Primate Evolution

Primate Evolution

Primate Taxonomy Primate Taxonomy Introduction This first lecture is designed to introduce the primate order in terms of its classification and to familiarise you with the animals so that the rest of the course makes some sort of sense. I will cover a working definition of what makes a primate, give you a general classification scheme, describe the major features that identify the groups within the classification and discuss some of the controversial areas of the classification.

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I shall treat the taxonomy as a synonym for classification which seems to be its commonest current usage, although you should be aware that some people consider taxonomy to be more about the principles behind the classification than the classification itself. Definition of a primate Most primatology textbooks include a definition of a primate in their introductory chapters. This discussion comes from Martin [Martin, 1986] and is as good a starting point as any. Mivart’s Definition Like many definitions, the definition of what makes a primate (as opposed to a rodent, or a carnivore etc. is complex. There is little argument as to the core groups of animals today that are primates as I will be illustrating later, but as one goes back in the fossil record, there is more dissension. Still, a purely descriptive definition is needed as a starting point and Mivart [Mivart, 1873] provided this in figure 1. Figure 1. Mivart’s definition of a primate [Mivart, 1873] Mivart’s Primate Definition Unguiculate, claviculate, placental mammals, with orbits encircled by bone; three kinds of teeth, at least at one time of life; brain always with a posterior lobe and calcarine ? sure; the innermost digit of at least one pair of extremities opposable; hallux with a ? at nail or none; a well developed caecum; penis pendulous; testes scrotal; always two pectoral mammae. Notes: Unguiculate – possessing nails, hooves or claws Claviculate – possessing a clavicle (collar bone) Page 1 of 21 Primate Taxonomy Le Gros Clark’s Definition Mivart’s definition is quite a good definition considering its age and certainly has the value of being nice and short. A rather more up to date version was produced by Le Gros Clark [Le Gros Clark, 1959] (see figure 2).

Figure 2. Le Gros Clark’s definition of a primate [Le Gros Clark, 1959] Le Gros Clark’s Definition 1. Preservation of generalised limb structure with primitive pentadactyly. 2. Enhancement of free mobility of the digits, especially of the pollux and hallux (both used for grasping). 3. Replacement of sharp, compressed claws by flat nails; development of very sensitive tactile pads on the digits. 4. Progressive shortening of the snout. 5. Elaboration of the visual apparatus, with the development of varying degrees of binocular vision.

Orbits ringed with bone. 6. Reduction of the olfactory apparatus. 7. Loss of certain elements of the primitive mammalian dentition. Preservation of a simple molar cusp pattern. 8. Progressive expansion and elaboration of the brain, especially of the cerebral cortex. 9. Progressive and increasingly efficient development of gestational processes. Considering each of these features in turn: Page 2 of 21 Primate Taxonomy Preservation of generalised limb structure with primitive pentadactyly Figure 3.

Diagram of the forelimb of a variety of tetrapods showing how the primate has retained the primitive pentadactyly limb (3 girdle bones; 1 upper limb bone; 2 lower limb bones; carpals/tarsals; meta-carpals/tarsals; phalanges) whereas various other mammalian orders have lost various bones (taken from Strickberger [Strickberger, 1990]). As you can see from figure 3 the primates have retained a limb bone structure that is very similar to that of the primitive tetrapod, whereas the other mammals shown have considerably reduced bone numbers.

Enhancement of free mobility of the digits, especially of the pollux and hallux (both used for grasping) Figure 3 also shows how the primate hand and foot has long, mobile digits compared to some other mammals. Coupled with this, primates can use a variety of power and precision grips to manipulate objects. Figure 4 shows a variety of primate autopodia. Loss of grasping capability in the foot is a feature peculiar to humans. Page 3 of 21 Primate Taxonomy Figure 4. Photographs of the autopodia (hands and feet) of a variety of primates.

Top left, hand of Pithecia pithecia (white-faced saki); top right, foot of Lemur catta (ring-tailed lemur); bottom left, foot of Pan troglodytes (common chimpanzee); bottom right, foot of Cercopithecus hamlyni (Hamlyn’s owl monkey). Page 4 of 21 Primate Taxonomy Replacement of sharp, compressed claws by flat nails; development of very sensitive tactile pads on the digits Figure 5. Photograph of a variety of primate hands showing the presence of flattened nails and sensitive dermatoglyphs.

