The Order Primates is a quite diverse and successful group of mammals. However, the 400 or so living primates are just the present expression of an evolutionary radiation that dates back over 55 million years. It is the thin veneer of a radiation with a much greater depth and breadth. To fully appreciate and understand the distribution and diversity of the living primates, it is useful to have a general understanding of their evolution. Over the course of their evolutionary history, primates have been documented on every major continent colonized by placental mammals, with Antarctica being the only exception. Fortunately, the primate fossil record is one of the best known among all major living mammalian groups.

On the basis of molecular clock estimates, it is suggested that primates first evolved somewhere between 70 and 80 million years ago in the Cretaceous epoch, when dinosaurs still dominated Earth’s fauna. If these molecular clock estimates are correct, this very early period in primate evolution is currently undocumented by the fossil record. More paleontological fieldwork in underrepresented areas and time periods such as the Cretaceous of Africa and Asia is necessary to further investigate primate evolution during this time.

The first fossils that share all of the anatomical features characterizing the living members of our Order and are thus clearly related to the modern primate groups appear more or less abruptly and simultaneously in Asia, Europe, and North America at the Paleocene/Eocene boundary, ~55 million years ago. There is also tantalizing evidence of modern primates from a collection of fossil teeth in northern Africa dated to ~65 million years ago, but the affinity of these fossils is still a matter of debate. Africa or Asia seems the most likely place for the origin of primates. Asia is particularly likely because that continent is where we find the mammalian orders most closely related to Primates, the tree shrews (Scandentia) and the flying ilemurs (Dermoptera). Following their origin, these early primates spread throughout the northern continents where the fossil record documents their appearance ~55 million years ago.

Early Eocene primates can be divided into two major groups, adapoids and omomyoids. Although the earliest members of these two groups are very similar in the anatomy of their teeth, their skulls suggest that they can be allied with modern strepsirhines and haplorhines, respectively. The lemur-like adapoids were relatively common approximately 55-35 million years ago in North America, Europe, and Asia. They are also found, although less commonly, in the late Eocene of northern Africa (~40-35 Ma). Adapoids were mostly diurnal, ate both fruits and leaves, and reached body sizes up to 7 kg (15.4 lb) or more. While most adapoids went extinct at the end of the Eocene epoch ~35 million years ago, at least one group of adapoids, the Asian sivaladapids, lasted until the end of the Miocene in Southeast Asia. The second major group of early fossil primates is the tarsier-like omomyoids. Omomyoids are fossil haplorhines that were also relatively common ~55-35 million years ago in the northern continents. They are often found in the same areas where adapoids are found. In contrast to adapoids, omomyoids were mostly nocturnal, subsisted on insects, gums, and fruits, and probably did not exceed 3 kg (6.6 lb) in body mass. These major ecological differences between the adapoids and the omomyoids almost certainly allowed them to occupy different niches within the same forests and avoid any direct competition for resources.

Modern strepsirhines (lemurs, lorises, and galagos) are differentiated from their fossil adapoid relatives by at least one significant, evolutionarily derived feature: the alignment of the first four to six teeth in the lower jaw (incisors and canines) into a procumbent tooth comb. While adapoids are found in North America, Europe, Asia, and Africa during the Eocene, the fossil record of the first tooth-comb strepsirhines is restricted to Africa between 41 and 37 million years ago. These early tooth-comb primates are found in Egypt and appear to be related to modern lorises and galagos. Later fossils related to lorises and galagos are known from the Miocene and Pliocene of East Africa and the late Miocene of Egypt. There are fossil lorises from the Miocene of Asia.

Currently, the fossil record of Malagasy lemurs is limited to the last 15 thousand years. Unfortunately, we have virtually no information about the origin and diversification of that group over the previous 50 million years. However, the appearance of tooth-comb primates in Africa in the Eocene as well as the lack of lemur or any modern mammalian fossils in Madagascar before the end of the Cretaceous strongly suggest that the living Malagasy lemurs are descended from African tooth-comb strepsirhines that “rafted” to Madagascar on a floating island of vegetation sometime during the Eocene epoch.

