body and brain size, in general.
Tendency for ischial callosities (i.e., bare-calloused sitting pads).
Sexual swellings in many species.
Characteristics That Distinguish the Two Subfamilies of Old World Monkeys
Colobinae Cercopithecinae
Complex stomachs Cheek pouches
High folivory High frugivory and omnivory
Dental specializations More generalized dentition
for leaf consumption
Primarily arboreal Arboreal and semi-terrestrial species
Elongated hind limbs Limbs more equal in length
with good leaping abilities
Long tails Variable tail length
Apes
Apes are distinguished by a suite of characteristics related to their ability to hang and swing by their arms, termed a suspensory hanging adaptation (see “Locomotor Categories”). Various aspects of their shoulder girdle, upper limb, and hand morphology reflect this capability. In addition, they have relatively larger brains and longer life stages relative to the other primates. The apes are divided based upon their body size into the small bodied “lesser apes” (gibbons and siamangs) and the larger bodied “great apes” (gorillas, orangs, and chimps). Except for the gorillas, the other species of apes are opportunistic omnivores, and their dentition reflects this fact. The lesser apes are monomorphic, that is, the sexes do not differ in size or morphology (except for genitalia). This is related to the fact that they have a monogamous social organization. The great apes, in contrast, are sexually dimorphic. This is taken to the extreme in the gorillas and orangs. Extreme sexual dimorphism is related to male-male competition for mates. The male gorillas and orangs defend their females from infanticidal and “raping” males respectively. The pronounced “fighting teeth” seen in male apes are also the result of male-male competition, but certainly also serve the chimps in their hunting activities.
Primate Evolution
The first primates are believed to have evolved during the Paleocene epoch (i.e., more than 55 million years ago). These early primates were likely descended from small nocturnal insectivorous mammals. The tree shrew is commonly viewed as an extant model for what these earliest primates or primate predecessors would have been like. The earliest fossil primates are from North America and Europe. They are divided into two super-families, adapoidea and omomyoidea. In general, the adapoids were lemur-like diurnal frugivores with some insectivory and folivory likely, whereas the omomyoids were small nocturnal insectivore-frugivores. It was during the Eocene epoch (~55-35 million years ago) that these early prosimians speciated on a grand scale, termed an adaptive radiation, expanding into numerous niches over a wide geographic area. They spread into Asia and mainland Africa. The ancestors of the Madagascar primates are believed to have rafted from the main-land on floating mats of vegetation. Eventually, due to global cooling, the ancestral primates of North America and Europe became extinct.
By the late Eocene, the first anthropoid primates had evolved and remains are found in Africa and possibly Asia. The richest location for early anthropoid fossils is the Fayum Beds of Egypt.
These ancestors of the monkeys and apes are believed to have descended from an omomyoid ancestry but there are numerous debates on the subject. It was during the oligocene epoch (~23-35 million years ago) that the anthropoid primates experienced a great adaptive radiation. Oligocene anthropoids are divided into three families, parapithecidae, oligopithecidae, and propliopithecidae. The New World monkeys are believed to have descended from the most primitive of the early anthropoids, the parapithecids. New World monkeys are more primitive than their Old World counterparts, and they share some characteristics seen in the Parapithecidae. How the ancestors of the New World monkeys migrated to the New World is unknown, but, once again, a rafting hypothesis is proposed. There is no evidence of a North American origin for New World monkeys as there have been no discoveries of anthropoid fossils. In addition, even if anthropoid primates evolved in parallel in the Old and New Worlds, the ancestral New World monkeys would still have had to raft from North to South America because their appearance in South America precedes a land bridge between the two continents. Thus, the most parsimonious explanation is an Old World origin.
The ancestors of the Old World monkeys and the apes branched off from the family propliopithecidae, during the oligocene. Aegyptopithecus zeuxis is believed by some to be a possible propliopithecid contender for a member of the ape lineage and possibly the Old World monkey lineage as well. While the earliest anthropoids were more monkey- than apelike, it is the apes (or hominoids) that were the first to successfully adapt to changing environmental conditions in Africa. It is during the Miocene epoch (~23-25 million years ago) that the adaptive radiation of the hominoids can be observed in the fossil record. There were approximately 20 genera evident during the Miocene. This is surprising considering that there are only four nonhuman genera today. These early apes exhibited a wide range of body size (3.5-50 kilograms) and foraging strategies. While the ancestry of the lesser apes (gibbons and siamangs) is difficult, the extant great apes can be traced to the genus Proconsul at approximately 18 million years ago. After that point, the ancestors of the orangutans, genus: Sivapithecus, left Africa and moved into western Asia. Apes also moved from Africa into Europe. It is difficult to trace the origins of humans and the modern African apes during the mid-Miocene as there are many conflicting viewpoints. However, suffice it to say that the ancestry of the hominids (i.e., the bipedal primates) branched off during this time and, along with the Old World monkeys, experienced an adaptive radiation into the Pliocene (1.6-5 million years ago) and Pleistocene (10,000 to 1.6 million years ago) epochs. In response to the success of the anthropoid lineage in Africa and Asia, the only surviving members of the suborder Prosimii in those areas are nocturnal. A nocturnal niche allowed them to survive alongside the larger and more cognitively superior anthropoid primates in that they could avoid competition for resources.
