The Nonhominid Primate Fossil Record Chapter Twelve The Nonhominid Primate Fossil Record

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 Earliest Primates The earliest primates were small, arboreal, nocturnal mammals living in tropical and subtropical forests. Their diet consisted of insects and plant material, such as fruit. They showed anatomical specializations for life in the trees, such as grasping hands and stereoscopic vision. Altiatlasius, dated at 60 million B.P., is a candidate for the earliest primate.

The Eocene Primates Two distinct groups of primates appeared at the beginning of the Eocence in both Europe and North America: Adapidae and Omomyidae. Characteristic Adapidae Omomuidae Average body size Snout Mandibular symphysis Tooth row Incisors Size of central incisor relative to lateral incisor Canines Lower anterior premolar Size of orbit Number of premolars Activity Postorbital closure Nails Tibia and fibula Tarsal bones Probable diet Above 500 grams Elongated Usually fused Almost parallel Spatulate and more vertically implanted Smaller Large, with marked sexual dimorphism Sectorial Small Usually four Diurnal Absent Present Unf used Not elongated Insects, fruit, leaves Below 500 grams Short Unfused and mobile V-shaped Pointed and protruding Equal or larger Small, with no sexual dimorphism Not sectorial Large Usually fewer than four Nocturnal Absent or beginning Fused Insects

The Evolution of Modern Prosimians and Tarsiers In some ways the adapids resemble modern lemurs and lorises, although they lacked many features of these animals, such as a dental comb. The omomyids resemble the tarsiers, although many of the tarsier-like features may be superficial. Fossils that appear to be related to modern lorises, galagos, and tarsiers are known from the fossil record beginning with the Early Oligocene. A true tarsier has been recovered from Southern China dating from the Middle Eocene.

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 from sites in Africa and Asia. Species from this time include Amphipithecus, Pondaungia, Eosimias, and Siamopithecus. Since the anthropoids were widely distributed by the Middle Miocene, their origins probably lie in a much earlier time. The Late Eocene witnessed an important adaptive radiation of early anthropoids.

The Anthropoids of the Fayum The Fayum of Egypt 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. Figure 12.13 from Primate Adaptations and Evolution by John Fleagle, copyright 1988, Elsevier Science (USA), reproduced by permission of the publisher. This material may not be reproduced in any form or by any means without the prior written permission of the publisher.

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. This is 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. However, one genus, Gigantopithecus, survived long enough to be contemporary with members of the genus Homo.

Hominoids of the Early Miocene The fossil hominoids of the Early Miocene radiation belong to a number of genera. The best-known are several species of Proconsul. (The Natural History Museum, London)

Hominoids of the Middle Miocene In the Middle Miocene, the Afro-Arabian tectonic plate came into contact with the Eurasian plate, creating a land connection between Africa and Asia. The migration of African hominoids into Asia was possible between 16 and 14 million B.P. The Middle Miocene hominoids of Africa are represented by Kenyapithecus and Otavipithecus. Kenyapithecus exhibits many features of the skull found in contemporary apes, but the postcranial skeleton lacks the specializations of modern apes.

Hominoids of the Late Miocene The Miocene hominoids reached their greatest diversity and geographical range in the Late Miocene. Important Late Miocene hominoids include Sivapithecus, Oreopithecus, and Gigantopithecus. (Reprinted with permission from Szalay, F.S., and A. Berzi, “Cranial Anatomy of Oreopithecus,” Science, 180 (1973), 184. Copyright © 1973 American Association for the Advancement of Science. Courtesy of F.S. Szalay.

The Origins of the Hominidae The hominids are thought to have diverged from the Miocene hominids between 6.5 and 5.5 million years ago. Unfortunately, there are few fossils from the time when this diversion most likely occurred. By the Pliocene, the earliest hominid fossils had appeared.