Despite the lack of any paleontological documentation of Madagascar primates in “deep time,” the abundant fossils from the last few thousand years document a diverse array of species much larger than any of the living lemurs. These include the massive koala-like Megaladapis, a whole radiation of sloth lemurs, and several species of more monkey-like quadrupedal lemurs. These last two groups are related to living indriids.

Like modern strepsirhines, modern haplorhines make their appearance in the Eocene fossil record. The Eocene fossil tarsiers Xanthorhysis and Tarsius eocaenus are found in Asia approximately 45 million years ago. Although they are known only from a few specimens (including a cranial fragment), all available fossil evidence suggests that, just like living tarsiers, these fossil species also possessed enormous, hypertrophied orbits. Thus, the living populations of tarsiers appear to be relict populations of an ancient lineage that has been on Earth for at least 45 million years.

The other group of haplorhine primates, anthropoids (monkeys, apes, and humans), also first appears in the Eocene fossil record. Over the past 25 years in particular, our knowledge of anthropoid origins has expanded greatly due to new fossil discoveries. A number of these putative fossil anthropoids, however, are quite controversial and are the source of much debate. The geographic origin of anthropoids is also a contentious issue, with competing theories alternatively suggesting either Asia or Africa as the most likely source. The tiny middle and late Eocene eosimiid primates from Asia, known from numerous jaws and teeth as well as many parts of the limb skeleton, are generally identified as the earliest anthropoid primates. However, the early Paleocene primate Altiatlasius from North Africa (~65 Ma) possibly represents not only the earliest modern primate but the earliest anthropoid as well.

The earliest undoubted anthropoids, known from skulls as well as jaws and teeth, are from late Eocene and early Oligocene (~37-30 Ma) deposits in the Fayum Depression of Egypt. These early anthropoids had full postorbital closure, although they had not yet evolved other modern anthropoid features such as fused mandibles. More contentious is the position of the large amphipithecids from Myanmar and Thailand in Southeast Asia. Many authorities consider them to be anthropoids while others believe they are adapoids that have independently evolved dental features similar to those of anthropoids.

The living anthropoids are divided into two major groups: the New World monkeys (platyrrhines) and the Old World monkeys, apes, and humans (catarrhines). These groups are believed to have diverged over 35 million years ago, and there are fossils from Egypt about this time that may document this split. The first fossil New World monkeys in South America are found in deposits dated around 25 million years ago. Since South America was an island continent from the beginning of the Age of Mammals until about 5 million years ago, one of the large questions in primate evolution is how platyrrhine monkeys got to South America. North America seems an unlikely source, since there are no anthropoids anywhere in North America or South America until the arrival of humans in the last 15 thousand years, and the mammals of North America and South America were very distinct until the last few million years when the two continents first connected. In contrast, the fossil record of North Africa during this time documents a radiation of fossil monkeys that are very similar in appearance to modern New World monkeys. Although separated by ~2,600 km today, South America and Africa were much closer together during the Eocene epoch 55 to 35 million years ago. It thus appears most likely that New World monkeys “rafted” to South America from Africa during this time. Multiple factors such as the prevailing Eocene paleowinds and paleocurrents across the Atlantic Ocean would have made the journey from Africa to South America somewhere between 8 and 15 days. In addition, the first appearance of monkeys in South America comes just after the major drop in sea level in the early Oligocene ~35 million years ago, a time when the distance between continents would have been further reduced. Moreover, monkeys are not the only African group that appears to have made the trip across the Atlantic: considerable morphological evidence also connects fossil South American rodents with close relatives in Africa as well.

The two groups of catarrhine primates, Old World monkeys and hominoids (apes and humans), probably diverged sometime in the later part of the Oligocene epoch (35-23 Ma). The first fossil Old World monkeys appear around 20 million years ago in East Africa and can be identified through their bilophodont teeth. Likewise, the first fossils commonly recognized as hominoids are also found in the early Miocene of East Africa around 20-25 million years ago, although it is more difficult to find features that definitively link them with extant apes. In the early Miocene of East Africa there is a diverse radiation of fossil apes with numerous species and genera at numerous sites, primarily in Kenya and adjacent parts of Uganda. In the middle Miocene, fossil sites in Africa yielding either monkeys or apes are much rarer, but there are indications that the two living groups of Old World monkeys—colobines and cercopithecines—diverged during this time period.

