Evolution and systematics

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 Pa-leocene epoch (55-65 mya), containing many North American and European representatives and allocated to the infraorder Plesiadapiformes (e.g., Ignacius, Palaechthon, Plesiadapis, Purga-torius), is commonly included in the order Primates. However, some authors have questioned the proposed link between Ple-siadapiformes 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-like species (infraorder Adapiformes) and tarsier-like species (infraorder Omomyiformes). Representatives of both of these groups are found in Europe and North America (e.g., European Adapis and American Notharctus among Adapiformes and European Necrolemur and American Tetonius for Omomyiformes).

Eulemur Flavifrons
A blue-eyed lemur (Eulemur macaco flavifrons) with its young. (Photo by Tom & Pat Leeson/Photo Researchers, Inc. Reproduced by permission.)

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., Aegyptop-ithecus versus Apidium). A few enigmatic Eocene forms with some monkey-like features had been reported from Asia (e.g., Amphipithecus and Pondaungia from Myanmar [formerly Burma]), but the remains were so fragmentary that their affinities were uncertain. Recovery of more complete specimens revealed that these Asian forms are, indeed, related to higher primates, and the discovery of monkey-like Siamop-ithecus from Eocene deposits in Thailand has provided additional confirmation. Thus, the earliest known relatives of higher primates come from Asia. 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 omomyi-forms, 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 dis covered on Madagascar, although a score of subfossil lemur species (predominantly large-bodied forms) dating back just a few thousand years have been discovered.

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 (Der-moptera), 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 Ar-chonta. 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-dating the demise of the dinosaurs at the end of the Cretaceous. However, comprehensive phylogenetic trees for placental mammals based on molecular evidence suggest that many orders, including the Primates, began to diverge during the Cretaceous, about 90-100 mya. Furthermore, a statistical analysis that takes into account the numerous gaps in the primate fossil record indicates that these gaps have led to marked underestimation of the age of the last common ancestor of primates of modern aspect. Calculations suggest that ancestral primates existed at least 82 mya.

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-invasive (epitheliochorial) type of placentation. Furthermore, they are generally characterized by the development of a toothcomb in the lower jaw, in which the bilaterally flattened crowns of the lower incisors and canines have become almost horizontal. This distinctive dental specialization can be traced back over 40 million years. By contrast, modern tarsiers and higher primates have completely lost the rhinarium and are accordingly labeled haplorhines. They uniformly exhibit a highly invasive (hemochorial) type of pla-centation, and this in fact provided the first evidence of a link between tarsiers and higher primates. Haplorhine primates lack any dental development resembling the toothcomb of strepsirrhine primates. 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 Adapi-formes 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-branch from the ancestral primate stock. Similarly, the relationship between Omomyiformes and modern tarsiers is tenuous. Although both groups show an intriguing similarity in possessing relatively large molar teeth and a bell-shaped upper dental arcade, the Omomyiformes merely have a postorbital bar and lack a postorbital plate. Thus, there is probably no more than a sister-group relationship between Omomyiformes and tarsiers. From the late Eocene through the lower Oligocene, there is increasing evidence of the development of higher primate characteristics in certain lineages. Deepening of the lower jaw (mandible) and the presence of a postorbital plate are identifiable in the late Eocene, and by the lower Oligocene there are fossil forms with spatulate (rather than peg-like) incisors and medial fusion of the right and left halves of the mandible. All of these are advanced features of the higher primates. From the beginning of the Miocene onwards, it is possible to identify representatives of all three natural groups of higher primates on the basis of defining characteristics.

For many years, it was customary to classify the primates into two suborders: Prosimii and Anthropoidea. This reflected a classical, grade-based approach to classification in which the most primitive surviving forms are allocated to a basic group along with all early fossil forms. The suborder Prosimii hence included the fossil Adapiformes and the Omomyiformes along with the extant lemurs, lorises, and tarsiers, while the suborder Anthropoidea included the extant monkeys, apes, and humans along with any fossil forms show-

A Japanese macaque (Macaca fuscata) eats phloem from the bark. (Photo by Nils Reinhard/OKAPIA/Photo Researchers, Inc. Reproduced by permission.)

ing certain advanced features that characterize this subgroup of primates. However, many authors now favor a cladistic type of classification in which the main subdivisions are designed to reflect directly the main divergences within the reconstructed phylogenetic tree. This has led to the widespread adoption of an alternative classification in which lemurs and lorises are allocated to the suborder Strepsirrhini and tarsiers and higher primates to the suborder Haplorhini. 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 strepsir-rhines 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 (aye-aye). The loris group can be divided into two families: Lori-dae (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-monkeys as sufficiently different to place them in a separate family Colobidae. Finally, the hominoids have been traditionally divided into three families: Hylobatidae (lesser apes, or gibbons), Pongidae (great apes), and Hominidae (modern humans and their fossil relatives).

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