Session 2: Primate Taxonomy

1. Key Concepts:

Primates hold a unique position in the ordered world of humans.  Owing to our status as primates, we relate to non-human primates in a manner unlike our interaction with any other animal group.    Our interest draws from our morphological and genetic similarity but also from our shared history.   The evolution of the primate radiation is our own evolutionary history.

 

2.  Definitions & Terms:

evolution - 

Darwin's own definition of "descent with modification" is still relevant.  At the most basic level, evolution is the process by which new species emerge from pre-existing species over time.  From a population genetics perspective, it is simply changes in allele frequencies over time.  It should be inferred from multiple lines of evidence and include the fossil record when relevant.

phylogeny - 

The study of ancestral relations among species, often illustrated with a "tree of life" branching diagram, which is also known as a phylogenetic tree.

taxonomy - 

The ordering and classification of life.

 

3.  What is a Primate?

Uniquely among Mammalia, primates have no single anatomical trait that is the identifying mark of a primate and no trait that unites them.  Broadly, they are generalist foragers with relatively large brains and intricate, complex social systems.

Primates, including fossil primates, typically have some or all of the following traits: A bony ring around the eye socket (post-orbital bar) or a bony enclosed eye socket Hands (manus) and/or feet (pes) that are constructed such that they can grasp objects (they exhibit prehension) Nails instead of claws on the ends of the digits (unguiculate) binocular, or 3-D, vision (overlapping visual fields) Large brain relative to body size (high encephalization quotient) Long gestation and slow-post natal growth compared to maternal body size

 

Why did primates evolve?

There are currently three major hypotheses for the explanation of primate evolution.  Although there is some level of overlap between them, each brings its own level of additional clarity to the question.

1. Arboreal Hypothesis -  Argues that the hallmark primate traits were the basic adaptations required for living in a three-dimensional and vertically stratified environment.  However, it fails to address those species that live in similar environments without these traits, such as squirrels and marsupials, or species with those traits living in different environments, such as some terrestrial carnivores.
   
2. Visual Adaptations Hypothesis - Argues that the basic adaptations for visually preying on small animals led to the earliest development of "primate" traits such as grasping hands and stereoscopic vision. 
   
3. Angiosperm Radiation Hypothesis - Argues that the rise of angiosperms, or flowering plants, led to a vast development and broadening of potential food sources.
4. The visual adaptations and angiosperm radiation hypotheses are arguments that both attempt to minimize the amount of evolutionarily parallel examples.  Neither could eliminate them entirely.
   

4. When primates became Primates:

The fossil record for primates is fragmentary and difficult to interpret.  It includes only fossilized prosimians, monkeys, and apes.  Yet, still it directly benefits our overall understanding of primate evolution.  The fossil record of primates, as with other taxa, is limited by the natural fossilization process.  In other words, the natural processes that lead to the creation of a fossil limit the types of biological materials capable of being fossilized (i.e. bones more than skin) as well as the locations fossils are most likely to be found in.  Thus, certain regions of the world have produced large numbers of fossilized primates while other regions of the world remain largely depauperate. 

The Fossil Evidence:

 

The Paleocene:  65 - 40 mya

 

The oldest known primate fossils date from this period.  They are small-bodied, likely both nocturnal and diurnal, and have been found in North America, Europe, North Africa, and Asia.  Examples of fossils of early candidate primates include:

• Pleseadapiformes (members of Archonta)
• Purgatorius
• Decoredon elongatus (or anhuiensis)
• Altiatlasius koulchii

 

 

The Eocene:  65 - 40 mya

 

Eocene primates include fossils from North America, Europe, Africa, and Asia.  The adapids resemble the Lemurs of Madagascar while the omomyids strongly resemble the nocturnal prosimians, especially the Tarsiers.  The early forms are found in Africa (genera Algeripithecus, Catopithecus, Moeripithecus, Oligopithecus) and Asia (Amphipithecus, Eosimias, Pondaungia, Siamopithecus, and Wailekia).  From this period, too, comes the earliest candidate fossil of early anthropoids (Simiiformes). 