While both Old World monkeys and apes appear first in Africa, both groups spread to Eurasia sometime thereafter. The fossil record suggests that apes made the journey first. Fossil apes appear in Eurasian deposits around 17 million years ago, soon after the appearance of a land bridge in the present-day Levant that connected the continents, interrupting the ancient Tethys Seaway and forming the Mediterranean Sea. During the early and middle Miocene, the climate was much warmer and much of Eurasia was forested. During the middle Miocene, apes became relatively common members of the Eurasian fauna, and the ancestors of the Asian lesser apes as well as orangutans made it to South and Southeast Asia during this time. The largest ape to have ever existed, the 200-300 kg Gigantopithecus, is found in late Miocene through Pleistocene deposits in Asia. While apes dominated the Eurasian primate fauna for millions of years, the end of the Miocene brought climate change, the disappearance of apes in Europe, and a new wave of primate immigrants from Africa. Fossil monkeys first make their appearance in Eurasia ~8 million years ago, when the earth’s climate was cooling and Eurasian tropical forests were disappearing, and have dominated primate faunas there ever since.

The origin of the lineage leading to the African great apes and humans is a matter of debate, with some researchers arguing that the group evolved in Africa in situ while others believe that the group migrated back into Africa from a Eurasian ancestry. While molecular dates suggest that African great apes evolved somewhere between 16 and 20 million years ago, there are very few African deposits containing apes between this time and the late Miocene 6 to 7 million years ago. Thus it is difficult to test hypotheses regarding the origin of the African great apes, and their initial evolution remains an active and interesting area of research.

Our own lineage is hypothesized to have split from the lineage leading to the chimpanzees between 5 and 7 million years ago. Recent fossil discoveries from this time period shed some light on hominin origins and the first bipedal apes. However, all of these earliest putative hominin fossils continue to be sources of controversy, and it is unclear which of them, if any, were actually bipedal or more closely related to humans than to other, earlier Miocene apes. Nevertheless, they do seem to show the human characteristic of a reduction in sexual dimorphism of the canine teeth. The first undoubted bipedal hominins are found in East Africa just prior to 4 million years ago and are placed in the genus Australopithecus. Australopithecus species are then found throughout East and South Africa, and they are thought to have given rise to our own genus, Homo, as well as to another group of hominins with enormous teeth, Paranthropus. The first fossils attributed to Homo occur around 2.3 million years ago in East Africa, and there seems to have been a diversity of species attributable to our genus in its early years. Soon after the first appearance of Homo, hominins left Africa and spread across much of Eurasia, apparently even making it to small islands in the Pacific such as Flores. Current fossil and molecular evidence suggests that our own species, Homo sapiens, evolved in Africa ~200 thousand years ago and then migrated out of Africa to replace all other hominin species about 50 thousand years ago, but arriving in North and South America  only in the last 15 thousand years.

In closing, primate evolution and the primate fossil record help to explain the current distribution of all the world’s primates. Despite all of our knowledge regarding primate evolution, it is clear that we still have much to learn. In the past 20 years alone, approximately 65 new living primate species and subspecies have been named or discovered, and countless fossil primates also continue to be named and discovered, illustrating the wonderful diversity of these animals, both past and present.

Biographies

John Fleagle is Distinguished Professor of Anatomical Sciences at Stony Brook University. He is the author of Primate Adaptation and Evolution (1988, 1999) and a coeditor of numerous volumes, including The Human Evolution Sourcebook (1993, 2006, with R. L. Ciochon), Primate Communities (1999, with C. Janson and K. Reed), and Primate Biogeography (2006, with S. Lehman). 

Christopher C. Gilbert is Assistant Professor in the Anthropology Department at Hunter College of the City University of New York.  He is the author of numerous scholarly articles found in publications such as  American Journal of Physical Anthropology, Journal of Human Evolution, and Proceedings of the National Academy of Sciences.  He is also a coeditor of a volume spanning the field of primate evolution in honor of Dr. Elwyn Simons, Elwyn Simons: A Search for Origins (2008, with J. Fleagle).