Perry, Jonathan and Stephanie Canington. 2019. “Chapter 8: Primate Evolution.” Explorations: An Open Invitation to Biological Anthropology, edited by Beth Shook, Katie Nelson, Kelsie Aguilera, and Lara Braff. Arlington, VA: American Anthropological Association.
Image Attributions
Anaptomorphus-descent-primates by Hubrecht A.A.W. The descent of the primates. Lectures delivered on the occasion of the sesquicentennial celebration of Princeton University. New York: Charles Scribner’s Sons, 1897, is in the Public Domain.
Anaptomorphus, Life restoration of Tetoniushomunculus (an omomyid) by W.B. Scott in A History of Land Mammals in the Western Hemisphere. New York: The Macmillan Company, is in the Public Domain.
Fossil Primates
Station 1: The Paleocene (Approximately 65 - 54 MYA): Primate-Like Mammals
Plesiadapiforms, the Archaic Primates
The Paleocene epoch saw the emergence of several families of mammals that have been implicated in the origin of primates. These are the plesiadapiforms, which are archaic primates, meaning they possessed some primate features and lacked others. The word plesiadapiform means “almost adapiform,” a reference to some similarities between some plesiadapiforms and some adapiforms (or adapoids; later-appearing true primates)—mainly in the molar teeth. Because enamel fossilizes better than other parts of the body, the molar teeth are the parts most often found and first discovered for any new species. Thus, dental similarities were often the first to be noticed by early mammalian paleontologists, partly explaining why plesiadapiforms were thought to be primates. Major morphological differences between plesidapiforms and euprimates (true primates) were observed later when more parts of plesiadapiform skeletons were discovered. Many plesiadapiforms have unusual anterior teeth and most have digits possessing claws rather than nails. So far, no plesiadapiform ever discovered has a postorbital bar (seen in extant strepsirrhines) or septum (as seen in haplorhines), and whether or not the auditory bulla was formed by the petrosal bone remains unclear for many plesiadapiform specimens. Nevertheless, there are compelling reasons (partly from new skeletal material) for including plesiadapiforms within the Order Primates.
Geographic and Temporal Distribution
Purgatorius is generally considered to be the earliest primate. This Paleocene mammal is known from teeth that are very plesiomorphic for a primate. It has some characteristics that suggest it is a basal plesiadapiform, but there is very little to link it specifically with euprimates (see Clemens 2004). Its ankle bones suggest a high degree of mobility, signaling an arboreal lifestyle (Chester et al. 2015). Purgatorius is plesiomorphic enough to have given rise to all primates, including the plesiadapiforms. However, new finds suggest that this genus was more diverse and had more differing tooth morphologies than previously appreciated (Wilson Mantilla et al. 2021). Plesiadapiform families were numerous and diverse during parts of the Paleocene in western North America and western Europe, with some genera (e.g., Plesiadapis; see Figure 8.4) living on both continents (Figure 8.5). Thus, there were probably corridors for plesiadapiform dispersal between the two continents, and it stands to reason that these mammals were living all across North America, including in the eastern half of the continent and at high latitudes. A few plesiadapiforms have been described from Asia (e.g., Carpocristes), but the affinities of these remain uncertain.
Figure 8.4: Families of plesiadapiforms with example genera and traits: a table. Credit: Plesiadapiforms table original to Explorations: An Open Invitation to Biological Anthropology (2nd ed.) by Jonathan M. G. Perry and Stephanie L. Canington is under a CC BY-NC 4.0 License. Content derived from Fleagle 2013.
Family
Genera
Morphology
Location
Age1
Paromomyidae
Ignacius
Long, dagger-like, lower incisor.
North America and Europe
Early Paleocene to Late Eocene
Carpolestidae
Carpolestes
Plagiaulacoid dentition. Limb adaptations to terminal branch feeding. Grasping big toe.
North America, Europe, and Asia
Middle Paleocene to Early Eocene
Plesiadapidae
Plesiadapis
Mitten-like upper incisor. Diastema. Large body size for group.
Although there is much morphological variation among the families of plesiadapiforms, some common features unite the group. Most plesiadapiforms were small, the largest being about three kilograms (approximately 7 lbs.; Plesiadapis cookei). They had small brains and fairly large snouts, with eyes that faced more laterally than in euprimates. Many species show reduction and/or loss of the canine and anterior premolars, with the resulting formation of a rodent-like diastema (a pronounced gap between the premolars and the incisors, with loss of at least the canine); this probably implies a herbivorous diet. Some families appear to have had very specialized diets, as suggested by unusual tooth and jaw shapes.
