New research published in Science Advances challenges the hypothesis that Australopithecus sediba, a fossil species discovered in South Africa, is the direct ancestor of the Homo genus, which includes modern humans. The study, conducted by paleontologists from the University of Chicago, suggests that Australopithecus afarensis, known for the famous “Lucy” skeleton, remains the most likely candidate as the ancestor to Homo.
Australopithecus sediba, dating back nearly two million years, was first discovered near Johannesburg in 2008. Since then, numerous fragments of this species have been unearthed, all dating to approximately two million years ago. However, the oldest known Homo fossil, a jawbone from an unnamed species found in Ethiopia, predates A. sediba by 800,000 years, raising questions about the direct lineage between the two.
While the researchers who discovered A. sediba proposed it as an ancestral species to Homo, the new statistical analysis challenges this notion. Lead author Andrew Du, Ph.D., explains that while it is theoretically possible for an ancestor’s fossil to postdate its descendant’s, the probability of such a pattern occurring is extremely low, bordering on zero.
The study reviewed other hypothesized ancestor-descendant relationships among hominin species and found that in most cases, the first-discovered fossil of a descendant was younger than its proposed ancestor, with an average lifespan of about one million years for hominin species. This further supports the argument against A. sediba being the direct ancestor of Homo, given the substantial 800,000-year gap.
Instead, the researchers argue that Australopithecus afarensis is a more plausible candidate for direct ancestry to Homo. A. afarensis fossils have been dated up to three million years old, overlapping with the age of the earliest Homo jawbone. Additionally, A. afarensis fossils were discovered in Ethiopia, close to where the Homo jawbone was found, and exhibit morphological similarities that support a direct descendant relationship.
Dr. Zeray Alemseged, the Donald M. Pritzker Professor of Organismal and Biology and Anatomy at UChicago, emphasizes that the combination of timing, geography, and morphology makes A. afarensis a stronger candidate than A. sediba. The statistical and chronological analyses presented in the study provide robust evidence supporting this argument, adding weight to the case for A. afarensis as the likely direct ancestor to Homo.
While debates about morphology and fossil features may continue, the study underscores the importance of rigorous statistical analyses in paleontological research and the value of considering multiple lines of evidence in reconstructing evolutionary lineages.