Elwyn L. Simons identified the earliest known primate ancestor of all modern monkeys, apes, and humans. This find was particularly remarkable, because it was of a nearly complete skull, unlike most finds, which usually consist of only teeth and jaws.
The Oligocene deposits of the Egyptian Fayum,
In 1961, nearly fifty years after the first discovery of these deposits, Elwyn L. Simons of the Peabody Museum at Yale University directed the first expedition to investigate the Fayum with the hope of learning more about these possible human ancestors. Excavations continued over the next two decades and were to lead to the best-known fossil primate of the Oligocene.
In the first years of excavation, a scattering of teeth and fragments of lower jaws convinced Simons that a new genus and species of primate had existed during the Oligocene. He named it Aegyptopithecus zeuxsis. Simons’s original assessment was vindicated in January, 1966, when a rare and spectacular discovery was made—a nearly complete skull of Aegyptopithecus. In subsequent years, additional finds became so numerous that a complete reconstruction of the entire animal was possible. Accurate reconstructions of fossil primates are extremely uncommon because most finds usually consist of only teeth and jaws. As a result, Aegyptopithecus has become famous.
Elwyn L. Simons.
Aegyptopithecus lived about 30 million years ago. It differs from the earlier and more primitive prosimians of the Eocene epoch by a variety of features of the head and teeth that are shared by all catarrhines. Its canine teeth are larger than its incisors, it has a mobile jaw, and its skull exhibits postorbital closure, a thin bony septum extending from behind the edge of the eyes to the sides of the skull. The brain of Aegyptopithecus is proportionally larger than the brains of Eocene prosimians, although it is smaller than that of any ape or monkey. The interior of its brain case shows that, while Aegyptopithecus has a smaller frontal lobe than later catarrhines, vision had become increasingly important, and sense of smell had declined. Aegyptopithecus has an extremely projecting snout, which, taken together with its very small brain, gives it the appearance of a lemur, a modern prosimian. Overall, the skull of Aegyptopithecus is very primitive in comparison with all other catarrhines.
The most important aspect of the Aegyptopithecus lower skeleton lies in its forelimbs. Although sharing certain similarities with later hominoidea, the primitive character of Aegyptopithecus forelimbs also resembles the forelimbs of the lemur. Unlike most later catarrhines, Aegyptopithecus was not an agile arm-swinger, but rather a slow, heavily muscled quadrupedal climber. The size of a fox or a large cat, it weighed about seven kilograms, making it a large primate for its time. Aegyptopithecus probably made its ponderous way through the leafy canopy of the Fayum forests, subsisting on a diet of fruit supplemented by leaves. Simons stated that, despite its many primitive features, Aegyptopithecus resembled modern and extinct apes more than it resembled Old World monkeys. He based this decision in particular on the similarity of its teeth with those of living chimpanzees and gorillas, which, like Aegyptopithecus, have large interlocking canines and large molars. Simons declared that Aegyptopithecus was the ancestor of the family of Miocene apes known as the dryopithecines, which throve in Africa between 27 million years ago and 4 million years ago. The dryopithecines, in turn, may be ancestral to gorillas, chimpanzees, and humans. Aegyptopithecus was, therefore, the earliest known hominoid and the likely ancestor of all modern apes and humans. Other scientists have disagreed with Simons’s assessment, claiming that Aegyptopithecus is an ancestor of both apes and monkeys. At issue is whether Aegyptopithecus is similar to later hominoids because it shares certain primitive features, or whether it exhibits features that are unique to later hominoids.
A development in evolutionary theory called cladism
Applying cladistic theory, some authors, such as John G. Fleagle
Cladists have pointed out that, in many respects, Old World monkeys are equally or more derived from the ancestral catarrhine condition than apes. Living cercopithecoidea are highly specialized in dentition, diet, habitat, posture, and lower skeleton. These scientists believe that living hominoids, including humans, may approximate the primitive ancestral catarrhines more than has previously been considered. If they are correct, then it is possible that Old World monkeys are descended from a hominoid ancestor. Simons disagrees with this hypothesis. He believes that Old World monkeys and apes had already split before the time of the Fayum fauna. Furthermore, he believes that the teeth and body of Aegyptopithecus resemble hominoids, not cercopithecoids, and that it is highly unlikely that Aegyptopithecus could have given rise to Old World monkeys.
