Statistical analysis of fossil data shows that it is unlikely that Australopithecus sediba, a nearly two-million-year-old, apelike fossil from South Africa, is the direct ancestor of Homo, the genus to which modern-day humans belong.
The research by paleontologists from the University of Chicago, published this week in Science Advances, concludes by suggesting that Australopithecus afarensis, of the famous “Lucy” skeleton, is still the most likely ancestor to the genus Homo.
The first A. sediba fossils were unearthed near Johannesburg in 2008. Hundreds of fragments of the species have since been discovered, all dating to roughly two million years ago. The oldest known Homo fossil, the jawbone of an as yet unnamed species found in Ethiopia, is 2.8 million years old, predating A. sediba by 800,000 years.
Despite this timeline, the researchers who discovered A. sediba have claimed that it is an ancestral species to Homo. While it is possible that A. sediba (the hypothesized ancestor) could have postdated earliest Homo (the hypothesized descendant) by 800,000 years, the new analysis indicates that the probability of finding this chronological pattern is highly unlikely. Paper. (open access) – Andrew Du and Zeresenay Alemseged. Temporal evidence shows Australopithecus sediba is unlikely to be the ancestor of Homo. Science Advances, 2019 DOI: 10.1126/sciadv.aav9038 More.
The discussion of probabilities is interesting:
“It is definitely possible for an ancestor’s fossil to postdate a descendant’s by a large amount of time,” said the study’s lead author Andrew Du, Ph.D., who will join the faculty at Colorado State University after concluding his postdoctoral research in the lab of Zeray Alemseged, Ph.D., the Donald M. Pritzker Professor of Organismal and Biology and Anatomy at UChicago.
“We thought we would take it one step further to ask how likely it is to happen, and our models show that the probability is next to zero,” Du said.
Du and Alemseged also reviewed the scientific literature for other hypothesized ancestor-descendant relationships between two hominin species. Of the 28 instances they found, only one first-discovered fossil of a descendant was older than its proposed ancestor, a pair of Homo species separated by 100,000 years, far less than the 800,000 years separating A. sediba and earliest Homo. For context, the average lifespan of any hominin species is about one million years.
“Again, we see that it’s possible for an ancestor’s fossil to postdate its descendant’s,” Du said. “But 800,000 years is quite a long time.” University of Chicago Medical Center, “Statistical study finds it unlikely South African fossil species is ancestral to humans” at Phys.org
So we’re back with Lucy again, for now at least. But the evidence for Lucy is hardly decisive either:
“This is probability statistics at its best,” said Yohannes Haile-Selassie, a physical anthropologist from the Cleveland Museum of Natural History, in an email to Gizmodo. “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,” said Haile-Selassie, who wasn’t involved with the new research.
“I hope Du and Alemseged’s work lays this issue to rest,” he wrote. “We still have to look for the ancestor of the genus Homo even though A. afarensis appears to be the best candidate for now. Let’s keep looking, the fossil record is always full of surprises!” George Dvorsky, “New Analysis Debunks Controversial Claim About the Origin of Humanity” at Gizmodo
The anthropologist is right, the fossil record is full of surprises. But the news that Lucy is only “the best candidate” is worth some reflection. It sounds like we have little to go on and Lucy is at best plausible.
Hat tip: Philip Cunningham
See also: Australopithecus sediba to be dumped from human family? (2017)
Choosing between Sediba and Naledi as human ancestor? (2015)
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