The Animal Kingdom is divided into 25 to 30 major groups called phyla (singular, phylum): the Arthropoda (insects and spiders), Mollusca, Coelenterata (sea “anemones and jellyfish”), Echinodermata (starfish and sea urchins), and many other phyla that most people would recognize only as “worms.” The phyla are divided into subordinate groups called Classes, the classes again into subordinate groups called Orders, the orders into Families, the families into Genera (singular, genus), the genera into Species. Within each category, subordinate categories can be inserted: thus, families can be divided into subfamilies where needed (another way of looking at this is to say that the genera of a family can be grouped into subfamilies). Super-families (groups of closely related families) can also be instituted. With finer and finer divisions we may well “run out of ranks,” and unranked groups are perfectly feasible, provided it is widely understood where they belong in the main classification.
The phylum to which Primates belong is called Chordata (crudely speaking, animals with backbones). The phylum Chordata is divided into approximately eight classes, one of them being Mammalia—the mammals, distinguished by having hair and mammary glands. The 20 or so orders of mammals include Carnivora (cats and dogs), Proboscidea (elephants), Chiroptera (bats)—and Primates.
The living groups of Primates are lemurs, lorises, tarsiers, New World (i.e., American) monkeys, Old World (i.e., African and Asian) monkeys, apes, and humans. Linnaeus, the originator of biological taxonomy, in 1758, did not adopt the rank of Family, but divided the order Primates directly into four genera: Homo (humans), Simia (apes and monkeys),Lemur, and Vespertilio (the latter being bats—Linnaeus’s successors expelled bats from the Primates). Although he thus separated humans from apes, it is very noteworthy that he did classify humans as Primates; unfortunately most of his successors removed humans and, instead of Primates, recognized two different orders—Bimana (humans) and Quadrumana (other Primates). It was not until a century later that the order Primates was definitively re-instituted (by Mivart, in 1863).
For nearly another century there was dispute regarding what should be the major subdivisions of the order Primates. Most authors tended to regard tarsiers as being related to lemurs and lorises, but Pocock, in 1918, showed that they have more in common with monkeys, apes, and humans, and divided the Primates into suborders Strepsirhini (lemurs and lorises) and Haplorhini (tarsiers, monkeys, apes, and humans). A few taxonomists adopted his classification, but most adopted the one formalized by Simpson in 1945, in which the sub-orders were Prosimii (lemurs, lorises and tarsiers) and Anthropoidea (monkeys, apes, and humans).
The reason why this matters is because these two competing schemes reflect different philosophies of what the function of taxonomy actually is. The tarsier shares with lemurs and lorises characteristics such as a small brain—a primitive retention from the common ancestor of all Primates. This certainly makes it look superficially more like a lemur than like a monkey—but it has never been clear why it should be important to classify animals together simply because, in certain selected characters, they are “primitive.” Since the 1960s, the tendency has been more and more to classify animals together because they share an exclusive common ancestor (phylogenetic systematics: originating with Hennig in 1952, and especially with the translation into English of his book Phylogenetic Systematics in 1966). It is now clear to most primatologists that the tarsier shares a common ancestor with monkeys, apes, and humans that is more recent than the one they all shared with lemurs and lorises—hence Pocock’s classification is now overwhelmingly preferred.
A still more important example of a change in a prevailing taxonomic scheme is how great apes (orangutan, gorilla, and chimpanzee) are classified. In most of the 20th century the great apes were classified as the family Pongidae, while humans alone were placed in a different family, Hominidae. As the phylogeny became clearer, starting with Goodman’s 1963 immunological work, and the philosophy of phylogenetic systematics spread, doubts began to be expressed about the correctness of recognizing the family Pongidae. It was the orangutan, not humans, whose ancestor separated first among the great ape and human group, so it should be the orangutan that should be classified separately. The growing tendency now is to recognize only a single family, Hominidae, with two subfamilies: Ponginae for the orangutan, and Homininae for gorilla, chimpanzee, and human.