Top right, Trachypithecus obscura (dusky leaf monkey); top left, Callithrix geoffroyi (Geoffroy’s marmoset); bottom left, Pan troglodytes (common chimpanzee); bottom right Sanguinus oedipus (cotton-top tamarin). The two photographs on the right hand side are of Callitrichidae which are the exceptions to the normal primate rule and have claws rather than nails on most of their fingers. Figure 5 shows a examples of the flattened nails that are characteristic of primate digits and at the same time shows that rules are always made to be broken since in some primates the nail has reverted to being a claw.

Page 5 of 21 Primate Taxonomy Progressive shortening of the snout Figure 6. Photographs of a selection of mammalian skulls. Top left, Ursus arctos (brown bear); top right, Pan troglodytes (common chimpanzee); bottom left, Macropus giganteus (grey kangaroo); bottom right Daubentonia madagascariensis (aye-aye) (Photos from Dorling-Kindersley [Dorling-Kindersley, 1994] and Alexander [Alexander, 1994]). Figure 6 shows a selection of mammalian skulls. The primate skulls are the two on the right and as you can see, they have relatively short snouts.

Obviously there is a huge amount of variation in gross skull shape across the mammalian class, but certainly, on average, primate skulls tend to be short. Elaboration of the visual apparatus, with the development of varying degrees of binocular vision. Orbits ringed with bone. Figure 6 also shows the characteristic morphology of the primate orbit. The eyes face forward for binoccular region, the eyes tend to be quite large, and the orbit is either a fully enclosed cup (as in the chimp in figure 6) or a lateral post-orbital bar (as in the aye-aye).

In most other mammals (for example the bear and kangaroo), the eye is simply plastered onto the side of the skull and held in place by soft tissue. Page 6 of 21 Primate Taxonomy Reduction of the olfactory apparatus The reduction in the size of the snout and the increased reliance on vision has also led to a relative reduction in the olfactory apparatus compared to many other mammals. This is again apparent from figure 6 and is associated with the reduction in size of the snout. Loss of certain elements of the primitive mammalian dentition.

Preservation of a simple molar cusp pattern. Figure 7. Diagram of the teeth of the upper jaws of a selection of mammals (diagrams from Hillson [Hillson, 1986]). As you can see from figure 7, primates (as indeed do most mammals – teeth are often used for identification) have a fairly distinctive dentition. Primitive mammals are thought to have had a dental formula of 3.. 1.. 4.. 3 which means that they had 3 incisors, 1 canine, 4 premolars and 3143 3 molars in each half of their upper jaw and the same in each half of their lower jaw.

Modern mammals have a variety of dental formulas, but the typical dental formulas for primates are 2. 1. 3. 3 and 2.. 1.. 2.. 3 . The shape of the teeth is also diagnostic although there is quite a lot of variation 2. 1. 3. 3 2123 between different species. Progressive expansion and elaboration of the brain, especially of the cerebral cortex Primates of all types have larger brains that would be expected of mammals of their size (brain size is strongly correlated with body size, so any comparative measure of brain size Page 7 of 21

Primate Taxonomy has to take this into account). The cerebral cortex is the anterior part of the brain that seems to be mainly involved in higher cognitive processes such as memory and language. You can see the increase in brain size by looking at the sizes of the cranium in figure 6 and figure 8 shows a graph of brain size against body size for a variety of animals. Figure 8. Graph showing the relationship between brain and body size for a variety of animals. Note that the scales are logarithmic (Graph from Dunbar [Dunbar, 1996]).

Progressive and increasingly efficient development of gestational processes Mammals are generally characterised by specialised gestational mechanisms that lead to the birth of live offspring. This process is brought about by means of the implanatation of the embryo into the wall of the uterus and in eutherian mammals the development of a chorioallantoic placenta to allow nutrient and waste transfer between the embryo and the mother [Hamilton et al. , 1947]. The structure of the placenta is complex, but it basically consists of a series of membranous barriers between the maternal and embryonic blood supplies.

