The Early Primate Fossil Record and the Origins of the Hominins Chapter Twelve The Early Primate Fossil Record and the Origins of the Hominins

Evolution of the Early Primates The earliest mammals evolved during the Mesozoic era. By Paleocene times, at the beginning of the Cenozoic era, all of the dinosaurs and many other forms of life were extinct and mammals began their adaptive radiation. By the start of the Eocene, most of the modern orders of mammals had appeared. At the end of the Mesozoic, great forests evolved with flowering trees that provided protection and food for early primates.

The Plesiadapiformes The earliest primates may be the Plesiadapiformes, who were small, arboreal quadrupeds. The Plesiadapiformes had a long snout, small brain, and claws, and lack both a postorbital bar and a grasping big toe. Because of this, many primatologists believe the plesiadapiformes were not direct ancestors of later primates. Two genera of plesiadapiformes are Purgatorius and Plesiadapis.

The Early True Primates and the Origins of the Lemuriformes and Tarsiiformes Two distinct groups of primates appeared at the beginning of the Eocene in both Europe and North America: the Adapidae and the Omomyidae. The members of the Adapidae resemble in many ways the modern lemurs and lorises and the Omoyidae resemble the living tarsiers. An extremely complete adapid fossil is Darwinius from Germany dated at 47 million years ago. Early primates resembling living lemurs, galagos, and lorises are known from the Eocene and Oligocene of Africa and Asia. Middle and Late Eocene fossils from Egypt and China represent the earliest tarsiers.

Darwinius

Evolution of the Anthropoidea The suborder Anthropoidea includes the living monkeys, apes and humans. The earliest anthropoids in the fossil record date from the Middle Eocene and are known from sites in Africa and Asia. There is debate over whether the anthropoids first evolved in Africa or Asia.

The Anthropoids of the Fayum Perhaps the richest site with early anthropoid fossils is the Fayum of Egypt that dates from the Late Eocene and Early Oligocene. The primates of the upper levels belong to the families Parapithecidae and Propliopithecidae. The Parapithecidae, including Aegyptopithecus, may represent a group of primates ancestral to both the cercopithecoids and the hominoids.

Aegyptopithicus Early Monkey-like Anthropoid

The Evolution of the New World Monkeys The New World monkeys, the ceboids, are probably derived from early African anthropoids that traveled across the then-narrower Atlantic Ocean on natural rafts. The earliest-known ceboid, Branisella, dates from the Late Oligocene of Bolivia. The evolution of the ceboids into their present subfamilies took place by the Middle Miocene. Although New World monkeys and Old World monkeys evolved independently, there are many similarities in their appearance due to parallelism.

The Evolution of the Old World Monkeys The Old World monkeys, the cercopithecoids, were relatively scarce in the Miocene. By the Pliocene and Pleistocene, they became common animals, especially in Africa. The earliest-known fossils of Old World monkeys belong to the genera Prohylobates and Victoriapithecus and make up the family Victoriapithecidae. Beginning in the Late Miocene, the monkeys underwent a divergence into two subfamilies, the Cercopithecinae and the Colobinae.

Evolution of the Hominoidea Fossil hominoids are well-known from the Miocene, during which time they underwent a major adaptive radiation. The earliest hominoids to appear in the fossil record date between 22 and 18 million B.P. from early Miocene fossil beds of east Africa and Saudi Arabia. They most likely evolved from Oligocene primates, such as the propliopithecids. Most of the Miocene hominoids disappeared by around 8 million years ago, during the Late Miocene. One genus, Gigantopithecus, survived long enough to be contemporary with members of the genus Homo.

Proconsul

The Miocene Hominoid Radiation The fossil hominoids of the Early Miocene radiation belong to a number of genera. They tend to fall into two major groups, represented by the genera Proconsul and Afropithecus. By the end of the Early Miocene, most of the African hominoids were extinct. But migration from Africa into Europe and Asia was now made possible because of the connection formed between Africa and Eurasia and at least one lineage survived and moved into Eurasia.

Miocene Hominoids In Eurasia, the hominoids underwent an extraordinary diversification and adaptive radiation. The hominoids of the Middle and Late Miocene can be divided into two general groups: Those with thick enamel on their molars (associated with drier habitats) Those with thin enamel on their molars (associated with wetter habitats). The hominoids of the Miocene include the genera Dryopithecus, Pliopitthecus, Sivapithecus, Oreopithecus, and Gigantopithecus.

Gigantopithicus

The Origins of the Hominins The origins of the hominins lie in the diverse group of hominoids found in the Miocene, although links with known fossil species are problematic. Several new species have been found recently that fill in a gap in the fossil record from about 8 to 4 million years ago. These species include Sahelanthropus tchadensis, Orrorin tugenensis, Ardiptihecus kadabba, and Ardipithecus ramidus.

Orrorin tugenesis, Sahelanthropus