8.1: What Is A Primate?

MORE ON PRIMATES

The first fifty million years of primate evolution was a series of adaptive radiations leading to the diversification of the earliest lemurs, monkeys, and apes. The primate story begins in the canopy and understory of conifer-dominated forests, with our small, furtive ancestors subsisting at night, beneath the notice of day-active dinosaurs.

From the archaic plesiadapiforms (archaic primates) to the earliest groups of true primates (euprimates), the origin of our own order is characterized by the struggle for new food sources and microhabitats in the arboreal setting. Climate change forced major extinctions as the northern continents became increasingly dry, cold, and seasonal and as tropical rainforests gave way to deciduous forests, woodlands, and eventually grasslands. Lemurs, lorises, and tarsiers—once diverse groups containing many species—became rare, except for lemurs in Madagascar where there were no anthropoid competitors and perhaps few predators. Meanwhile, anthropoids (monkeys and apes) emerged in the Old World, then dispersed across parts of the northern hemisphere, Africa, and ultimately South America. Meanwhile, the movement of continents, shifting sea levels, and changing patterns of rainfall and vegetation contributed to the developing landscape of primate biogeography, morphology, and behavior. Today’s primates provide modest reminders of the past diversity and remarkable adaptations of their extinct relatives. This chapter explores the major trends in primate evolution from the origin of the Order Primates to the beginnings of our own lineage, providing a window into these stories from our ancient past.

Definition: plesiadapiforms

The Order: Plesiadapiformes. Archaic primates or primate-like placental mammals (Early Paleocene–Late Eocene).

Definition: euprimates

The Order: Primates. True primates or primates of modern aspect.

Definition: anthropoids

Group containing monkeys and apes, including humans.

Definition: Old World

The continents of Africa and Eurasia.

How to Diagnose a Primate

When you examine the skeleton of a mammal, how do you know if you are looking at a primate? Some physical traits are useful in the diagnosis of primates and have been used to make decisions about which living and fossil mammals belong in our definition of the Order Primates. However, primates are hard to diagnose. There is no obvious diagnostic trait of our own order. From the first modern attempts to classify primates, scientists have struggled to come up with traits that are possessed exclusively and universally by primates. In the end, most have generated lists of traits that are of variable utility in making a correct diagnosis.

Definition: diagnosis

The identifying features that allow you to recognize a member of a group.

In the 19th century, British naturalist St. George Jackson Mivart articulated the most famous diagnosis of the Order Primates. This “primate pattern” is a list of the following traits: nails, clavicles, placentation, orbits encircled by bone, three tooth types (i.e., incisors, canines, premolars/molars), posterior lobe of the brain, calcarine fissure of the brain, opposable thumb and/or big toe, nail on the big toe, well-developed cecum, pendulous penis, testes within a scrotum, and two nipples in the pectoral region. Many primatologists have pointed out that no single feature on this list is unique to primates. Also, nails appear twice. Taken together, perhaps it is a useful list. Unfortunately, some of these traits (e.g., three types of teeth) are neither clear nor true of all primates. Other traits, like nipple number and location, are quite variable among primates. Still others, for example the pendulousness of the penis, can be assessed in only males.

Modifications of this approach by subsequent scientists have included lists of trends, like that suggested by Le Gros Clark. Clark’s trends emphasize the flexibility and generalized nature of the limbs, mobility and dexterity of the digits, reduction of the snout with elaboration of the visual system, retention of simple teeth, and elaboration of the brain with prolonged period of juvenile dependence. Later, Robert D. Martin emphasized distinctive reproductive characteristics of primates, along with details of cranial anatomy and grasping extremities (Martin 1968, 1990).

Most modern workers have focused on the grasping extremities and flattened nails, as well as branching of the carotid artery supply to the brain and of the formation of the auditory bulla of the cranium. In extant primates, the brain receives its blood supply via two principal routes (one pathway to the back of the brain and one toward the front). For all taxa, the paired vertebral arteries provide most of the blood to the back of the brain. Blood supply to the front, however, is more complex and involves branches of the internal carotid artery (ICA) and external carotid artery (ECA). For haplorhines (tarsiers, catarrhines, and platyrrhines), the main artery to the front of the brain is a branch of the ICA called the promontory artery (though most human gross anatomy textbooks simply refer to it as the internal carotid artery). In most lemuriforms, this is the job of a second branch of the ICA known as the stapedial artery (which tends to be absent in adult haplorhines). Finally, in lorisiformes and cheirogaleid lemuriformes, the front of the brain is supplied by the ascending pharyngeal artery (a branch of the ECA). These differences provide a valuable method for reconstructing phylogenetic relationships between fossil primates and living taxa.

