Pieces of the Human Evolutionary Puzzle: Who Was Australopithecus sediba?

Few things remain as mysterious—or controversial—as our own history as a species. However, a series of papers released in Science may add another piece to the puzzle: Four papers draw back the curtain on Australopithecus sediba, announced earlier this year, detailing morphological features of the hand, foot, pelvis, and skull that may establish this species within the ancestral lineage of modern humans.

                    Fig.Courtesy of Steven Churchill, Duke University.

In a subterranean cave at Malapa, South Africa, approximately 25 miles (40km) from Johannesburg, the remains of numerous hominins identified as Australopithecus sediba have lain between layers of flowstone—a type of rock that forms in caves, similar in composition to stalagmites and stalactites, except as the name implies, this rock forms in a layer that “flows” across the surface. Flowstone is rich in uranium, which decays into lead. By measuring the amount of uranium and lead present in the stone, scientists were able to date the flowstone to between 1.977 and 1.98 million years ago. This date has been applied to the fossils themselves (which are too old to be directly dated), making them older than the oldest known fossils of the genus Homo (1.90 Ma).

  Fig.Courtesy of Peter Schmid

But what makes the fossils curious is their unique blend of traits. The foot and ankle, for example, suggest that Au. sediba was certainly bipedal, but it had not completely given up its arboreal tendencies. Au. sediba has a rudimentary arch and evidence indicates that it may have possessed an Achilles tendon which would have aided in bipedalism but its ape-like heel would have been poor at absorbing weight from walking or running. The ankle demonstrates less flexibility than that of apes, but more than is possible for the modern human foot. These elements in combination lead researchers to suggest that Au. sediba may have been unique in both its climbing and walking styles.

The pelvis may challenge The Obstetric Hypothesis, according to Steven Churchill and colleagues, which maintains that the larger brains of early Homo facilitated changes to the pelvis making them short, broad, and more bowl-like rather than broad, flat and flaring like those observed in the fossils of the australopithecines. The pelvis of Au. sediba is unusual because the species was small-brained, so the changes to the pelvis—at least in this case—were not to accommodate larger brained offspring, and may require us to rethink our understanding about the development of the modern pelvis. Given the evidence for bipedalism in this species, the relationship between the pelvis and terrestrial habitation may warrant closer investigation.

But it is the hands that appear to have captured the imagination of many. Our hands allow us to connect with and shape our world—they provide the means by which we initially experience and understand our environments. Paleoanthropologist Tracy Kivell reports that one of the most complete fossil hominin hands following the appearance of stone tools has been recovered from Malapa, and adds volumes to our understanding of this hallmark feature of humanity. Au. sediba had shortened fingers, and most importantly, and elongated thumb that would have allowed for greater flexibility in manipulating objects. Apes, on the other hand, have long fingers for grasping branches and locomotion. With few exceptions, they do not display the dexterity that humans are capable of. Au. sediba’s hand would have been powerful and flexible enough to allow the hominin to climb trees, but also to have finessed contact with objects.

Kivell, Churchill, and co-authors Job Kibii, Peter Schmid, and Lee Berger​, believe that Au. sediba may change our understanding of the development and use of tools in our evolutionary history. Though they note that Au. sediba does not appear to have all of the morphological features commonly associated with tool production and use, the elongated fingers and prominence of the thumb suggest that the physical requirements for stone tool production may be more complicated than initially established. In the absence of actual stone tools to confirm Au. sediba’s manipulative skills, the morphological traits exhibited by the fossilized hands strongly suggest the potential exists.

Dr. Lee Berger (University of the Witwatersrand, Johannesburg), who discovered the fossil site in 2008, believes this combination of traits makes Au. sediba a strong contender as a direct ancestor to Homo erectus. However, all of the papers are careful to note that these traits may be homoplasies—developed simultaneously in unrelated lineages. Au. sediba may not be the missing link, but it raises interesting questions about the organization of our evolutionary tree, and gives us another possible ancestor to probe.