How 2-million-year-old ancestor moved: Sediba's ribcage and feet were not suitable for running

The fossils discovered four years ago in Malapa near Johannesburg show a mixture of primitive features of australopiths and advanced features of later human species. The researchers led by Prof Lee Berger of Wits University are therefore of the opinion that the new species is currently the best candidate for a direct ancestor of our own genus Homo. Researchers are now presenting new studies, including those of Peter Schmid, who taught and did research at the University of Zurich until he retired. Also involved were UZH students Nakita Frater, Sandra Mathews and Eveline Weissen.

Schmid has described the remains of Au. sediba's thorax. "They show a narrow upper ribcage, as the large apes have such as orangutans, chimpanzees and gorillas," says Peter Schmid. The human thorax on the other hand is uniformly cylindrical. Along with the largely complete remnants of the pectoral girdle, we see the morphological picture of a conical ribcage with a raised shoulder joint, which looks like a permanent shrug. The less well-preserved elements of the lower thorax on the other hand indicate a slim waist, similar to that of a human being.

Conical ribcage makes it difficult to swing arms when walking

The narrow upper thorax of apes enables them to move the shoulder blade, which is important for climbing and brachiation in trees. Its conical shape makes it difficult, however, to swing their arms when walking upright or running, plus they were a similar length to an ape's. This is why Schmid assumes that Au. sediba was not able to walk or run on both feet as well as humans. "They probably couldn't run over longer distances, especially as they were unable to swing their arms, which saves energy," says Schmid.

An examination of the lower extremities shows a heel, metatarsus, knee, hips and back, which are unique and unprecedented. Sediba must have walked with feet turned sharply inwards. This inward turn distinguishes it from other australopiths. The conclusion to be drawn is that our early ancestors were able to move around in a different way.

Arms for climbing and brachiation

Au. sediba was an experienced climber. This is shown by the remains of the upper arm, radius, ulna, scapula, clavicle and fragment of sternum found in Malapa. These clearly belong to a single individual, which is unique in the entire previously known fossil record of the earliest hominins. With the exception of the hand bones described above, the upper extremity is exceptionally original. Au. sediba, like all the other representatives of the Australopithecus genus, had arms that were suitable for climbing as well as possibly for brachiation. Perhaps this capability was even more pronounced than has been assumed for this genus hitherto.

Differences from Australopithecus afarensis

Based on the dental crowns the researchers assume that Au. sediba does not belong phylogenetically to the eastern African australopiths but is closer to Au. africanus and thus forms a southern African sister group. This has an impact on our modern understanding of the evolution of early hominins from the late Pliocene. As such, Au. sediba and maybe even Au. africanus were not descended from Au. afarensis.

The lower jaw of the female skeleton was also examined along with previously unknown incisors and premolars. As noted already on the skull and other areas of the skeleton, the mandibular remains show similarities with other australopiths. They differ, however, in size and shape as well as in ontogenetic growth changes of Au. africanus. These results support the hypothesis that Au. sediba is taxonomically different from Au. africanus. In the relevant differences the parts of the lower jaw appear most to resemble those representatives of early Homo.

An analysis of the cervical, thoracic, lumbar and sacral region of the spinal column shows that Au. sediba had the same number of lumbar vertebrae as modern man. The strong hollow back suggests that he was more advanced in this area than Au. africanus and may be more likely compared with Homo erectus.

The new studies show a unique image of a human species with a mosaic-like physique. Some body parts are similar to those of earlier and others to those of later hominins. "The numerous similarities with Homo erectus suggest that Au. sediba represents the most appropriate early form of the genus Homo," says Peter Schmid. The previous candidates are too fragmentary to be capable of occupying this position.