Classification
Traditionally, the order Primates was divided into Prosimii (the primitive primates: lemurs, lorises, and tarsiers) and Anthropoidea (the bigger-brained monkeys and apes, including humans). It is now known that one of the “prosimians,” the tarsier, is actually more closely related to the “anthropoids,” so the classification of the primates has had to be revised. The two suborders recognized today are Strepsirrhini (lemurs and lorises) and Haplorrhini (tarsiers, monkeys, and apes, including humans). The present classification was adopted in the early 1970s, when the logic of phylogenetic systematics was beginning to be appreciated, and the taxonomy of the order Primates was reorganized so as to make taxa equivalent, as far as possible, to clades. In this classification, the order is divided into the suborders Strepsirrhini and Haplorrhini, and these in turn are divided into infraorders.
The two suborders differ in such features as the nose, eyes, and placenta. The muzzle of strepsirrhines is moist and bare, like a dog’s; haplorrhines have a nose covered with downy hair. Strepsirrhines have a reflective layer, the tapetum lucidum, behind the retina, which increases the amount of light for night vision, while haplorrhines have no tapetum but, instead, an area of enhanced vision, the fovea. This difference is consistent, even though not all strepsirrhines are nocturnal or all haplorrhines diurnal. Finally, the uterine wall of haplorrhines is elaborated and vascularised each month for possible pregnancy, forming a hemochorial placenta (with intimate contact between maternal and fetal bloodstreams) if pregnancy occurs and requiring it to be shed, in menstruation, if there is no fertilization. Strepsirrhines have no uterine preparation, no menstruation, and a much-less-intimate epitheliochorial placenta. All indications are that these two suborders separated very early, perhaps 60 million years ago. The Strepsirrhini divide further into Lemuriformes (lemurlike) and Lorisiformes (lorislike) infraorders, while the Haplorrhini divide into Tarsiiformes (tarsiers) and Simiiformes (“anthropoids”—i.e., monkeys, apes, and humans). The Simiiformes divide in turn into Platyrrhini (New World monkeys) and Catarrhini (Old World monkeys and hominoids). A group of fossil mammals called the Paromomyiformes, known mainly from the Paleocene, have usually been classified as primates, but the eminent primate specialist Robert D. Martin has long argued that their connection with authenticated primates is tenuous, to say the least, and, in the 1990s, the paleontologist K.C. Beard discovered hand bones and other material that suggest strongly that some of these fossils may actually belong not to Primates but to the order Dermoptera.
There has been a good deal of discussion about the relationships of primates to other mammals. In the 1930s it was proposed that the tree shrews (small Southeast Asian mammals, family Tupaiidae), hitherto classed in the order Insectivora, belong to the order Primates—or at least that they are closely related. This has turned out to be wrong; Martin has shown in detail how they differ from Primates and how the error arose. Tree shrews are nowadays placed in their own order, Scandentia. In the 1980s the mammalogist Jack Pettigrew discovered that the Megachiroptera, the suborder of bats (order Chiroptera) that contains the fruit bats or “flying foxes” (family Pteropodidae), share aspects of the visual system with primates, and he proposed that they are in effect flying primates, having evolved wings independently from other bats (suborder Microchiroptera, the echolocating, mostly insectivorous smaller bats). Later he found that the colugos, so-called “flying lemurs” (sole living representatives of the order Dermoptera) of the Southeast Asian rainforests, share the same features. Molecular data, however, confirm that, whereas primates and colugos really are closely related, the bats form a monophyletic group. Therefore, either the primates, colugos, and all bats shared a common ancestor with those specializations of the visual system, which the Microchiroptera then lost, or these features were developed independently by (1) primates and colugos and (2) Megachiroptera. Molecular data are equivocal as to whether bats are the sister group to primates and colugos, so these two competing hypotheses cannot yet be tested.
Like any classification, the present arrangement is essentially a hypothesis of relationships among superfamilies, families, and genera, and it is continually being tested by the discovery of new fossils, reanalyses of data from conventional sources, and analyses of new data such as DNA sequences. A new source of information has been the molecular clock; Morris Goodman, a molecular biologist who as long ago as 1963 (before the cladistic revolution) was already insisting that taxonomy should reflect phylogeny, recently proposed that an attempt be made to give the same taxonomic rank to groups of equivalent time depth (so reducing the subjectivity of higher categories), and his idea has begun to take hold in some quarters. In line with the arguments of such authorities as R.D. Martin and K.C. Beard, the (predominantly Paleocene) Plesiadapiformes (or Paromomyiformes) are not, here, regarded as Primates.