The two blood supplies never mix and chemical transfer is by diffusion. In some mammals, including primates, a number of these membranes have disappeared leading to a reduction in diffusion barriers and more efficient transfer across the placenta. Napier and Napier’s Defintion Napier and Napier [Napier and Napier, 1967] have added two extra items to this list (see figure 1). Page 8 of 21 Primate Taxonomy Figure 9. Napier and Napier’s definition of a primate. Napier and Napier’s De? nition Le Gros Clark’s de? nition plus: 10. Prolongation of postnatal life periods. 1. Progressive development of truncal uprightness leading to a facultative bipedalism. Prolongation of postnatal life periods Primates as a group tend to have long periods of postnatal care. Offspring remain dependent on their parents for long periods allowing opportunities for social learning and requiring large amounts of parental investment. For the ecologists among you, primates tend to be K-selected rather than r-selected. Figure 10. Photograph of a family group of yellow baboons (Papio hamadryas cyanocephalus) [Rowe, 1996].

Progressive development of truncal uprightness leading to a facultative bipedalism This refers to the suggestion that primates tend to adopt postures with the trunk held vertically and that a number of primates are able to walk bipedally when necessary. Facultative Page 9 of 21 Primate Taxonomy bipedalism – the ability to walk on two legs when required – is found in the apes and is occasionally seen in other primates. Habitual bipedalism – walking on two legs all the time – is only found in humans and their immediate ancestors. Figure 11.

Photographs of two primates walking bipedally: left, Pongo pygmaeus (orangutan); right Pan paniscus (pygmy chimpanzee) [Rowe, 1996]. At first view, these various definitions seems OK (apart from the dreadful language which makes the whole thing read like a life insurance document), but if fact there are problems with them: Firstly, there is no unique characteristic that defines a primate. It is a list of shared characteristics and trends – most of which are not even derived, but are retentions of ancestral features which is definitely not good.

Many mammalian orders can be characterised by a single anatomical feature that they uniquely possess such as wings for bats which makes classification much easier. Secondly, many of these features are behavioural or depend on soft tissue anatomy. They will not help us identify a fossil primate. Thirdly, many of these features are not really features of primates in general but trends that run through the order as the primates get more and more human-like. These are not necessarily very useful in an individual case. Fourthly, there are many primates that do not have all the features listed.

Indeed, there is no feature that all primates share. This is a set of features that you would use as evidence. A primate is likely to show many of these features, but we cannot a priori say which ones. Page 10 of 21 Primate Taxonomy Thus it can be very difficult, especially when dealing with very early mammals and fragmentary fossils, to decide whether a specimen is a primate or not. There are even whole groups (extant ones such as the tree shrews and extinct ones such as plesiadapids) that some people classify as primates whilst others do not.

Similarly, there are several mammalian groups (insectivores, bats, flying lemurs) that have been considered the closest living relatives to primates and the jury is certainly still out on this question. Primate Classification Classification serves two distinct purposes. Firstly it is there so we all know which animals we are talking about when we are discussing them – whether this is for scientific use in journals, knowing which shelf to look on in a museum or setting laws for the transport of live animals across national boundaries.

Secondly, it should not be an arbitrary grouping but should reflect the evolutionary relationships of the animals. In theory these two uses should not interfere with each other but in practice we do not accurately know the evolutionary relationships among the primates so a classification based entirely on current consensus thinking of evolutionary relationships would change on a monthly basis as the next scientific paper on the subject came out. This means that we have a fairly well established classification for the primates that changes relatively slowly that only approximately relates to current thinking on evolutionary relationships.

Even so, you will find several different classification schemes in use in textbooks. The classification scheme I shall adopt for this course is the one from Szalay and Delson [Szalay and Delson, 1979] (reprinted in the back of Conroy [Conroy, 1990]). It is probably not the most commonly used scheme, but I feel it reflects the anatomy quite well. The scheme in Fleagle [Fleagle, 1999] is the one more generally encountered and I will periodically comment on the differences. Before we go on, a note on typography1. The way that taxonomic terms are written in important (translation: you will lose marks if you get it wrong! . Species names are written in italics with the name of the genus starting with a capital (uppercase) letter and the name of the species starting with a lowercase letter such as Varecia variegata. Further subspecies designations are added after this (also in italics) such as Varecia variegata rubra. After the first usage in an essay or paper, the genus name can be abbreviated to its first letter (still in capitals) such as V. variegata. Within the primate order, the species names are all unique so there should be no chance of confusion.

