Au. Sediba Shows Evolution Is “Experimental”

12 Apr

A. SedibaLucy (A 288-1 Australopithecus) has a new rival for humanity’s admiration – australopithecus sediba.

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.

Scientists have used the new additions to the fossil record, to complete humanity’s family tree and to provide an example of how evolution experiments in fits and starts, not necessarily toward perfection or even to a final point, like homo sapiens sapiens. Au. sediba inspires the sort of oddity only science fiction writers could admire, but fiction needs to up its game now.

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 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.”

This just shows how fallible life on earth is, but also more creative than the best funded of crackpots.

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