August 28th, 2015
“For only the past 30,000 years has there been a single species of hominin dominating the planet. Now Homo sapiens threatens to wipe out nearly every other species, as well as itself, making them just as extinct as the fossils described in this book.”—Donald R. Prothero
Donald R. Prothero concludes The Story of Life in 25 Fossils: Tales of Intrepid Fossil Hunters and the Wonders of Evolution by examining the discovery “Lucy,” the oldest human skeleton. In the following excerpt, Prothero examines Lucy’s legacy and the efforts of paleontologists and anthropolgists to discover more fossils of human ancestors:
The rush to find hominins from the “Dark Continent” soon spread across East Africa, especially in regions with long sedimentary records in fault basins along the Great Rift Valley. Louis Leakey’s son Richard, who was initially uninterested in anthropology, eventually adopted his father’s mantle. Seeking to escape his father’s shadow, he began to excavate in Lake Rudolf (now Lake Turkana) in northern Kenya in the 1970s. There, many more skulls were found, including the best-preserved specimen of Homo habilis, the oldest species in our genus, Homo. Richard moved on to prominent positions in the Kenyan government (especially fighting the poaching of rhinos and elephants). His wife, Meave, working with local people, carried on the Leakey legacy. His mother, Mary, continued to make significant finds, especially the spectacular trackway of hominins at Laetoli in Tanzania.
Kenya and Tanzania were in the news almost every year with the spectacular finds of the Leakeys. In the late 1960s, Louis Leakey had lunch with President Jomo Kenyatta of Kenya and Emperor Haile Selassie of Ethiopia. The emperor asked Leakey why there had been no discoveries in Ethiopia. Louis quickly persuaded him that fossils would be found if he gave the order to let scientists explore for them. Soon anthropologist F. Clark Howell of Berkeley was working on the northern shore of Lake Turkana, where the Omo River flows out of Ethiopia. Howell and his colleague Glynn Isaac spent many years collecting in the Omo beds, which have abundant volcanic ash dates. Unfortunately, these deposits were formed in flash floods that produced gravelly and sandy streams, which tend to break up and abrade fossils, so no well-preserved hominin specimens were found.
Meanwhile, other rising young anthropologists were eager to make their own discoveries in a region that had been almost exclusively the territory of the Leakeys and their allies. Two of them were Donald Johanson and Tim White. Both were seeking to make their professional fortunes by exploring sites not under the control of the Leakeys. Through French geologists Maurice Taieb and Yves Coppens and anthropologist Jon Kalb, they learned about beds in the Afar Triangle, the rift valley that is opening between the tectonic plates where the Gulf of Aden meets the Red Sea. These beds already had yielded numerous fossils of mammals, suggesting that they were at least 3 million years old, which made them potentially older than any hominin fossil found so far in Kenya or Tanzania. Johanson, White, Taieb, and Coppens received permission to work in these beds and began to excavate at Hadar in 1973.
After months of exploring and prospecting for fossils, and finding a few hominin fragments, on November 24, 1974, Johanson took a break from writing field notes to help his student Tom Gray search an outcrop. He spotted the glint of bone out of the corner of his eye, dug out the fossil, and immediately recognized that it was a hominin bone. They continued to unearth more and more bones, until they found almost 40 percent of a skeleton of a hominin. It was the first skeleton, rather than isolated bones, found of any hominin older than the Neanderthals of the late Pleistocene. That night as they celebrated over the campfire, they were playing a tape of the Beatles when “Lucy in the Sky with Diamonds” came on. Singing lustily along, a member of the crew named Pamela Alderman suggested that the fossil be nicknamed “Lucy.” Later, it was formally named Australopithecus afarensis, in reference to the Afar Triangle, where it was found.
A year after the discovery of “Lucy,” the crew returned to Hadar, where they found a large assemblage of A. afarensis bones. Nicknamed the “First Family,” it was the first large sample of fossils of both juvenile and adult hominins from beds dating to 3 million years ago, and it gave anthropologists a look at how much variability was typical in a single population. This can be important when deciding whether a newly discovered fossil that is slightly different from specimens found earlier should be considered a new species or genus or just a member of a variable population.
When the analysis of “Lucy” was conducted, Johanson and White determined that the skeleton was that of an adult female that had stood about 1.1 meters (3.5 feet) tall. The most important evidence was the knee joint and the hip bones, which show the critical features that prove that A. afarensis walked upright with its legs completely beneath its body, as do modern humans. It had a relatively small brain (380 to 430 cc) and small canines, like those of advanced hominins, yet still had a pronounced snout, rather than a flat face. This was yet another blow to the “big brains first” theory of human evolution, which was still in vogue in the mid-1970s. Its shoulder blade, arms, and hands are quite ape-like, however, so A. afarensis still climbed trees, even if it was fully bipedal. Yet the foot shows no signs of a grasping big toe, so its legs and feet were adapted entirely for walking on the ground and its toes could not grasp branches.