Common names of animals do not have capital letters (red ruffed lemur in this case). When referring to a whole genus, the genus name is used on its own, with a captial letter and in italics (Varecia). Higher taxonomic levels (subfamilies, families, superfamilies, infraorders and suborders) usually have two forms: a In practice, typographical rules are rarely carved in stone. Books and journals have a ‘house style’ which tightly defines the typographic rules they use. The recommendations here are widely followed but you will encounter differences. Page 11 of 21 Primate Taxonomy latinised form and an anglicised form. The latinised form is probably the correct form but these names are such a mouthful that the anglicised names are generally more commonly used. Italics are not used, and only the latinised form has a capital letter. Most people would put Varecia in the family Lemuridae, the superfamily Lemuroidea, the infraorder Lemuriformes, and the suborder Strepsirhini (and upwards: the order Primates, class Mammalia, Phylum Chordata, kingdom Metazoa, superkingdom Eukaryotes).

The anglicised versions are thus: lemurid, lemuroid, lemuriform and strepsirhine. There are other levels in the hierachy that can optionally be used. Subfamily is probably the commonest –- it is not used in our reference classification for Varecia (although you could make use Lemurinae) but, for example, humans are in the subfamily Hominae, anglicised name hominin. All the various taxonomic levels can have a name and a date written after them to denote the person who first used that particular name but unless you are writing a taxonomic textbook (when it is very useful), it is robably not worth doing this. Note that the taxonomic level of a term can usually be identified by the ending of the word. Thus subfamilies end in inae (in), families end in idea (id), superfamilies end in oidea (oid). Unfortunately, the other taxonomic levels are less consistent, but if your etymological taste buds have been tickled the you might want to check out Gotch [Gotch, 1995] for a highly detailed discussion. Suborders Table 1. The primate suborders Order Suborder Strepsirhini Primates Haplorhini

The first major split in the primate order is between the Strepsirhini and the Haplorhini (see table 1). Unfortunately, this is also the first (and probably most important) difference of opinion between the various classification schemes. Fleagle [Fleagle, 1999] uses Prosimii and Anthropoidea as his first major split and you will hear and read the terms prosimian and anthropoid all over the place. I will certainly talk about prosimians a lot – mostly because it is much easier to say than strepsirhines and is much more widely used.

Unfortunately strepsirhine is not synonymous with prosimian and haplorhine is not synonymous with anthropoid although in many instances the difference is irrelevant. The difficulty lies in a rather specialised and rather small infraorder called the Tarsiiformes which are classified as prosimian on the one hand and haplorhine on the other (as shown in table 2). What this means in practice is that you should use the strepsirhine/haplorhine distinction formally, but its OK to use the prosimian/anthropoid terms in less formal settings provided you are not talking about tarsiers!

The other occasion when the term prosimian is quite useful is that because it is rather less precise in its anatomical definition, we can be fairly sure that early fossil primates fall into the prosimian category but there is a certain amount of uncertainty about whether some Page 12 of 21 Primate Taxonomy of them are haplorhine or strepsirhine. Mind you, this is not necessarily a good thing. Woolly classification is not terribly helpful. Table 2.

Comparison of the two common primate suborder classifications Order Suborder Suborder Infraorder Strepsirhini Prosimii Tarsiiformes Primates Haplorhini Anthropoidea The other thing to note when reading about strepsirhines and haplorhines is that strepsirhines are often referred to as ‘primitive’. This is a valid term when talking about fossil forms and is fine when talking about particular traits (where it means a trait that is found in an ancestral form which has been retained in a current form). It is not a particularly useful term to apply to a whole animal.

What people generally mean when they talk about ‘primitive’ and ‘advanced’ extant primates is less or more like humans and that is an incorrect usage. Humans do indeed have some advanced features (features not found in their ancestors) such as large brains and feet adapted to habitual bipedalism, but then all extant species tend to have a few specialised features that set them apart from other species (specialised throat anatomy to allow extremely loud vocalisations in Indri, multi-chambered stomachs for leaf fermentation in Colobus to name but a few).

