A 3 million-year-old cold case: What was the cause of Lucy's death?

Lucy is an iconic early hominin specimen found in 1974 in the Afar Desert of Ethiopia. Technically known as A.L. 288-1, Lucy is certainly one of the most famous fossils of all time. She is notable for her completeness (about 40% of her skeleton is preserved) and for providing our first glimpse into the paleobiology of Australopithecus afarensis, an important branch in the human evolutionary tree.

Reconstruction of Lucy by John Gurche on display at the Cleveland Museum of Natural History.

While Lucy is incredibly well preserved, every element exhibits signs of breakage, which until now have been attributed to compression during the fossilization process (Johanson et al., 1982). In other words, it was thought that her bones were intact when she died but were slowly crushed in the ensuing 3 million years as sediments were piled on top, eventually forming sedimentary rocks. But last month, a study published in Nature by John Kappelman of the University of Texas at Austin and colleagues challenged that interpretation, concluding that the fractures observed in Lucy were in fact perimortem (i.e., created at the time of her death) rather than postmortem. Specifically, Kappelman et al. (2016) posit that they were caused by a fatal fall from a tree.
The study by Kappelman et al. used computed tomography (CT scanning) to study the fractures of Lucy’s bones in detail. They argue that the pattern of breakage, particularly in the ankle and shoulder joints, indicates a fall from a high place – most likely a tree – and that Lucy would have died soon after such an impact. Their conclusion hinges on interpreting these fractures as ones that only could have occurred in living bone.

Lucy’s death scenario as depicted by Kappelman et al. (2016:fig. 2).

The scenario proposed by Kappelman is novel and received wide coverage in the media, but I wasn’t convinced by the data they present. I had a difficult time imagining how the force of impact could have caused such widespread damage in her skeleton. Based on the visual depiction included in the article, most of the force from the fall would have been absorbed by the ankles and knees upon impact. This is not what is seen in Lucy’s skeleton; the fractures are throughout the skeleton. Kappelman et al. correlate the great number of fractures with the severity of the fall, but a fall so severe strains credibility.
Furthermore, similar patterns of breakage are present in many fossil specimens from the same and other rock formations in Ethiopia, and most of these specimens belong to terrestrial animals that do not have the ability to climb trees – animals like horses and rhinos. I’ve collected many such specimens from a variety of localities, and I agree with the authors of the original description of Lucy (Johanson et al., 1982), who interpreted the breakages seen throughout her skeleton as postmortem and not dissimilar from those of other fossils from the same site.
Another shortfall I found in the paper was a detail of the death scenario itself. If one assumes that Kappelman et al.’s hypothesis is correct, why must Lucy have fallen from a tree? Couldn’t she have fallen from a cliff? This would be an unimportant distinction if Kappelman et al. didn’t go on to use their scenario to make over-reaching interpretations about the paleobiology of Lucy’s species, A. afarensis.
Although there is scientific consensus that A. afarensis spent most of its time walking on two legs (e.g., it was a habitual biped), there continues to be debate about how well it was able to climb trees. Kappelman and colleagues argue that because Lucy fell from a tree, A. afarensis was partially arboreal. However, it is difficult to extrapolate the behavior of a species based on an inference from a single individual. Moreover, one could easily reach the opposite conclusion about A. afarensis after reading this paper: perhaps Lucy fell from a tree because she and her kin were not very good at climbing!
- Dr. Denise Su is Curator of Paleobotany and Paleoecology at the Cleveland Museum of Natural History
References Cited:
Johanson, D. C., C. O. Lovejoy, W. H. Kimbel, T. D. White, S. C. Ward, M. E. Bush, B. M. Latimer, and Y. Coppens. 1982. Morphology of the Pliocene partial hominid skeleton (A.L. 288-1) from the Hadar formation, Ethiopia. American Journal of Physical Anthropology 57:403-451.
Kappelman, J., R. A. Ketcham, S. Pearce, L. Todd, W. Akins, M. W. Colbert, M. Feseha, J. A. Maisano, and A. Witzel. 2016. Perimortem fractures in Lucy suggest mortality from fall out of tall tree. Nature 537:503-507.
Posted: 10/18/2016 6:50:35 AM by croftdarinadmin | with 0 comments
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