Researchers have unveiled the second oldest skeleton of a possible human ancestor, a 3.6-million-year-old male of the species Australopithecus afarensis. The roughly 40% complete skeleton has been nicknamed Kadanuumuu, which means "big man" in the Afar language of the Afar Depression of Ethiopia where it was found. "It was huge—a big man, with long legs," says lead author Yohannes Haile-Selassie, a paleoanthropologist at the Cleveland Museum of Natural History in Ohio.
The new skeleton is almost half a million years older than Lucy, the famous A. afarensis specimen discovered in 1974. It had long legs and a torso and a pelvis more like a modern human than an African ape, showing that fully upright walking was in place at this early date, Haile-Selassie says. Although headless, the skeleton also preserves parts not found before in Lucy's species. "It is important because it provides the ribs and scapula," or shoulder blade, says paleoanthropologist Carol Ward of the University of Missouri, Columbia.
In 2005, a sharp-eyed member of Haile-Selassie's team, Alemayehu Asfaw, spotted a fragment of a lower arm bone on the ground at Woranso-Mille, about 48 kilometers north of Lucy's grave at Hadar. Over the next 4 years, the team unearthed the shoulder blade, collarbone, ribs, and neck vertebra, the first time those bones were found together from one adult individual of A. afarensis. The team also found a pelvis, an arm, and leg bones.
When the bones were laid out, Haile-Selassie found that the robust male stood between 1.5 and 1.7 meters tall, about 30% larger than Lucy. The shoulder blade looks more like that of a gorilla and a modern human than that of a chimpanzee. The curvature of the second rib suggests a wide rib cage at the top and a barrel shape overall, similar to that of modern humans and distinct from the more funnel-shaped rib cage of a chimpanzee, the authors report online this week in the Proceedings of the National Academy of Sciences.
Co-author Owen Lovejoy of Kent State University in Ohio argues that these nonchimpanzee-like features support a hypothesis he has championed that the last common ancestor shared between hominins and other chimpanzees didn't look much like chimpanzees, as many had once thought. He says this skeleton also gives a leg up to researchers who had proposed that Lucy's legs were proportionately longer compared with her arms than a chimpanzee's.
Paleoanthropologist Terry Harrison of New York University agrees that the new skeleton "tells you it's not apelike in limb proportions." But paleoanthropologist William Jungers of Stony Brook University in New York state is skeptical of claims that the common ancestor of chimps and humans didn't look like a chimpanzee. He points out that the ribs are damaged and the limb proportions depend primarily on just one complete leg bone; thus, the skeleton can't say much new about leg length. Expect more data—and more debate—from this newest member of Lucy's family.
For more information on this find, see the 25 June issue of Science.
This article has been corrected. It originally stated that this was the first time that the shoulder blade, collarbone, ribs, and neck vertebra were found from one individual A. afarensis; those bones were found together in an infant skeleton, so this is the first time they have been found in an adult A. afarensis.