Some 3.2 million years after she died and 42 years after scientists discovered her fossilized bones, the autopsy results of the famous human ancestor known as Lucy are in. The findings bear on a long-standing question of just how committed Lucy’s species, Australopithecus afarensis, was to life on the ground versus in the trees.

In 1974 paleoanthropologist Donald Johanson, now at Arizona State University in Phoenix, and his then-graduate student Tom Gray found the first of Lucy’s bones eroding out of a hillside at a site called Hadar in Ethiopia’s remote Afar region. Eventually the team would recover much of her skeleton. Many of the bones were broken, as fossils often are. In their analysis of the remains, the researchers concluded that the breakage had occurred after death during the fossilization process.

For the new analysis, paleoanthropologist John Kappelman of the University of Texas at Austin and his colleagues examined Lucy’s skeleton as well as computed tomographic (CT) scans of the bones, which showed their internal structure. Their results confirm that many of the fractures evident in the bones occurred postmortem. But some of them appear to have happened before Lucy died. Comparing the fractures with those seen in modern human accident victims, Kappelman and his collaborators determined that the pattern of damage in Lucy’s bones was most consistent with that of people who experience a severe impact after falling from a considerable height. The researchers detail their findings a paper published online today by Nature.

To get that far off the ground, the researchers postulate, Lucy probably climbed up one of the tall trees that paleohabitat reconstructions indicate would have been found in her woodland home. Roughly the size of a chimpanzee, Lucy would have been able to climb quite high—perhaps to forage for fruit or to build a nest on a branch for sleeping, as chimps do. Chimpanzees can climb to heights of 135 meters in search of fruit and will sleep in nests 21 meters up.

Damage to Lucy’s leg and arm bones suggests to Kappelman and his colleagues that she landed feet-first and then stretched out her arms in an attempt to break her fall—but to no avail. Between the severe fractures and presumed organ damage, she probably died soon after hitting the ground.

The team’s assessment does not sit well with Johanson. He notes that other A. afarensis fossils from Hadar show the same sort of damage seen in Lucy’s bones, as do human fossils from elsewhere. “There’s a myriad of explanations for bone breakage,” Johanson says. “Lucy [might] have been run over by a stampede of larger animals, elephants or large bovids or antelopes, prior to her being washed into a water environment where fossilization began.” The weight of the sediments that eventually covered her bones could have damaged the bones, too, he adds. “Such alternative explanations are not explored in the Nature article. In this regard the suggestion that she fell out of a tree is largely a ‘Just-So Story’ that is neither verifiable or falsifiable, and therefore unprovable,” Johanson contends. [Disclosure: The writer of this article authored a book with Johanson about Lucy.]

If Lucy did tumble from a tree, however, some of the traits that may have helped make A. afarensis so successful as a species might have led to her downfall. Although apelike in some ways—with long arms and curved fingers good for arboreal locomotion—A. afarensis had a number of traits in its pelvis, legs and feet that would have facilitated upright walking on the ground. Bipedal locomotion is a hallmark of human evolution, an innovation that may have helped our ancestors cope with shifting environmental conditions that replaced wooded habitats with open ones. Those same adaptations that made A. afarensis a capable biped may have compromised her ability to navigate the trees, Kappelman’s team concludes.
 
Experts have been arguing about the lifestyle of A. afarensis since the 1980s. Some think it had transitioned fully to life on the ground and that its tree-friendly traits were just evolutionary holdovers from its arboreal ancestors. Others think the persistence of those traits implies that A. afarensis still clung in part to life in the trees. The new study is unlikely to settle the debate. “Australopithecus afarensis was essentially a terrestrial animal as witnessed from a biomechanical analysis of the many postcranial bones known for Lucy’s species, as well as the 3.6-million-year-old human footprints found in a volcanic ash layer at Laetoli, Tanzania,” Johanson asserts. “The a priori assumption that Lucy spent much time in the trees is not supported.”