Everybody knows "Lucy." For nearly four decades, this famous partial skeleton of Australopithecus afarensis, dated to 3.2 million years ago, has been an ambassador for our prehistoric past, and her species has stood as the most likely immediate ancestor of our own genus—Homo.
But in a spate of new studies, paleoanthropologist Lee Berger, of the University of the Witwatersrand, and a team of collaborators have put forward a controversial claim that another hominin—Australopithecus sediba—might be even closer to the origin of our lineage, possibly bumping Lucy from the critical evolutionary junction she has occupied for so long.
Berger and colleagues named Australopithecus sediba in 2010. The 1.98-million-year-old hominin, known from partial skeletons of an adult female and a juvenile male, along with an isolated tibia, was discovered two years earlier at the South African cave site of Malapa.
Since that initial announcement, Berger and coauthors have been further analyzing the anatomy and geological context of the fossils, with their studies culminating in a series of six papers published Thursday in Science.
Together, the papers on the teeth, jaw, limbs, and spine of Australopithecus sediba highlight the fact that this early human possessed a strange mixture of traits seen in both early australopithecines and Homo. These findings make the fossils a significant point of contention among those devoted to understanding where and when our genus evolved.
What Teeth and Bones Say
Not surprisingly, perhaps, Australopithecus sediba's closest relative appears to be Australopithecus africanus, a species that also lived in South Africa from around three million to two million years ago.
In a paper examining 22 discrete traits on sediba's teeth, Joel Irish of Liverpool John Moores University and colleagues found that the species more closely resembles A. africanus than other early hominins. But the teeth also show some features shared with early members of our own genus, such as Homo habilis.
Analysis of jawbone by Darryl de Ruiter of Texas A&M University and colleagues also argues for a distinct species status for sediba,countering earlier claims that the fossils may represent simply a late form of africanus. According to Berger, the dental features makeAustralopithecus sediba "the best candidate" for the ancestor of the Homo lineage, although he notes that this connection is contingent on finding more complete fossils of other hominins.
Other aspects of the skeleton retain a more archaic anatomy. The upper arms of Australopithecus sediba, anthropologist Steven Churchillof Duke University and collaborators report, had the anatomy and proportions of a limb still suited to climbing through the trees.
Australopithecus sediba was probably a climber "of some sort," Berger says, but he notes that "climbing trees is not the only option available to a hominin living on karstic terrain," or landscape pocked by limestone gullies and caves. (Exactly how the hominin got around and what the environment was like 2 million years ago is part of the next phase of research, Berger says.)
Additionally, University of Zurich anthropologist Peter Schmid and co-authors report that the chest of Australopithecus sediba retained the funnel-like, flared shape of other early australopithecines. Compared with the living skeletal extremes of chimpanzees and our species, the upper body of Australopithecus sediba was still much like that of the nonhuman apes.
Curiously, less-well-preserved parts of the lower rib cage have a much more human-like appearance. Scott Williams of New York University and colleagues report that the spine of Australopithecus sediba was also human-like, with a relatively long and flexible lower back that shares more in common with the spines of Homo erectus than with those of other australopithecines, including the curvature of the spine that is a hallmark of upright walking.
But while sediba was clearly a biped, it did not walk at all like we do. According to Jeremy DeSilva of Boston University and his co-authors, the heel bone of the female skeleton of Australopithecus sediba suggests she would have turned her foot inward as she stepped, with the outside edge of the foot contacting the ground along with the heel.
"Contacting the ground on the outside edge of a twisted-in foot causes the foot to rapidly and excessively rotate so that the inside of the foot is driven into the ground," Berger says, which begins a "chain reaction" of rotation of the shin, femur, and torso to keep balance.
No other known hominin walked like this, hinting that the way humans walk isn't the outcome of an ever-improving evolutionary trajectory, but one result out of several possible alternatives that evolved among our ancient relatives.
Sediba's odd mode of walking, Berger says, "might be a compromise locomotion of a hominin that had features of the foot that are adaptive for both upright walking and tree climbing."
An Enduring Controversy
Because of all these varied skeletal clues, Australopithecus sediba is said to possess a "mosaic" of traits that mix the archaic and the derived. But are the ways that Australopithecus sediba resembles early Homo species true indicators of a close evolutionary relationship—or are they traits that evolved independently in both lineages?
Few scientists believe this question has even begun to be settled. Berger himself has more confidence.
"My stance is that [Australopithecus] sediba exhibits so many derived, Homo-like traits across the whole of the body that it must be considered as, at the very least, a possible ancestor of the genus Homo," he says.
This hypothesis faces difficulties, Berger says, because of a "nostalgia" for previous hypotheses and because sediba's remarkably informative skeletons are being compared "with a fragmentary and disassociated record of a small number of bits and pieces, many of which have simply been cobbled together into the basket we call early Homo."
Berger also discounts the record of possible earlier Homo fossils—such as a 2.33-million-year-old jaw found in Ethiopia—as "shockingly bad" and therefore argues that such fragmentary finds do not rule out Australopithecus sediba as a Homo ancestor.
Most other researchers, however, concur that the Ethiopian jaw is indeed Homo and that the trail of our own genus significantly precedes the Malapa finds.
Berger doubts that the new papers will convince those who disagree with him, but affirms that "across the body, head to toe, sediba has a remarkable number of shared derived characters with definitive members of the genus Homo, including H. erectus, Neanderthals, and humans," thus underscoring a possible evolutionary connection.
Paleoanthropologist John Hawks of the University of Wisconsin-Madison points out that the dental details are the best evidence for a possible connection between the Malapa hominins and early Homo. "The new papers really spell out the shared features in the mandibles and teeth in a way that supports their position with A. africanus as a sister taxon to Homo."
A Complex Picture
Still, Hawks cautions, "I think the story could be more complicated." Relatively little is known of early Homo species, Hawks points out, and "knowing what we do about the mixture of later humans—including Neanderthals—it's possible that early Homo and later australopithecine relationships included widespread mixture also."
Regardless of what Australopithecus sediba turns out to be, however, the fossils offer an important caution about interpreting more fragmentary human remains found elsewhere.
"That mosaic of anatomy is the most important insight from this site. It says that when you find a fragment that looks like Homo, you can't expect the rest of the skeleton will look like Homo," Hawks says. "No single fragment can look more like Homo than these skeletons do overall, yet these skeletons have many features that don't look like Homo. And that's what we expect from an evolving lineage."
Smithsonian National Museum of Natural History paleoanthropologist Rick Potts is uncertain of how Australopithecus sediba might be relevant to the origin of Homo, especially since the earliest Homo fossils are hundreds of thousands of years older, but notes that the combination of features in Australopithecus sediba "is astonishing."
That's what makes the placement of the hominins so difficult. "From what we know so far," Potts says, "I think Australopithecus sediba is best seen as a compelling example of the highly experimental nature of evolution in the several hundred thousand years around the time of the origin of Homo."
Ultimately, he says, determining the place of Australopithecus sediba will hinge upon "debates about whether it is the overall morphological pattern that is key to assessing where something like Australopithecus sediba sits in human evolution or [whether] it is the discovery of isolated traits in each area of the skeleton."
The hominin "is so curious in its totality," Potts says, "it might lead to some rethinking of how we classify fossil humans and place them in our evolutionary tree."