The Miocene Epoch (23 million to 5.3 million years ago) is probably the most fruitful for paleoprimatology. During this time, dramatic changes in geomorphology, climate, and vegetation took place. The Miocene was a period of volcanism and mountain building, during which the topography of the modern world was becoming established. Of particular relevance to the story of primate evolution are the vegetational changes resulting from the formation of mountain ranges. Grasses, known only since the Paleogene Period (66 million to 23 million years ago), flourished in the new conditions and in many areas that had previously been forested. Grasslands are known regionally by such names as savanna, Llanos, and prairies. A new type of primate—the ground inhabitant—came into being during this period. The generalized nature of the bodily form of primates, combined with their specialized brain, made this critical step possible.

In the last few decades, considerable additions to the knowledge of ape and human evolution have accrued from Miocene fossil beds in East Africa and Europe. In East Africa, as long ago as the 1930s, the excavations of the inshore islands and Kenyan shores of Lake Victoria by Louis Leakey and a number of colleagues began to illuminate knowledge of human and ape evolution. Renewed excavations at these sites, 17 million to 19 million years old, and exploration of new sites (one of them as much as 24 million years old) in northern Kenya have modified the older conclusions. The genus Proconsul is known from a nearly complete skeleton and several other postcranial bones, a large number of jaw and facial fragments, and several partial skulls—only one of them complete, and it had been distorted by pressure of the surrounding rocks during fossilization. Subsequent reconstruction reveals a skull more monkeylike than apelike in its contours; this, along with the forelimb skeleton, which is known in great detail for this species, indicates a body form that most closely resembles that of living monkeys. Leakey concluded that Proconsul diverged from the modern ape/human lineage before any of the living members of this group began to diverge from each other, and this led him to classify it in a separate family, Proconsulidae. Since the 1980s a number of other genera (Limnopithecus, Dendropithecus, Afropithecus, Kamoypithecus, and others) have been added to the family. The location of the actual ancestors of living hominoids remained mysterious until previously known specimens from the Moroto District, in eastern Uganda, were reexamined and fresh material was discovered. In 1997 the description of a new genus and species, Morotopithecus bishopi, was announced, and this 20-million-year-old fossil is claimed to show the earliest traces of modern hominoid skeletal features. As, at the same time, traces of the earliest Old World monkeys are known, it appears that, while the Proconsulidae flourished with many genera and species, the hominoids (apes) and cercopithecoids (monkeys) were emerging and beginning to specialize. When, in the Middle Miocene, the proconsulids finally disappeared, it was the Old World monkeys that immediately diversified and took their place; the hominoids, until the rise of the human line, tended to remain mostly an inconspicuous group, remaining rather scarce in the fossil record. The separation of the gibbons (Hylobatidae) from the great ape/human stock (Hominidae) is at present not documented by fossils; indeed, whether there are any fossil gibbons known at all before the Pleistocene is still disputed.

In Europe, an archaic family, Pliopithecidae, was widespread. Remains of the best-known genus, Pliopithecus, from the Czech Republic have provided a remarkably complete picture of the habits of this group, which, on this evidence, appears to have possessed bodily forms of a tailed quadruped retaining numerous characteristics of New World monkeys. Long considered to be ancestral gibbons, the pliopithecids are now known to be far removed from gibbons, or indeed any other living primates. Their ancestors diverged from primitive catarrhines before even the Proconsulidae became separate. Alongside them in Spain, France, and Hungary occur remains of Dryopithecus, which are now classified in the Hominidae; they are close to living human/ape ancestry and show further advances over Morotopithecus in the development of the skeletal features characterizing modern hominoids.

In the Siwālik Hills of northern India and Pakistan, remains of several species of the Middle–Late Miocene Sivapithecus have been known since the 1870s. It was long suspected that this genus was related to the living orangutan, and this hypothesis was splendidly corroborated in the 1970s with the discovery of the first facial skeleton, which exquisitely combines primitive hominid features with derived orangutan-like states. If the orangutan lineage was now separate, it would be expected that ancestors of the human/gorilla/chimpanzee line would be found at contemporary sites farther west, and this turns out probably to be the case with the discoveries of two additional genera: the poorly known eight-million-year-old Samburupithecus, from northern Kenya, and the increasingly complete craniodental discoveries of Graecopithecus, from several sites of about the same age in Greece.

One of the most famous of the Late Miocene fossils was the “abominable coalman,” so called because the best-preserved remains, a complete skeleton, were found during the 1950s in a lignite mine in northern Italy. Oreopithecus possessed a number of dental and bony characters that are typically hominid. The canines were relatively short and stout; the face was abbreviated; and the pelvis was broad and even showed characteristics associated with bipedal walking, as did the vertebral column. The arms were long (the intermembral index being well above 100) and the fingers long and curved. The limb proportions are those of a brachiator. An early argument was that it was a special human ancestor; reanalysis suggested that it might be an Old World monkey that had developed brachiating features convergently with gibbons; new studies have placed Oreopithecus firmly in the Hominidae, but, within this family, its exact position is still unclear. The human line is not thought to have separated from that of the chimpanzee by this period, yet the Oreopithecus pelvis undeniably shows biomechanical stress patterns expected of a partial biped. That the end of the Oreopithecus story has not yet been heard is certain. A final twist is that the sites at which it is found seem to have formed an isolated swampy region, probably an island, on which the (somewhat impoverished) fauna had been evolving in isolation for some considerable time, perhaps even a million years or more.

Pliocene

The Pliocene Epoch (5.3 million to 2.6 million years ago) was very similar to the present in terms of its geomorphology and climate. Discounting the effects of recent human influence on the distribution of forest and savanna in the tropics, the face of the land cannot have differed much from today. Thus, one would expect that, during the Pliocene (given the effectiveness of environmental selection), essentially modern forms of primates would have made their appearance. Yet no fossils referable to modern ape lineages are known during the Pliocene, and monkey families are scarcely better known. Libypithecus and Dolichopithecus, both monkeys, were probably ancestral colobines, but neither genus can be placed in a precise ancestral relationship with modern members of this subfamily. What did characterize the Pliocene was the rise in Africa of the human line, with Ardipithecus ramidus at 4.4 million years ago in Ethiopia.