Over the last decade, some paleontologists have proposed that a tree-climbing, bipedal species whose fossils were found in South Africa was an ancestor of humans. But a new analysis, published yesterday (May 8) in Science Advances, finds that scenario to be “highly unlikely,” given that the only fossils of Australopithecus sediba are 800,000 years younger than the oldest Homo specimen.
The authors say the result supports the idea that the now-extinct hominin A. afarensis is probably the true ancestor of humans.
“I had no doubt in my mind—nor did many in our field—that A. sediba could not have been the ancestor of Homo, not only because the earliest known representative of Homo is 800,000 years older, but also because A. sediba does not have all of the morphological features that one would expect to see from the earliest Homo,” Yohannes Haile-Selassie, a physical anthropologist from the Cleveland Museum of Natural History who was not involved in the new study, writes in an email to Gizmodo. In particular, a jawbone of the oldest Homo fossil more closely resembles that of an A. afarensis jaw.
The proposal that humans descended from A. sediba came from an analysis of 2-million-year-old fossils uncovered in South Africa in 2008. The species’ blend of humanlike and australopith features—such as a small brain but a modern pelvic shape—led the study’s authors to conclude that A. sediba likely gave rise to Homo. Theirs has not been a universally accepted idea, partly because a fossil from Homo described in 2015 dates back to about 2.8 million years.
In the new analysis, Andrew Du and Zeresenay Alemseged of the University of Chicago calculated the probability of finding a fossil from an ancestor that is 800,000 years younger than a fossil from a descendent. Based on the probable duration of each group’s presence on Earth and the amount of time they might have overlapped, the team determined that it would happen 0.09 percent of the time. “[O]ur models show that the probability is next to zero,” Du says in press release.
A. afarensis, on the other hand, has been dated back to 3 million years, and in close proximity to the oldest Homo fossil. “Given the timing, geography and morphology, these three pieces of evidence make us think afarensisis a better candidate than sediba,” Alemseged says in the statement.