Much of the difficulty in placing Aegyptopithecus and related forms is caused by the fact that most primate fossils consist of teeth and jaws. In living animals, these elements often do not differ between species as much as do other parts of the body. The absence of such body parts in other fossils makes comparison with Aegyptopithecus difficult. Consequently, it is often impossible to identify derived traits or to identify the last common ancestor shared by two fossil types. This paucity of fossil information means that debates about the evolution of the primates is not likely to cease for a long time.
Adding to the difficulty imposed by the fossil record, according to Fleagle and Richard F. Kay
It was the discomfort experienced by many scholars in classifying this most unapelike creature with unquestioned apes that led to the reassessment by some of hominoid and cercopithecoid evolution. To cladists, Aegyptopithecus is the only known primate having the potential to give rise equally well to apes and to Old World monkeys. It is what might be expected of an extremely primitive catarrhine that could connect the early prosimians of the Eocene epoch to the apes and monkeys of the Miocene epoch. Aegyptopithecus may be neither an ape nor a monkey, but a forerunner of both.
The taxonomic position given to Aegyptopithecus by cladists is based on morphology. This position fits the evidence from molecular biology. Biomolecular specialists such as Vincent Sarich have insisted for many years that the cercopithecoid-hominoid split occurred between 20 million years ago and 26 million years ago in the early Miocene epoch. If this is so, then temporally, geographically, and anatomically, Aegyptopithecus was in the right place at the right time.
Because of the information stemming from the study of Aegyptopithecus, it has been necessary to reconsider the relationship between the cercopithecoidea and the hominoidea. Old World monkeys—clearly less closely related to human beings than apes—were also assumed to be older, more primitive, and less evolved than apes. Scholars such as Fleagle believe that this is a false and anthropomorphic concept resulting from the idea that human beings represent the pinnacle of biological evolution. Aegyptopithecus has forced a reevaluation of this idea and the humbling possibility that, at least in some respects, humans and apes may be more primitive than humans’ distant relatives, the Old World monkeys.
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Cachel, Susan. Primate and Human Evolution. New York: Cambridge University Press, 2006. A general textbook on human and primate evolution. Recommended for all students and general readers. Part of the Cambridge Studies in Biological and Evolutionary Anthropology series. -
Delson, Eric, ed. Ancestors: The Hard Evidence. New York: Alan R. Liss, 1985. Contains good examples of portions of the debate between Simons and his opponents. Section 2 includes articles by Simons, Fleagle and Kay, and David Pilbeam. Pilbeam’s article is an evaluation of the controversy. Simons’s introductory text (cited below) should be read before attempting these articles. Illustrated, bibliographies. -
Fleagle, John G., and Richard F. Kay. “New Interpretations of the Phyletic Position of Oligocene Hominoids.” In New Interpretations of Ape and Human Ancestry, edited by Russell L. Ciochon and Robert S. Corruccini. New York: Plenum Press, 1983. Although quite technical, this article is indispensable for an understanding of new ideas regarding the evolution of apes and monkeys and the role of cladistic theory. Copious illustrations, bibliography. -
Sarich, Vincent. “A Molecular Approach to the Question of Human Origins.” In Primate Evolution and Human Origins, edited by Russell L. Ciochon and John G. Fleagle. New York: Aldine de Gruyter, 1987. A reprint of an early statement of the molecular evidence for primate evolution. Addressing his discussion to readers unfamiliar with biomolecular studies, Sarich discusses problems in identifying ancestral species solely through fossils and explains the molecular approach and its potential utility. A useful introduction to molecular dating. Illustrations, bibliography. -
Simons, Elwyn L. Primate Evolution: An Introduction to Man’s Place in Nature. New York: Macmillan, 1972. Although dated, this textbook is still an excellent introduction to primate evolution, particularly with respect to basic principles. It should be read before attempting more technical material. Illustrations, with a list of taxonomic synonyms, a formal classification of living and extinct primates, and a bibliography. -
_______. “Ways of Seeing.” In Passionate Minds: The Inner World of Scientists, edited by Lewis Wolpert and Alison Richards. New York: Oxford University Press, 1997. Lighter reading by Simons, who discusses his approach to science and how he approaches his work.
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