The hominoids--the group including humans and living and fossil apes--originated sometime during the Oligocene period, between 34 and 24 million years ago. But it was during the Early Miocene that ancient hominoids began the impressive differentiation that created many diverse ape lineages that came to inhabit most of the tropical and subtropical Old World. During most of the Miocene, climatic fluctuations appear to have been much less marked than during the subsequent Pliocene and Pleistocene. Tropical and subtropical forests covered large parts of the Old World. The East African Rift Valley and the mountains of South Asia, including the great Himalayas, had only begun to form, so that the climatic patterns of today, dominated by strong dry and monsoonal wet seasons, had not yet emerged. Early apes adapted to exploit the large forested environment, becoming established in Africa, across South and East Asia, and in large parts of Europe and West Asia.
Although fossils representing many Miocene ape lineages have been found, paleontologists do not have a clear idea of their relationship to the living species of hominoids. The common ancestors of orangutans, chimpanzees, gorillas, and humans all existed during the later parts of the Miocene, between 15 million and 6 million years ago. Thus, the earliest hominoid fossils, which date from earlier than 25 million years ago, lived long before the divergences of the living great apes. Some of these ancient apes may be our ancestors, but most of the great hominoid diversity of the Early Miocene ultimately became extinct.
Our own lineage, the hominids, arose during the latest part of the Miocene in Africa. In some senses, fossil apes are a sidebar to the story of our own evolution. Paleontologists do not know which, if any, of the fossil apes of the Miocene may have given rise to the hominids, but the impressive diversity of fossil apes has yielded insight into the evolutionary pathways taken by our ancestors. Most important, the Miocene apes show us the primitive conditions of many of the skeletal features that were later to undergo great changes during human evolution.
Fossils from the Oligocene that may be hominoids are rare. The most notable, Aegyptopithecus, may represent the ancestral catarrhines that later gave rise to both hominoids and Old World monkeys. But by the early Miocene, a great diversity of apes had arisen with a range of body sizes and dietary adaptations. Fossil genera such as Proconsul, for example had monkey-like locomotor adaptations and ape-like jaws and teeth. Others, like Morotopithecus, may show the development of the suspensory locomotor pattern of later great apes, while retaining dental anatomy somewhat distinct from the later apes.
Great ape fossils and relatives
By 13 million years ago, a series of fossil apes from Europe and Asia show clear signs that they belong to the group including living great apes and humans. Some of these apes had a full adaptation to suspensory locomotion of the form found in living chimpanzees and orangutans. Others--especially the larger apes--may have had a mixture of suspensory and quadrupedal adaptations. All these apes appear to have matured relatively slowly, like the living apes, and may have had brains essentially the same relative size as chimpanzees (Begun, 2003). Thus it is likely that these fossil apes represent the origin of the great ape adaptive pattern.
Shortly after the first of these apes arose, the early great apes divided into two lineages. One of these invaded South Asia, and ultimately its descendants colonized the tropics of China, Southeast Asia,and Indonesia. The other lineage spread across Europe and diversified quickly into several species of a single genus, Dryopithecus, and its larger descendant, Ouranopithecus. These European apes share many features with the living chimpanzees and gorillas, which may indicate that they are the closest fossil relatives of the living African apes and humans.
Asia has produced a great quantity of great ape fossils from the Miocene up through the Pleistocene. Ultimately, these apes were a distinct clade from those found in Europe and Africa, and they had no close relationship to the hominids. The most recent members of this clade are the living and fossil orangutans, of the genus Pongo. Today, orangutans survive as remnant populations only on the islands of Sumatra and Borneo, though fossil Pongo is found as far north as China. From China in the east to Turkey in the west, the fossil apes of Asia were a large and diverse group.
The common ancestors of humans, chimpanzees, and gorillas belong to a clade separate from the Asian fossil apes. The earliest representative of this clade is probably Dryopithecus, which itself is likely to have been very similar to the common ancestor of both clades. Dryopithecus and later European genera, called dryopithecines, belong to an adaptive radiation that resulted in a substantial diversity in body size and possibly in locomotor patterns. This radiation almost entirely known from European sites, since African fossil apes have not yet been found from the important time span from 10 million years ago to 7 million years ago, during which the African apes and hominids diverged. Nevertheless, the known dryopithecines present evidence of the anatomy of close relatives to later hominids, much closer in time to the beginnings of human evolution than are living species. It is likely that when African great ape fossils from this time period are discovered they will share many features with the European ape radiation. Thus, the anatomy of these ancient apes may partially represent the starting point from which human evolution began.