Locomotor Categories
While primates (1) exhibit a generalized skeleton, (2) are capable of locomoting using a variety of means, and (3) can, essentially, walk bipedally for at least short periods of time, they tend to habitually practice one form of locomotion and can, thus, be categorized as such. Primate skeletons exhibit some specializations to their particular mode of locomotion. The locomotor categories seen in the primates are slow climbing, vertical clinging and leaping, arboreal quadrupedalism (also termed branch-running), semi-terrestrial quadrupedalism (also termed ground-running), brachiation, knuckle walking, and fist walking.
Slow Climbers
The only species that practice this form of locomotion are the pottos and the slow loris. These animals are solitary nocturnal foragers that rely on crypsis, or camouflage, to avoid detection by predators and prey. They are drab in color, move slowly through the trees, and can freeze in place for long periods of time due to a vascular specialization in their extremities, termed “rite mirabile.” They lift one limb at a time and move forward in a slow mechanical fashion. They can also move on the underside of branches.
Vertical Gingers and Leapers
The earliest primates are believed to have practiced this mode of locomotion based upon fossilized skeletal evidence. It is, thus, not surprising that many of the living prosimians are either categorized as vertical clingers and leapers (VCLs) or have the capability to do so. VCL involves leaping from a vertical substrate by powerfully extending the elongated hind limbs, arcing up and out, and landing on another vertical substrate, usually feet first. The most specialized of the VCLs are the tarsier. Galagos/bush-babies and the indriids (indris, avahis, and sifakas) are also primarily VCLs. Many other species are capable of VCL in varying degrees (for example, most of the remaining Madagascar prosimians as well as the tamarins and marmosets of the New World). Due to their elongated legs, the true VCLs hop or gallop when on the ground.
Arboreal Quadrupeds/Branch-Runners
The majority of primates are arboreal quadrupeds. All of the New World monkeys are categorized as such, along with the majority of Old World monkeys and many species of prosimians. The name of this locomotor category reflects the activity, e.g., walking/running through the trees, on a variety of substrates. Arboreal quadrupeds tend to have longer legs than arms and, thus, have good leaping capabilities.
Semi-Terrestrial Quadrupeds/Ground-Runners
All but one species of Old World monkeys (the Hanuman langurs of Asia) are members of the subfamily cercopithecinae (the cheek pouch monkeys).
Several species of Old World monkeys (the baboon, patas, vervet, certain guenon and mangabey monkeys of Africa, and the macaques, predominately of Asia) spend much of their time on the ground and are, thus, categorized as semi-terrestrial quadrupeds. Their arms tend to be longer than their legs, and the general morphology of their shoulder and hip regions more closely resembles other terrestrial mammals (for example, cats and dogs). Some species have specialized adaptations for speed, termed cursorial adaptations. The baboons, and especially the patas monkeys, can run very fast. They walk/run on the tips of their digits and have relatively long limbs. Macaques walk on the palms of their hands and, accordingly, most species spend much of their time in trees.
Brachiators
Brachiation is a unique mode of locomotion that involves swinging by the arms on the underside of branches. One hand grasps onto a branch, pulling the body forward, and the other hand then reaches out for the next branch. Two groups of primates have evolved the ability to brachiate, independently of one another—the apes and spider monkeys. All of the apes (including ourselves) exhibit what is termed a “suspensory hanging adaptation.” This adaptation involves a very mobile shoulder girdle that allows a 360-degree range of motion. Thus, we can hang and swing by our arms and hands.
The lesser apes such as gibbons and siamangs are sometimes known as “true” brachiators due to their enhanced brachiating capabilities that are the result of a suite of derived characteristics, relative to the other apes. Their trunks are shorter, their arms and fingers are longer, and their thumbs are reduced so that their hands resemble hooks. They swing fluidly through the forest, but must walk or run on the ground with their arms in the air, due to their length. Brachiating is not the primary mode of locomotion in the great apes, and they are not as adept. Adults are seldom observed to brachiate due to their large size, but young animals are often seen to hang and swing by their arms, especially when playing.
Spider monkeys are sometimes referred to as “semibrachiators” due to the incorporation of their prehensile tails when brachiating. Like the rest of the New World monkeys, they are also fully capable of traveling along branches quadrupedally and leaping between trees.
Knuckle- and Fist-Walkers
The primary mode of locomotion for chimps and gorillas is walking on the knuckles of their hands and the soles of their feet. The middle phalanges (finger bones) are placed flat on the ground and the metacarpo-phalangeal joints (joint between the metacarpals [bones of the body of the hand] and the first phalanges) are hyper-extended. Orangs, in contrast, ball up their hands into fists when they loco-mote quadrupedally. When in the trees, however, they use their hands and feet to climb through the branches. This is referred to as quadrumanous, or four-handed, locomotion. In order to move from tree to tree, they often use their weight to sway/bend the tree until it is closer to another tree. Male orangs are so large that they often have to drop down to the ground to move from place to place. Male gorillas are primarily terrestrial due to their large size.
References:
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