Definition: catarrhines

The Order: Primates; Suborder: Anthropoidea; Infraorder: Catarrhini. The group containing Old World monkeys and apes, including humans.

Definition: platyrrhines

The Order: Primates; Suborder: Anthropoidea; Infraorder: Platyrrhini. The group containing New World monkeys.

In all extant primates, the auditory bulla is ossified and is formed by an extension of the petrous part of the temporal bone (or, more simply, petrosal bone). This last trait, a petrosal bulla, is perhaps the best candidate for a universally applicable diagnostic trait of primates. Unfortunately, it is often extremely difficult to assess in an adult cranium and perhaps even more difficult to assess in a fossil that has various cracks and deformities associated with preservation and preparation.

Although taxonomists crave neat and complete lists of traits to aid in sorting animals into bins, the true definition of a phylogenetic group is always one of descent from a common ancestor. The Order Primates is made up of all of the descendants of some common ancestor in the remote past. This last common ancestor probably did not possess all of the traits common to primates today and might have been indistinguishable from other primitive placental mammals living in the Cretaceous Period.

MAJOR HYPOTHESES ABOUT PRIMATE ORIGINS

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 an ecological scenario that could explain the rise and success of our own order. Three major hypotheses have been advanced to explain the origin of primates and to explain what makes our own order unique among mammals (Figure 8.1.1); these are described below.

Figure \(\PageIndex{1}\): Three major hypotheses are A) the arboreal hypothesis, B) the visual predation hypothesis, and C) the angiosperm-primate coevolution hypothesis.

Arboreal Hypothesis

In the 1800s, many anthropologists viewed all animals in relation to humans. That is, animals that were more like humans were considered to be more “advanced” and those lacking humanlike features were considered more “primitive.” This way of thinking was particularly obvious in studies of primates. Thus, when anthropologists sought features that separate primates from other mammals, they focused on features that were least developed in lemurs and lorises, more developed in monkeys, and most developed in apes (Figure 8.1.2). Frederic Wood Jones, one of the leading anatomist-anthropologists of the early 1900s, is usually credited with the Arboreal Hypothesis of primate origins (Jones 1916). This hypothesis holds that many of the features of primates evolved to improve locomotion in the trees. For example, the grasping hands and feet of primates are well suited to gripping tree branches of various sizes and our flexible joints are good for reorienting the extremities in many different ways. A mentor of Jones, Grafton Elliot Smith, had suggested that the reduced olfactory system, acute vision, and forward-facing eyes of primates are an adaptation to making accurate leaps and bounds through a complex, three-dimensional canopy (Smith 1912). The forward orientation of the eyes in primates causes the visual fields to overlap, enhancing depth perception, especially at close range. Evidence to support this hypothesis includes the facts that many extant primates are arboreal, and the primitive members of most primate groups are dedicated arborealists. The Arboreal Hypothesis was well accepted by most anthropologists at the time and for decades afterward.

Figure \(\PageIndex{2}\):  Primate family tree showing major groups. Disconnected lines show uncertainty about relationships. Note two lines leading to tarsiers from different possible groups of origin. The timescale is shortened for the epochs since the Miocene.

Visual Predation Hypothesis

In the late 1960s and early 1970s, Matt Cartmill studied and tested the idea that the characteristic features of primates evolved in the context of arboreal locomotion. Cartmill noted that squirrels climb trees (and even vertical walls) very effectively, even though they lack some of the key adaptations of primates. As members of the Order Rodentia, squirrels also lack the hand and foot anatomy of primates. They have claws instead of flattened nails and their eyes face more laterally than those of primates. Cartmill reasoned that there must be some other explanation for the unique traits of primates. He noted that some non-arboreal animals share at least some of these traits with primates; for example, cats and predatory birds have forward-facing eyes that enable visual field overlap. Cartmill suggested that the unique suite of features in primates is an adaptation to detecting insect prey and guiding the hands (or feet) to catch insects (Cartmill 1972). His hypothesis emphasizes the primary role of vision in prey detection and capture; it is explicitly comparative, relying on form function relationships in other mammals and nonmammalian vertebrates. According to Cartmill, many of the key features of primates evolved for preying on insects in this special manner (Cartmill 1974).

