A few years ago we wrote about Australopithecus sediba, a hominid fossil that was discovered in South Africa in 2008. There was a lot of hype about this hominid when it was first published in 2010. Its discoverer, Lee Berger, called sediba “possibly the best candidate ancestor for our genus.”
The original hope for sediba is that it would help fill a “gap” in the fossil record that pertains to the precise time when evolutionary paleoanthropologists believe our genus Homo evolved from some australopithecine ancestor. ABC News made this point when sediba was first announced, calling the fossil a “game-changer”:
Scientists have long talked about a “missing link” between very old fossils, more than 3 million years old, and much newer ones that they believe are clearly ancestors of today’s human beings. There is a gap in the fossil record, so far unexplained. Does Australopithecus sediba help fill the gap? Not on its own, say most researchers, but it helps.
The media touted sediba as a spectacular confirmation of this prediction. The Washington Post ran the headline, “Scientists identify ancestor that bridges gap in human evolution, a potential ‘game-changer,’” while the Associated Press quoted paleoanthropologist Darryl J. DeRuiter, stating:
This is what evolutionary theory would predict, this mixture of Australopithecene and Homo … It’s strong confirmation of evolutionary theory.
Now, things have changed radically. As the journal Science reports, “A famous ‘ancestor’ may be ousted from the human family,” explaining that Australopithecus sediba is far removed from the human portion of the hominid tree:
Instead of belonging to the human lineage, the new species of Australopithecus sediba is more closely related to other hominins from South Africa that are on a side branch of the human family tree, according to a new analysis of the fossil presented here last week at the annual meeting of the American Association of Physical Anthropologists.
This isn’t exactly anything new. The article makes another point we’ve made here at Evolution News in the past, namely that “With its fossils dated to 1.98 million years ago, Au. sediba is too young to be directly ancestral to all members of the genus Homo.” However, this new study doesn’t argue against sediba as a human ancestor simply because the age of the species is wrong, but also on the basis of the fossil’s morphology:
In a talk here, though, paleoanthropologist Bill Kimbel of Arizona State University in Tempe analyzed the most complete skull of Au. sediba and systematically shot down the features claimed to link it to early Homo. Kimbel noted that the skull was that of a juvenile — a “7th grader” — whose face and skull were still developing. In his analysis, with paleoanthropologist Yoel Rak of Tel Aviv University in Israel, he concluded that the child already showed traits that linked it most closely to the South African australopithecine Au. africanus, a species that lived in South Africa 3 million to 2.3 million years ago. And had it survived to adulthood, its humanlike facial traits would have changed to become even more like those of Au. africanus.
For example, the breadth of the young Au. sediba’s cheekbones appears narrow, as in early Homo. But by studying other australopithecine, ape, and Homo fossils to see how features of the cheekbones change as individuals grow and chewing muscles develop, Kimbel and Rak could predict how the boy’s face and skull would have looked if he’d grown up to be an adult. The resemblance to Au. africanus is so striking, in fact, that Kimbel thinks Au. sediba is a closely related “sister species” of Au. africanus — and not a long-lost human relative. “We don’t believe … that Au. sediba has a unique relationship to the genus Homo,” says Kimbel.
Other researchers who have long been skeptical that Au. sediba was an ancestor of Homo found Kimbel’s talk persuasive: “Spot on,” says paleoanthropologist Bernard Wood of George Washington University in Washington, D.C. Paleoanthropologist Ian Tattersall of the American Museum of Natural History in New York agrees with Kimbel that Au. sediba is most closely related to Au. africanus and that neither species is ancestral to early Homo.
Those are some very big names who think that Au. sediba is merely an extinct side-branch of the hominid tree and not ancestral to Homo. So much for sediba being a human ancestor who is “what evolutionary theory would predict.”
But if not Au. sediba, from what did our genus Homo evolve? When Au. sediba was first reported, the science media admitted that we simply don’t know:
The oldest Homo specimens are scrappy and enigmatic, leaving researchers unsure about the evolutionary steps between the australopithecines and Homo. … “The transition to Homo continues to be almost totally confusing,” says paleoanthropologist Donald Johanson of ASU Tempe, who has seen the new fossils.
(Michael Balter, “Candidate Human Ancestor From South Africa Sparks Praise and Debate,” Science, Vol. 328:154-155 (April 9, 2010).)
With the fall of Australopithecus sediba, that confession of ignorance seems to be left firmly in place.