To describe an animal as a whole as advanced requires some sort of subjective value judgement on some non-specific notion of importance of their specialised traits and it generally all boils down to, “if it’s a feature that humans are proud of then it’s advanced”. Infraorders Table 3. The primate infraorders Order Suborder Infraorder Strepsirhini Lemuriformes Tarsiiformes Primates Haplorhini Platyrrhini Catarrhini The next taxonomic level is infraorder (see table 3). This lumps all the extant strepsirhines into a single infraorder, the lemuriforms.

There are other strepsirhine suborders in the classification but they only contain fossil forms so we will not cover them in this section. They have truly frightening names and the stability of the fossil classification is rather poor. Tarsiiforms only contain the tarsiers which small, nocturnal primates with a very restricted geographical range in Southeast Asia although there are some important fossil members of Page 13 of 21 Primate Taxonomy this group. Platyrrhines and catarrhines (note the double r as opposed to the single r in strepsirhine and haplorhine) consist of the true monkeys, apes and humans and split neatly on geographic lines.

The platyrrhines are confined exclusively to South and Central America (the New World) and catarrhines, with the exception of humans who get everywhere, live in Africa and Asia2. Thus platyrrhines are known as New World Monkeys (NWM) since they are all monkeys found in the Americas. Catarrhines consist of Old World Monkeys (OWM: monkeys found in Asia and Africa), Apes and Humans. Geography is extremely important in classification. Because speciation is generally associated with geographical isolation followed by diversification, different groups of animals tend to occur in geographically isolated units.

An example of this is Madagascar [Garbutt, 1999]. This island off the East coast of Africa actually broke away from the Indian subcontinent and drifted southwards. It has almost certainly never been connected to Africa by any land bridge and has been isolated from India for 80 million years. The assumption is that it was colonised by primates due to extremely rare rafting episodes where small numbers of early primates were accidentally washed from the East African shore to Madagascar clinging to pieces of floating vegetation.

This is a highly unlikely process but we think it probably occurred twice in Madagascar’s early history since there are two quite different primate groups now found there. A somewhat similar story may also have happened in South America since it also has a highly specific mammalian fauna and was isolated from North America when primates first appeared on the continent. Unfortunately it is slightly less convincing in this case – a 400 km rafting feat seems reasonably feasible whereas a 2000 km journey from Africa seems unlikely (600 km is the longest rafting journey where we have reasonable evidence that it occurred [Conroy, 1990]).

The alternative suggestion is that the primates ‘island hopped’ through the Caribbean from North America which had large primate populations at the time [McKenna, 1980]. Superfamilies Table 4. The primate superfamilies Order Suborde Infraorder Superfamily Strepsirhini Lemuriformes Lemuroidea Lorisoidea Tarsiiformes Tarsioidea Platyrrhini Ceboidea Primates Haplorhini Catarrhini Cercopithecoidea Hominoidea Table 4 shows the superfamily divisions. Because Tarsiiformes and Platyrrhini only contain a single superfamily (in most classification schemes) even when the fossil forms are considered, you are unlikely to see these terms used.

Lemuroids are geographically restricted to the island of Madagascar and consist of the large-bodied, diurnal lemurs. Lorisoids are more 2 Primate fossil forms of all primates are more widespread being found in North America and Europe as well. Page 14 of 21 Primate Taxonomy widespread consisting of the small-bodied, nocturnal primates found in Madagascar, Africa and Asia. Some classification schemes (for example Simons [Simons, 1972]) put all the Malagasy strepsirhines into Lemuroidea arguing that Madagascar was actually colonised by a single rafting event and there is certainly dispute in this area (see Conroy [Conroy, 1990] for discussion).

The cercopithecoids correspond to the Old World Monkeys and the hominoids are apes and humans. Families Table 5. The primate families. Order Suborder Infraorder Strepsirhini Lemuriformes Tarsiiformes Lorisoidea Tarsioidea Primates Haplorhini Platyrrhini Catarrhini Superfamily Lemuroidea Ceboidea Cercopithecoidea Cercopithecidae Hominoidea Family Lemuridae Indriidae Daubentoniidae Cheirogaleidae Lorisidae Tarsiidae Cebidae Atelidae Hominidae Once we get to the family level (see table 5) there starts to be a great deal of disagreement between various taxonomists.

