In 1974, in Ethiopia’s Afar Triangle, Donald Johanson discovered the bones of a human ancestor that had lived at least 3.5 million years earlier. The bones were from a small female who walked erect but who had a brain size about one-third that of the average modern human. Forty percent of the skeleton was found, making it the most complete fossil of a human ancestor ever discovered older than seventy-five thousand years. The skeleton was named Lucy, after the Beatles song “Lucy in the Sky with Diamonds.” In 1981, with the help of noted science writer Maitland Edey, Johanson wrote Lucy: The Beginnings of Humankind. The book describes the finding of Lucy’s skeleton and discusses how the discovery forced scientists to rethink their theories about human evolution. For example, it suggested that human ancestors walked erect millions of years before their brain size approached the modern norm.
Particularly controversial, however, was Johanson’s decision to create a new species for Lucy, which he named Australopithecus afarensis. Two Australopithecus species were already known, africanus and robustus, and it was commonly thought that africanus was the direct ancestor of the human line. (Species actually in the human line are designated “Homo,” as in Homo erectus; modern humans are Homo sapiens.) Johanson believed that Australopithecus afarensis was the...
In Ethiopia, just north of Addis Ababa in the Afar triangle, a woman named Lucy died, probably either of accidental drowning or of a severe illness. She was in her mid- or late twenties. She had walked erect, just as modern humans do. Her brain, however, was not much bigger than that of a chimpanzee, and she was not a user of tools. Her apelike face was not much more than three feet from the ground. She died in 3,500,000 B.C. It is possible to read about Lucy—she was named after the Beatles’ song—because in 1974 her fossilized skeletal remains were discovered by Donald C. Johanson and an associate.
How was Lucy found? By accident, certainly, for Johanson and his associate practically stumbled over her one November day, but if finds in paleoanthropology—the study of humankind’s ancestors—are largely accidental, they are nevertheless accidents that occur within a highly planned environment. As in archaeology, digs are carefully mapped and gridded and all finds are diligently marked within these maps and grids. Moreover, the modern paleoanthropological dig is a multidisciplinary enterprise which relies on the services of geologists and of specialists in the evolution of creatures other than man. These expensive, sometimes multinational and unwieldy teams of scientists must be financed and administered. Finally, a suitable site must be selected; Afar was suitable because the dryness of the climate encouraged the preservation of fossils and the geology, although complex, made accurate dating at least possible.
It may seem preposterous that an erect-walking human ancestor existed nearly four million years ago, but the date is airtight, confirmed by five separate methods, each supportive of the others. In geological dating, the age of fossil-bearing rocks is deduced from purely geological principles. Potassium-argon and fission-track dating are measurements of the radioactive decay in these rocks. Paleomagnetism relies on the fact that over millions of years there are occasional measurable reversals in the earth’s magnetic poles and that these are reflected in its rocks. Finally, biostratography correlates hominid forms with other fossils, such as those of the pig, the dating of which is more nearly certain. On the basis of these five methods scientists agree that Lucy is 3.5 million years old.
She was also bipedal—that is, she walked erect, unlike apes and monkeys. Monkeys do not walk at all and chimpanzees and gorillas are knuckle walkers. According to one theory, man’s ancestors became bipedal because they needed their hands for tool use. A very plausible idea, but one that cannot be sustained in the light of the evidence Lucy provides. Lucy was bipedal, but she was not human; no tools were found with her or with similar hominids—they seem to be a human invention. In any case, her small brain made serious tool use out of the question.
It is easier, as it turns out, to undermine the tool-use theory than to construct a new theory that the evidence of Lucy will support. In the book, however, some measure of support is given to a very speculative idea which relates bipedalism to the survival of the species. According to this speculation, all species tend to survive either by giving birth to an enormous number of young to whom they give...