   The Anthropoid primates include monkeys, apes, and humans.  The features of these primates include:

 

 

The Oligocene: 40 - 2 mya

 

Fossil evidence from the oligocene, including fossils found at El Fayum include the Parapithecids (Apidium, Qatrani and Parapithecus).  All of the parapithecids share common dentition pattern of 2-1-3-3.  The Propliopithecids (Aegyptopithecus with a 2-1-2-3 dentition pattern) are also from this time.  The discovery of the Ceboidea fgroup, which dates from ~27 mya (2-1-3-3 dental formula), has led to the following questions regarding early primate evolution:

1. Neotropical primates evolved convergently with African and Asian forms from a common ancestor in the Paleocene?
2. Neotropical primates derived from North American forms and moved into South America via migration?
3. Neotropical primates arose from Parapithecids, or parapithecid like ancestors in Africa and migrated, somehow, to South America? (favored)

 

The Miocene:  22 - 5 mya

 

Hominoid Radiations:  Early hominoid fossils arise from multiple localities and include African forms [Early- Proconsulids (Proconsul) "dental apes," Middle- Nyanzapithecus, Morotopiothecus, and Micropithecus, and Late- Afropithecus, Kenyapithecus, and Samburupithecus] and Eurasian forms [Dryopithecus (thick enamel), Pliopithecus and Laccopithecus, Ouranopithecus (related to modern hominoids?), and Gigantopithecus, Lufengpithecus and Sivapithecus].   The hominoids have specific morphological features that set them apart from other primate radiations. These include:

 

The Pliocene-Pleistocene: the last 5 mya

 
Recent adaptive radiations have come about largely because of a reduction in forested environments.  This has led to an increase in diversity of monkeys and a decrease in diversity of Hominoids.

 

5.  Taxonomy of Primates:

The focus of primatology today is largely on extant species of primates, their phylogenies, behavioral ecology, social systems, and conservation.  However, understanding the historical evidence of the origin of primates is key to understanding current selective pressures of extant species of primates.  Today, primates can be broadly categorized as prosimians, new world monkeys, old world monkeys, or hominoids.  Although these categorizations are extremely broad, and the phylogenies are far from resolved, they will, in the context of the historical evidence, enable us to resolve more pertinent issues to primates today.

 

The Prosimians:

The prosimians include the lemuroids, the lorisids, galagids, and Tarsius.  Lemurs, often considered to be the most evolutionarily basal of all living primates, are found only on the island of Madagascar.  The Lorisids and Galagids are found on mainland Africa and Asia and are typically nocturnal.  Tarsiers are limited to Southeast Asia but are crucial to our understanding of primate evolution.

 

 Image by A. Fuentes

 

The New World Monkeys:

The New World Monkeys include the Atelines (Aloutta, Ateles, Lagothrix, Brachyteles), the Pithecines (Pithecia, Chiropotes, Cacajao), the Cebines (Cebus, Saimiri), and the Callitrichids (Callithrix, Cebuella, Saguinues, Leontopithecus, Callimico).  All platyrrhines are tree-dwelling or depending, but they may range widely due to their flexibility and resource exploitation skills. 

 

 Image by A. Fuentes

 

The Old World Monkeys:

The Old World Monkeys are comprised of the Cercopithecoidea, which has two major clades.  These are the Cercopithecinae and the Colobinae.  The cercopithecoids are the more generalist group are exemplified by the Macaca and Papio radiations.  The colobines, alternatively, are much more specialized and concentrate on exploitation of local leafy food materials.  The colobines are exemplified by Colobus and Presbytis

.

Image by A. Fuentes

The Hominoids:

 The living hominoids include the pongids, hominids, and hylobates (humans, chimpanzees, bonobos, gorillas, orangutans, and gibbons). 

 

6.  Special Cases:

As mentioned earlier, the Tarsiers present an interesting case to understanding the evolution of primates.  While studies based on behavioral ecology and niche positioning would place the Tarsiers firmly as Strepsirhini group, phylogenetically tariers may be more closely related to anthropoids and placed as a member of the Haplorhini group.

 

 

 

7.  Additional Material:

   Required Readings:

    From Primates in Perspective.  2007.  eds. C.J. Campbell, A. Fuentes, K. C. MacKinnon, M. Panger,
       and S. K.     Bearder.  Oxford University Press.

• Chapter 1:  A Brief History of Primate Field Studies - Robert W. Sussman
• Chapter 2:  Primate Evolution - Walter Hartwig
  

   Suggested Readings:

• The Nonhuman Primates.  1999.  eds. Dolhinow, Phyllis and Fuentes, Agustin.  Mountain View, CA: Mayfield Publishing.