I’ll stick with Szalay and Delson [Szalay and Delson, 1979] for the simple reason that they have relatively few families and that makes it a little easier to learn but you will find that family level classification varies from paper to paper. Lemurids comprise the typical lemur forms (with lemur in their common name) except for the wooly lemurs which are in the Indriids along with indries and sifakas. Daubentoniidae has a single extant member: the very peculiar aye-aye which is so utterly strange that it was classified as a rodent for many years.

Lorisids include the slow moving, tail-less lorises, the much more acrobatic bushbabies and the dwarf lemurs. Cebids include the smaller bodied New World Monkeys (capuchins, squirrel monkeys, marmosets and tamarins). The larger bodied South American monkeys (spider and howler monkeys, sakis and uakaris) and the only nocturnal monkey (the aptly named night monkey) are all Atelids. All extant old world monkeys (guenons, baboons, macaques and leaf monkeys) are cercopithecoids, and in this classification, apes and humans share the hominid family.

This latter grouping sits quite happily with primatologists who have long known that humans are just naked apes but anthropologists (a very influential bunch) always want to distance humans from non-human primates (NHP) and tend to reserve Hominidae for humans and fossil forms after the split from the great apes. They use another family, Pongidae, specifically for the great apes. Taxonomically this is certainly a bad idea (especially given the recent molecular evidence), but many people find it politically more acceptable and you will certainly find the name pongid commonly used in the literature. Page 15 of 21

Primate Taxonomy Here are some examples from these families (all taken from Rowe’s excellent book [Rowe, 1996]). Figure 17. Black and white ruffed lemur, Varecia variegata. Figure 18. Indri, Indri indri. Page 16 of 21 Primate Taxonomy Figure 19. Aye-aye, Daubentonia madagascariensis. Figure 20. Coquerel’s dwarf lemur, Microcebus coquereli. Page 17 of 21 Primate Taxonomy Figure 21. Slender loris, Loris tardigradus. Figure 22. Philippine tarsier, Tarsius syrichta. Page 18 of 21 Primate Taxonomy Figure 23. Buffy headed marmoset, Callithrix flaviceps. Figure 24. Black and gold howler monkey, Alouatta caraya.

Page 19 of 21 Primate Taxonomy Figure 25. Diana monkey, Cercopithecus diana. Figure 26. Orang-utan, Pongo pygmaeus. Page 20 of 21 Primate Taxonomy Bibliography Alexander, R. M. N. Bones: the Unity of Form and Function. London: Weidenfeld and Nicolson, 1994. Conroy, G. C. Primate Evolution. London: Norton, 1990. Dorling-Kindersley. Ultimate Visual Dictionary. London: BCA, 1994. Dunbar, R. Grooming, Gossip and the Evolution of Language. London: faber and faber, 1996. Fleagle, J. G. Primate Adaptation and Evolution. London: Academic Press, 1999. Garbutt, N. Mammals of Madagascar. Sussex: Pica Press, 1999. Gotch, A.

F. Latin Names Explained. London: Cassel, 1995. Hamilton, W. J. , J. D. Boyd, and H. W. Mossman. Human Embryology (Prenatal Development of Form and Function). Cambridge: W. Heffer& Sons Ltd. , 1947. Hillson, S. Teeth. Cambridge: Cambridge University Press, 1986. Le Gros Clark, W. E. The Antecedents of Man. Edinburgh: Edinburgh University Press, 1959. Martin, R. D. “Primates: a Definition. ” In Major topics in primate and human evolution, eds. B. Wood, L. Martin, and P. Andrews. 1-31. Cambridge University Press: Cambridge, 1986. McKenna, M. Early History and Biogeography of South America’s Extinct Land Mammals.

New York: Plenum, 1980. Mivart, S. G. “On Lepilemur and Cheirogaleus, and on the Zoological Rank of the Lemuroidea. ” Proceedings of the Zoological Society of London (1873): 484-510. Napier, J. R. , and P. H. Napier. A Handbook of Living Primates. London: Academic Press, 1967. Rowe, N. The Pictorial Guide to Living Primates. New York: Pogonias Press, 1996. Simons, E. Primate Evolution. New York: Macmillan, 1972. Strickberger, M. W. Evolution. Boston: Jones and Bartlett, 1990. Szalay, F. , and E. Delson. Evolutionary History of the Primates. New York: Academic Press, 1979. Page 21 of 21


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