Arguably the most interesting and unusual family of plesiadapiforms is the Carpolestidae. They are almost exclusively from North America (with a couple of possible members from Asia), and mainly from the Middle and Late Paleocene. Their molars are not very remarkable, being quite similar to those of some other plesiadapiforms (e.g., Plesiadapidae). However, their lower posterior premolars (p4) are laterally compressed and blade-like with vertical serrations topped by tiny cuspules. This unusual dental morphology is termed plagiaulacoid (Simpson 1933). The upper premolar occlusal surfaces are broad and are covered with many small cuspules; the blade-like lower premolar might have cut across these cuspules, between them, or both.
Many plesiadapiforms have robust limb bones with hallmarks of arboreality. Instead of having nails, most taxa had sharp claws on most or all of the digits. The extremities show grasping abilities comparable to those of primates and some arboreal marsupials. Nearly complete skeletons have yielded a tremendous wealth of information on locomotor and foraging habits. Many plesiadapiforms appear to have been able to cling to vertical substrates (like a broad tree trunk) using their sharp claws, propelling themselves upward using powerful hindlimbs, bounding along horizontal supports, grasping smaller branches, and moving head-first down tree trunks. In carpolestids in particular, the skeleton appears to have been especially well adapted to moving slowly and carefully in small terminal branches (Figure 8.6).
Debate: Relationship of Plesiadapiforms to True Primates
In the middle of the twentieth century, treeshrews (Order Scandentia) were often considered part of the Order Primates, based on anatomical similarities between some treeshrews and primates. For many people, plesiadapiforms represented intermediates between primates and treeshrews, so plesiadapiforms were included in Primates as well.
Studies of reproduction and brain anatomy in treeshrews and lemurs suggested that treeshrews are not primates (e.g., Martin 1968). This was soon followed by the suggestion to also expel plesiadapiforms (Martin 1972) from the Order Primates. Like treeshrews, plesiadapiforms lack a postorbital bar, nails, and details of the ear region that characterize true primates. Many paleoanthropologists were reluctant to accept this move to banish plesiadapiforms (e.g., F. S. Szalay, P. D. Gingerich).
Later, K. Christopher Beard (1990) found that in some ways, the digits of paromomyid plesiadapiforms are actually more similar to those of dermopterans (Order Dermoptera), the closest living relatives of primates, than they are to those of primates themselves (but see Krause 1991). At the same time, Richard Kay and colleagues (1990) found that cranial circulation patterns and auditory bulla morphology in the paromomyid, Ignacius (see Figure 8.4), are more like those of dermopterans than of primates.
For many anthropologists, this one-two punch effectively removed plesiadapiforms from the Order Primates. In the last two decades, the tide of opinion has turned again, with many researchers reinstating plesiadapiforms as members of the Order Primates. New and more complete specimens demonstrate that the postcranial skeletons of plesiadapiforms, including the hands and feet, were primate-like, not dermorpteran-like (Bloch and Boyer 2002, 2007). New fine-grained CT scans of relatively complete plesiadapiform skulls revealed that they share some key traits with primates to the exclusion of other placental mammals (Bloch and Silcox 2006). Most significant was the suggestion that Carpolestes simpsoni possessed an auditory bulla formed by the petrosal bone, like in all living primates.
The debate about the status of plesiadapiforms continues, owing to a persistent lack of key bones in some species and owing to genuine complexity of the anatomical traits involved. Maybe plesiadapiforms were the ancestral stock from which all primates arose, with some plesiadapiforms (e.g., carpolestids) nearer to the primate stem than others.
Below is a reconstruction of Plesiadapis. These small quadrupeds came in many diverse forms, and represent part of the mammalian radiation that occurred in the Paleocene. They may be relatives of primates (a side branch), or some of them were perhaps even ancestral to primates as some of them have the auditory bulla, which contains the middle ear and is distinctive of primates.
Plesiadapiforms lived in western North America, western Europe, Asia, and Africa at the start of the Paleocene. While at the right time to be early primates, they lack true primate characteristics.
Using the resources provided for you, what are four differences between plesiadapiforms and primates?
1.
2.
3.
4.
Station 2: The Eocene (Approximately 55 - 34 MYA): The First True Primates
Adapoids and Omomyoids, the First True Primates
Geographic and Temporal Distribution
The first universally accepted fossil primates are the adapoids (Superfamily Adapoidea) and the omomyoids (Superfamily Omomyoidea). These groups become quite distinct over evolutionary time, filling mutually exclusive niches for the most part. However, the earliest adapoids are very similar to the earliest omomyoids.
The adapoids were mainly diurnal and herbivorous, with some achieving larger sizes than any plesiadapiforms (10 kg; 22 lbs.). By contrast, the omomyoids were mainly nocturnal, insectivorous and frugivorous, and small.
Both groups appear suddenly at the start of the Eocene, where they are present in western North America, western Europe, and India (Figure 8.7). This wide dispersal of early primates was probably due to the presence of rainforest corridors extending far into northern latitudes.
