How did Lucy become a fossil? Investigating the life, death, and preservation of a famous hominin

Session Date: 
Apr 6, 2024

Lucy’s discovery in 1974 transformed understanding the anatomy of Australopithecus afarensis and other early hominins. Geological research—stratigraphy and geochronology—answered the question, “When did Lucy live?” by placing her just above the Kada Hadar Tuff, dated at 3.21 Ma. As the initial excitement subsided, we began to ask new questions. How did hominins like Lucy live and interact with the African fauna and flora of 3 million years ago? Where did they live and how did they die? Geology, taphonomy, and paleontology tackle such questions with evidence from the sediments where burial occurred, the patterns of skeletal preservation, and associated fossils. Starting with the burial environment and circumstances of Lucy’s “interment,” we can work back to questions about what happened immediately after death and look for clues about how she died. Her fossils were found scattered on an outcrop surface, with none in situ. The Hadar region at this time was a floodplain with a perennial river linked to seasonal tributary and distributary channels, much like the modern Awash River. Based on detailed geological description of the AL-288 site by Tesfaye Yemane (1997), Lucy’s remains eroded from a sand layer interpreted as a crevasse-splay deposit on a floodplain surface. Sediments above the sands are fine-grained and laminated, indicating that a lacustrine interval followed the flooding event(s). The source of the fossils has been re-interpreted as an overbank channel associated with the crevasse splay (Campisano, Pers. Comm. 2024). Less than 40% of Lucy’s skeleton was preserved, with a mix of different elements and better representation of upper versus lower body parts. The co-occurrence of low-density vertebrae and ribs with heavier limb elements indicates that some body parts were articulated when buried, also that burial was rapid without sorting by water flow. What happened to the many missing parts of Lucy’s skeleton? The absent elements suggest removal or partial destruction by scavengers as well as later losses due to Pliocene and/or modern surface processes. Only one tooth mark has been reported, but the absence of obvious carnivore modification doesn’t mean that predators and/or scavengers were not involved. Modern taphonomic studies show that tooth mark frequency depends on the carnivore species and other circumstances. The cause of Lucy’s death has generated considerable debate. Proposals include a fall from a high tree, mudslide, and crocodile attack. Detailed analysis of fractures in the bones show similarities to damage caused by compressive peri-mortem trauma but also could have been caused by sedimentary compaction after burial but before final mineralization. Without secure information on the bones in their burial environment, including orientations and associations of different parts, the evidence for a traumatic fall is questionable. Geological context argues against a mudslide or other flooding event as a cause of death, and the toothmark is inconclusive with regard to predation. Such debates about Lucy’s death and preservation highlight her continuing impact on paleoanthropology and will no doubt synergize new research as part of her legacy for human evolution.