The Hunt for the Dawn Monkey: Unearthing the Origins of Monkeys, Apes, and Humans

5. Received Wisdom

5 Received Wisdom By the late 1980s, paleontologists had been successfully recovering fossil primates for more than 150 years. From the pioneering efforts of Georges Cuvier and Jacob Wortman to the more recent expeditions of Elwyn Simons, a burgeoning inventory of Greek and Latin names charted the latest revelations from the fossil record. Despite the confusing proliferation of new species and genera, the signal concerning our own deep evolutionary history seemed easy enough to decode. The earliest fossil primates from the Eocene all belonged to one of two major groups. The first of these consisted of vaguely lemurlike adapiforms such as Adapis and Notharctus. The second group—omomyids and microchoerids— included smaller, more tarsierlike primates such as Tetonius and Necrolemur . Yet nowhere among this vast assemblage of Eocene primates was there clear and persuasive evidence for the lineage leading to modern monkeys, apes, and humans. Instead, the oldest fossils documenting the emergence of anthropoids hailed from early Oligocene sites in the Fayum region of Egypt. The simple fact that anthropoids followed prosimians in the grand succession of the fossil record reinforced the notion that life evolves in ladderlike fashion from primitive to advanced—or from tree shrew to lemur to tarsier to monkey to ape to human in the widely adopted scheme of Sir Wilfrid E. Le Gros Clark. 115 Around this same time, paleoanthropologists settled on a broad consensus about the major features of anthropoid evolution, even if occasional fights still broke out regarding specific details. In a nutshell, the party line went as follows. The anthropoid lineage originated somewhere in Africa near the Eocene-Oligocene boundary, about thirty-four million years ago. These early African anthropoids evolved from anatomically advanced prosimians that differed only marginally from their anthropoid offspring. Therefore, as the fossil record improved, it should become increasingly difficult to separate the earliest anthropoids from their immediate prosimian forebears. Nevertheless, at some point along this prosimian-anthropoid continuum, our distant ancestors crossed a critical threshold—a basic shift in anatomy and ecology that would force everyone to agree that the evolutionary product was an anthropoid rather than a prosimian. One of the last remaining hurdles was to identify the prosimian ancestors of these earliest anthropoids. Here, all bets were off, though, because different scientists championed one or the other of the major Eocene prosimian groups as anthropoid ancestors. Some argued that anthropoids descended from an advanced adapiform, while others thought that anthropoids evolved from some unknown omomyid. Even these persistent—and often bitter—debates over which prosimian group evolved into early anthropoids represented genuine progress, however . By the late 1980s, no serious scholar questioned the common ancestry , or monophyly, of all living anthropoids. According to this view, South American primates ranging in size and behavior from pygmy marmosets to spider monkeys sprang from the same ancestral stock that— on the other side of the planet—evolved into macaques, baboons, orangutans , and humans. This grand unification of New World monkeys with their brethren from across the Atlantic was an old idea whose time had finally arrived. It supplanted an earlier consensus in paleontology that viewed the two major groups of living anthropoids—South American monkeys, on the one hand, and Old World monkeys, apes, and humans, on the other—as distant relatives, having evolved along parallel lines from separate prosimian ancestors. Let’s briefly consider some of the factors behind this dramatic shift of scientific opinion. To some extent, the newfound support for anthropoid monophyly was contingent upon more earth-shaking developments in a separate field of science. A German scientist named Alfred Wegener began promoting the concept of continental drift as early as 1912. At the core of Wegener’s model was a truly radical idea—that the continents themselves were capable of moving about the surface of the globe like a fleet of rubber ducks 116 RECEIVED WISDOM in a bathtub. Although Wegener highlighted a variety of evidence— including the geographic distribution of certain fossils—as support for continental drift, his views were largely dismissed by the scientific community of the time. Among other problems, Wegener could never explain the natural forces that might cause continents to drift. Decades later, geologists showed that seafloor spreading was the driving force in plate tectonics, confirming Wegener’s theory of continental drift and his place in the history of earth sciences (although he himself did not live to see...