In North America and Europe, both groups achieved considerable diversity in the Middle Eocene, then mostly died out at the end of that epoch (Figure 8.8). In some Eocene rock formations in the western United States, adapoids and omomyoids make up a major part of the mammalian fauna. The Eocene of India has yielded a modest diversity of euprimates, some of which are so plesiomorphic that it is difficult to know whether they are adapoids or omomyoids (or even early anthropoids).
Figure 8.8: Families of adapoids and omomyoids with example genera and traits: a table. Credit: Adapoids and omomyoids table original to Explorations: An Open Invitation to Biological Anthropology (2nd ed.) by Jonathan M. G. Perry and Stephanie L. Canington is under a CC BY-NC 4.0 License(opens in new window). Content derived from Fleagle 2013.
Family
Genera
Morphology
Location
Age1
Cercamoniidae
Donrussellia
Variable in tooth number and jaw shape.
Europe and Asia
Early to Late Eocene
Asiadapidae2
Asiadapis
Plesiomorphic teeth and jaw resemble early Omomyids.
Asia
Early Eocene
Caenopithecidae3
Darwinius
Robust jaws with crested molars. Fewer premolars.
Europe, Africa, North America, and Asia
Middle to Late Eocene
Adapidae
Adapis
Fused mandible. Long molar crests. Large size and large chewing muscles.
Europe
Late Eocene to Early Oligocene
Sivaladapidae
Sivaladapis
Some large with robust jaws.
Asia
Middle Eocene to Late Miocene
Notharctidae4
Notharctus
Canine sexual dimorphism. Lemur-like skull. Clinging and leaping adaptations.
North America and Europe
Early to Middle Eocene
Omomyidae5
Teilhardina
Small, nocturnal, frugivorous or insectivorous. Tarsier-like skull in some.
North America, Europe, and Asia
Early Eocene to Early Miocene
Microchoeridae6
Necrolemur
Long bony ear tubes. Tarsier-like lower limb adaptations for leaping.
Europe and Asia
Early Eocene to Early Oligocene
1 Derived from Fleagle 2013.
2 See Dunn et al. 2016 and Rose et al. 2018.
3 See Kirk and Williams 2011 and Seiffert et al. 2009.
4 See Gregory 1920.
5 See Beard and MacPhee 1994 and Strait 2001.
6 See Schmid 1979.
Adapoids and omomyoids barely survived the Eocene-Oligocene extinctions, when colder temperatures, increased seasonality, and the retreat of rainforests to lower latitudes led to changes in mammalian biogeography. In North America, one genus (originally considered an omomyoid but recently reclassified as Adapoidea) persisted until the Miocene: Ekgmowechashala (Rose and Rensberger 1983). This taxon has highly unusual teeth and might have been a late immigrant to North America from Asia. In Asia, one family of adapoids, the Sivaladapidae, retained considerable diversity as late as the Late Miocene.
Adapoid Diversity
Adapoids were very diverse, particularly in the Eocene of North America and Europe. They can be divided into six families, with a few species of uncertain familial relationship. As a group, adapoids have some features in common, although much of what they share is plesiomorphic. Important features include the hallmarks of euprimates: postorbital bar, flattened nails, grasping extremities, and a petrosal bulla (Figures 8.9 and 8.10). In addition, some adapoids retain the ancestral dental formula of 2.1.4.3; that is, in each quadrant of the mouth, there are two incisors, one canine, four premolars, and three molars. In general, the incisors are small compared to the molars, but the canines are relatively large, with sexual dimorphism in some species. Cutting crests on the molars are well developed in some species, and the two halves of the mandible were fused at the midline in some species. Some adapoids were quite small (Anchomomys at a little over 100 g), and some were quite large (Magnadapis at 10 kg; 22 lbs.). Furthermore, the spaces and attachment features for the chewing muscles were truly enormous in some species, suggesting that these muscles were very large and powerful. Taken together, this suggests an overall adaptive profile of diurnal herbivory. The canine sexual dimorphism in some species suggests a possible mating pattern of polygyny, as males in polygynous primate species often compete with each other for mates and have especially large canine teeth.
Like adapoids, omomyoids appeared suddenly at the start of the Eocene and then became very diverse with most species dying out before the Oligocene. Omomyoids are known from thousands of jaws with teeth, relatively complete skulls for about a half-dozen species, and very little postcranial material. Omomyoids were relatively small primates, with the largest being less than three kilograms (approximately 7 lbs.; Macrotarsius montanus). All known crania possess a postorbital bar, which in some has been described as “incipient closure.” Some—but not all—known crania have an elongated bony ear tube extending lateral to the location of the eardrum, a feature seen in living tarsiers and catarrhines. The anterior teeth tend to be large, with canines that are usually not much larger than the incisors. Often it is difficult to distinguish closely related species using molar morphology, but the premolars tend to be distinct from one species to another. The postcranial skeleton of most omomyoids shows hallmarks of leaping behavior reminiscent of that of tarsiers. In North America, omomyoids became very diverse and abundant. In fact, omomyoids from Wyoming are sufficiently abundant and from such stratigraphically controlled conditions that they have served as strong evidence for the gradual evolution of anatomical traits over time (Rose and Bown 1984).