Angiosperm-Primate Coevolution Hypothesis

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. Primate ecologist Robert Sussman (Sussman 1991) argued that the few primates that eat mostly insects often catch their prey on the ground rather than in the fine branches of trees. Furthermore, predatory primates often use their ears more than their eyes to detect prey. Finally, most early primate fossils show signs of having been omnivorous rather than insectivorous. Instead, he argued, the earliest primates were probably seeking fruit. Fruit (and flowers) of angiosperms (flowering plants) often develop in the terminal branches. Therefore, any mammal trying to access those fruits must possess anatomical traits that allow them to maintain their hold on thin branches and avoid falling while reaching for the fruits. Primates likely evolved their distinctive visual traits and extremities in the Paleocene (approximately 65 million to 54 million years ago) and Eocene (approximately 54 million to 34 million years ago) epochs, just when angiosperms were going through a revolution of their own—the evolution of large, fleshy fruit that would have been attractive to a small arboreal mammal. Sussman argued that, just as primates were evolving anatomical traits that made them more efficient fruit foragers, angiosperms were also evolving fruit that would be more attractive to primates to promote better seed dispersal. This mutually beneficial relationship between the angiosperms and the primates was termed “coevolution” or more specifically “diffuse coevolution.”

Definition: diffuse coevolution

The ecological interaction between whole groups of species (e.g., primates) with whole groups of other species (e.g., fruiting trees).

At about the same time, D. Tab Rasmussen noted several parallel traits in primates and the South American woolly opossum, Caluromys. He argued that early primates were probably foraging on both fruits and insects (Rasmussen 1990). As is true of Caluromys today, early primates probably foraged for fruits in the terminal branches of angiosperms, and they probably used their visual sense to aid in catching insects. Insects are also attracted to fruit (and flowers), so these insects represent a convenient opportunity for a primarily fruit-eating primate to gather protein. This solution is, in effect, a compromise between the visual predation hypothesis and the angiosperm-primate coevolution hypothesis. It is worth noting that other models of primate origins have been proposed, and these include the possibility that no single ecological scenario can account for the origin of primates.

REFERENCES

Cartmill, Matt. 1972. Arboreal Adaptations and the Origin of the Order Primates. In The Functional and Evolutionary Biology of Primates, edited by Russell Tuttle, 97–122. Chicago: Aldine-Atherton.

Cartmill, Matt. 1974. Rethinking Primate Origins. Science 184 (4,135): 436–443.

Jones, F. Wood. 1916. Arboreal Man. London: Edward Arnold.

Martin, R. D. 1968. Towards a New Definition of Primates. Man (N.S.) 3 (3): 377–401.

Martin, R. D. 1990. Primate Origins and Evolution, a Phylogenetic Reconstruction. Princeton: Princeton University Press.

Rasmussen, D. Tab. 1990. Primate Origins: Lessons From a Neotropical Marsupial. American Journal of Primatology 22 (4): 263–277.

Smith, G. Elliot. 1912. The Evolution of Man. Smithsonian Institute Annual Report 2012: 553–572.

Sussman, Robert W. 1991. Primate Origins and the Evolution of Angiosperms. American Journal of Primatology 23 (4): 209–223.

FIGURE ATTRIBUTIONS

Figure 8.1.1 Hypotheses about primate origins a derivative work by Jonathan M. G. Perry and Stephanie L. Canington is under a CC BY-NC 4.0 License. [Includes Garnett’s Galago by Ltshears, CC BY-SA 3.0; Nycticebus bancanus by Gunay M.E., CC BY-SA 4.0; Anja réserve (Madagascar) – 06 by Wayne77, CC BY-SA 4.0].

Figure 8.1.2 Primate family tree by Jonathan M. G. Perry is under a CC BY-NC 4.0 License.