The known fossil record of undoubted primates dates back to the beginning of the Eocene epoch, some 55 million years ago (mya). A group of fossil mammals from the preceding Paleocene epoch (55–65 mya), containing many North American and European representatives and allocated to the infraorder Plesiadapiformes (e.g., Ignacius, Palaechthon, Plesiadapis, Purgatorius), is commonly included in the order Primates.
However, some authors have questioned the proposed link between Plesiadapiformes and Primates and the principal similarities involve the molar teeth. It is, in any case, generally agreed that the Plesiadapiformes branched away before the origin of modern primates. They are hence no more than a sister group and have accordingly been given the label “archaic primates.” Modern primates and their direct fossil relatives (“primates of modern aspect” or Euprimates) can only be traced back to the basal Eocene.
Close to 500 fossil primates of modern aspect have been recognized, and this total will surely increase. Surprisingly, the earliest representatives, from the Eocene epoch, have been discovered primarily in North America and Europe, where numerous species have been documented. This is unexpected, because primates today are very largely confined to the southern continents (South America, Africa, and Asia). Most of the Eocene primates that have been found are of course relatively primitive and hence most closely resemble modern prosimians. Indeed, it is possible to find both lemur-
For a long time, the earliest known direct fossil relatives of higher primates dated back only to the beginning of the Oligocene, about 35 mya. These early Oligocene anthropoids are all derived from a single fossil site in Egypt, the Fayum, and include a dozen genera belonging to two distinct groups with different dental formulae (e.g., Aegyptopithecus versus Apidium). A few enigmatic Eocene forms with some monkey-
Fissure fillings from the Chinese middle Eocene site of Shanghuang have also yielded several fossils that have expanded our understanding of early primate evolution. In addition to adapiforms and omomyiforms, the Shanghuang deposits contain a possible early anthropoid (Eosimias) and an apparent direct relative of modern tarsiers (Tarsius eocaenus). Overall, an impressive range of early fossil primates of modern aspect is known from the Eocene and early Oligocene, primarily from the northern continents. However, there is a period of 6 million years during the middle of the Oligocene epoch (26–32 mya) from which not a single fossil primate species has been recovered. A few primate fossils have been discovered in late Oligocene deposits, and from the Miocene upwards (i.e., over the last 25 million years) the primate fossil record is again relatively good. Miocene deposits have yielded direct relative of modern lorises and bushbabies, of New World monkeys, of Old World monkeys, and of apes (hominoids). Nevertheless, there are still some marked gaps in the fossil record. For instance, no single fossil lemur has ever been discovered on Madagascar, although a score of subfossil lemur species (predominantly large-
The order Primates is one of a score of major groups that radiated from the ancestral stock of placental mammals that existed at some time during the Cretaceous. One key question therefore concerns the relationship between primates and other mammals. Primates of modern aspect undoubtedly constitute a monophyletic group. In other words, they are all derived from a single, distinct common ancestor. Various attempts have been made to link this monophyletic group of primates to other orders of mammals. For some time, the tree shrews (now allocated to the separate order Scandentia) were actually included in the order Primates, but it eventually emerged that the similarities between tree shrews and primates are attributable to retention of primitive mammalian features and convergent adaptations for arboreal life. There has also been much support for recognition of a superorder Archonta containing primates, tree shrews, colugos (Dermoptera), and bats (Chiroptera). (In the original proposal, Archonta also included elephant shrews, but they were subsequently quietly dropped.) One problem with recognition of the Archonta is that it perpetuates the disputed link between primates and tree shrews by other means. Furthermore, it continues the practice of suggesting links on the basis of likely retention of primitive mammalian features and convergent adaptations for arboreal life. A quite different suggestion, based on certain features of the visual system, is that primates are the sister group of fruit bats (Megachiroptera). Among other things, this “flying primate hypothesis” has the corollary that the bats are not monophyletic and that flight evolved twice, once in ancestral fruit bats and once in the ancestor of the remaining bats (Microchiroptera). Comprehensive analyses of relationships between mammalian orders using large molecular data sets have now fairly clearly ruled out any connection between tree shrews and primates or between bats and primates. Indeed, several molecular studies have indicated that tree shrews may have some link to rabbits, while a whole host of morphological and molecular evidence resoundingly indicates that the bats form a monophyletic group. Hence, the “flying primate hypothesis” has been largely discredited and there is little support for recognition of a superorder Archonta. On the other hand, there are indications from the molecular data that there might be some kind of link between colugos and primates.
Because the earliest known undoubted fossil primates are only 55 million years old, it has been widely accepted that the common ancestor of primates of modern aspect dates back only to the Paleocene, some 60–65 mya, thus post-
Relationships within the order Primates are now relatively well established, at least as far as the living representative are concerned. Numerous sources of evidence, including morphology, chromosomes, and molecular data, all point to a basic divergence between one lineage leading to lemurs and the loris group and another leading to tarsiers and higher primates. Modern lemurs, lorises, and bushbabies have retained the rhinarium (a hairless area of moist skin surrounding the nostrils) and are referred to as strepsirrhines. They uniformly exhibit a non-
On the other hand, they all have a virtually complete bony wall (postorbital plate) behind the orbit, whereas strepsirrhine primates merely have a bony strut (postorbital bar) around the outer margin of the orbit. The relationships between Eocene primates and modern primates are uncertain. Although the Adapiformes resemble modern lemurs in many respects, this is mainly because both possess relatively primitive primate features. Significantly, the Adapiformes lack any dental development that can be linked to the distinctive toothcomb of modern strepsirrhines. Hence, it seems likely that the Adapiformes may be a sister group of the strepsirrhines or perhaps just a side-
For many years, it was customary to classify the primates into two suborders: Prosimii and Anthropoidea. This reflected a classical, grade-
This approach is not followed here for entirely practical reasons. In the first place, if a classification directly matches an inferred phylogenetic tree, it must logically be changed every time the tree is changed. This is a prescription for classificatory instability. Secondly, most primate fossils (particularly the earlier representatives) are known only from isolated molar teeth and there is no known way of reliably distinguishing all strepsirrhines from all haplorhines on the basis of molar features alone. In any event, almost all primate classifications in general use have a primary subdivision into two suborders. The consensus view is that these contain a total of at least 14 families with extant representatives. Reflecting the diversity of the lemurs of Madagascar, five of these families belong to that group alone: Cheirogaleidae (dwarf and mouse lemurs); Lemuridae (true and gentle lemurs); Lepilemuridae (sportive lemurs); Indriidae (indri group); and Daubentoniidae (ayeaye). The loris group can be divided into two families: Loridae (lorises); Galagonidae (bushbabies). There are only five species of modern tarsiers, and these are all allocated to the single family Tarsiidae. The New World monkeys have classically been divided into two families: Cebidae (true New World monkeys) and Callitrichidae (marmosets, tamarins and Goeldi’s monkey). The Old World monkeys are all morphologically very similar and they are generally placed in the single family Cercopithecidae. However, some authors regard the leaf-