Teilhardina (Figure 8.11; see Figure 8.2) is one of the earliest and arguably the most plesiomorphic of omomyoids. Teilhardina has several species, most of which are from North America, with one from Europe (T. belgica) and one from Asia (T. asiatica). The species of this genus are anatomically similar and the deposits from which they are derived are roughly contemporaneous. Thus, this small primate likely dispersed across the northern continents very rapidly (Smith et al. 2006).
In the Eocene, there were two early primate groups represented by the fossil record: omomyids and adapids.
The skull of Anaptomorphus (an omomyid). Hubrecht A.A.W. The descent of the primates. Lectures delivered on the occasion of the sesquicentennial celebration of Princeton University. New York: Charles Scribner’s Sons, 1897. U.S. Public Domain.
Life restoration of Tetoniushomunculus (an omomyid) from W.B. Scott's (1858–1947). A History of Land Mammals in the Western Hemisphere. New York: The Macmillan Company. U.S. Public Domain.
Examine the pictures above to determine which of the following traits omomyids and adapids have that indicate they are primates. Please mark “yes” if at least one of the above primates exhibits the trait.
Traits shared with primates
Yes or No?
Partially or fully enclosed eye orbits?
Eyes that are convergent (look forward)?
Small incisors and large canines?
Short snout?
Increased brain size?
Grasping hands?
Station 3: The Oligocene (Approximately 34 - 24 MYA): An Adaptive Radiation of Anthropoids
In the Oligocene, tropical rain forests extended far into areas that are now temperate zones. One of the best Oligocene fossil deposits is in the Egyptian desert—at a place called the Fayum. At one time this was an ideal habitat for anthropoid primates; a sluggish river delta surrounded by lush forest provided niches for several primate species.
In the 1960’s Dr. Elwyn Simons and a Yale expedition discovered a nearly complete skull of Aegyptopithecus. Simons maintained that Aegyptopithecus was the earliest ape, a member of the superfamily Hominoidea. Today most dispute this assertion and argue that Aegyptopithecus has a mosaic of features suggesting it was probably a primitive catarrhine. Compare the provided strepsirrhine, monkey, and ape skulls to Aegyptopithecus.
Which traits might Simons have regarded as hominoid (ape & human) like?
We now know that the ancestor of apes and Old World monkeys had the Y-5 molar cusp pattern. For the hominoid (ape & human) clade, then, is Y-5 a derived trait or primitive trait?
Station 4: The Miocene: Proconsul
Proconsul is well known from Early Middle Miocene sites (22-17 mya) in East Africa. Examine the teeth and jaws of Proconsul
In its teeth and jaws, does Proconsul resemble an ape, a monkey, or human?
As ape-like as the jaws and teeth appear to be, the postcrania (skeleton) is very monkey-like. Detailed studies of the forelimb of Proconsul have shown that it lacked the brachiation ability present in living hominoid elbows and wrists. This is evidence that the common ancestor of living hominoids (gibbons, great apes, and humans) appeared after Proconsul.
Describe the overall shape of skull when viewed from side
Eye orbit shape
Closeness of eyes
There are a lot of similarities between Sivapithecus and orangutans of today! There are some differences, though, including differences in their arm bones. Sivapithecus was probably closely related to, but perhaps not directly ancestral to, orangutans.
Gigantopithecus, which means “giant ape”, has been found in China, India, and Vietnam, dating as far back as 8 MYA, but as recent as 500,000 YA.
Compare Gigantopithecus teeth and jaw to a Gorilla and a human. What similarities do you see? What differences do you see?
What type of diet do you think Gigantopithecus subsisted on? Why do you think that?
Station 7: The Miocene (Approximately 24 - 5 MYA): A Radiation of Apes
The Miocene was warmer and wetter than the present period. Apes were found in relative abundance in Africa and Eurasia in the Miocene. Proconsul was among the earliest (in Africa), and after Proconsul the apes spread and diversified: Dryopithecus in Europe, Sivapithecus and Gigantopithecus in Asia.
When you have finished looking at all of the fossils from the Miocene:
1) Identify hominoid clade, and
2) After completing stations 4-6, describe where you think the fossil primates Proconsul, Sivapithecus, and Gigantopithecus best fit. Note: because they are fossils, they do not need to be listed at the top with the other primates, but can be drawn or described as a part of any lineage or as a side branch. Aegyptopithecus is drawn in as an example