This illustrates one of the ways in which new thinking has gradually been pervading taxonomy. A family is a monophyletic group (descended from an exclusive common ancestor), but why is this group to be ranked as a family, rather than, say, a super-family, or a subfamily, or a genus? Why would we want to recognize a separate family (Hylobatidae) for the lesser apes or gibbons, rather than incorporating them, too, in the Hominidae? Within the Hylobatidae, should we recognize just one genus (Hylobates) or four (one each for the concolor, siamang, hoolock, and lar-group gibbons)? There is no consistent thinking about this, but in 1977 Goodman and colleagues proposed that taxonomic rank should be dependant on time depth: a genus should have separated from its closest relatives at least by the Miocene-Pliocene boundary, and a family should have diverged at least by the Oligocene-Miocene boundary. There is no agreement about this yet, but it is approximately consistent with customary usage.
The Biggest Question of All: What Is a Species?
In 1963 Simpson noted that a species is a unit that evolves separately from others and has “its own evolutionary role and tendencies.” There are different ways in which taxonomists have tried to represent this important insight operationally.
In the 1940s Dobzhansky and Mayr formulated the Biological Species Concept (BSC): species are reproductively isolated from each other. This is often misunderstood to mean that two different species are incapable of interbreeding, or that, if they do interbreed, their hybrids are sterile. Actually, as Mayr insisted time and again, it means that they do not normally interbreed in the wild. (Among Primates, baboons and macaques interbred to give fertile hybrids in captivity; it would be meaningless to classify them all as one species.) It is interesting that, 20 years before Simpson, these authors were already attempting to express an aspect of his evolutionary conception.
There are many reasons why the BSC is not universally applicable. Perhaps the most cogent one is that many species are allopatric to (geographically isolated from) each other, so the question of reproductive isolation is simply inapplicable. An increasing number of taxonomists therefore feel that a more objective, falsifiable, and repeatable criterion must be adopted, and the use of the Phylogenetic Species Concept (PSC), first articulated by Cracraft in 1983, is now widely accepted. Under this concept, a species is a diagnosable entity, meaning that it has fixed heritable differences from other species. The nearest thing to a textbook in the field, Groves’s Primate Taxonomy (2001), adopted this concept and, under it, recognized over 300 living species of Primates.
An outline classification of living Primates, down to genus level, is shown in the following outline:
Fossil taxa can be slotted into this scheme where they are required. In the Homininae, there are several fossil genera that are closely related to Homo: Australopithecus, Paranthropus, Kenyanthropus, and Ardipithecus. If they are valid, this would require that the three genera of Homininae each be given its own tribe (Gorillini, Panini, Hominini), and the other fossils would then be inserted into the Hominini alongside Homo. An isolated fossil genus, sister to a whole suborder or family, can be placed, unranked, as a “plesion” of that group.
Notice several distinctions about the classification. First, a few of the old names have been modified: Strepsirrhini rather than Strepsirhini (because it conforms better to the Greek from which it is derived). Notice, too, the use of several standard endings: -oidea for super-families, -idea for families, -inae for subfamilies, -ini for tribes (if even finer division is wanted, subtribes, with the endingina, can be inserted below tribes). These are standard throughout the animal kingdom. Suborders, infraorders, and so on have no standard endings (though they are, nonetheless, given them within most orders). Notice, too, that they all begin with an uppercase; and, finally, that generic names are written in italics.
A classification should not be thought of as a “given”; it is a scientific hypothesis, like any other, and should be expected to change as further evidence accumulates.
Species, finally, have two names. First is the name of the genus, followed by the name special to that species: Homo sapiens, Pan troglodytes, Pan paniscus, Saimiri sciurea, Saimiri boliviensis, Saimiri oerstedi. The specific name always begins with a lowercase letter, even though it denotes a country (Bolivia) or a person (Orsted). Also, if subspecies are needed (in completely differentiated geographic segments of a species), a third word is added on at the end (for example, Saimiri sciurea sciurea [the squirrel monkey that occurs in the region where squirrel monkeys were first discovered], Saimiri sciurea cassiquiarensis [the squirrel monkey from the Rio Cassiquiare]).
If handled properly, a classification reveals the following: first, it clarifies what the species are (the units), and second, it explains what the monophyletic groups (clades) are; in other words, it tracks the course of evolution. It is still disputed whether it should also to give the reader an idea of the timing of separation of two groups (i.e., what makes a well-thought-out classification an indispensable basis for all other studies in primatology: ecology, behavior, conservation, biogeography, genetics, and pharmacology).