Overview:

The fossil skeletons of Au. sediba from Malapa cave are so complete that scientists can see what entire skeletons looked like near the time when Homo evolved. Details of the teeth, the length of the arms and legs, and the narrow upper chest resemble earlier Australopithecus, while other tooth traits and the broad lower chest resemble humans. These links indicate that Au. sediba may reveal information about the origins and ancestor of the genus Homo. Functional changes in the pelvis of Au. sediba point to the evolution of upright walking, while other parts of the skeleton retain features found in other australopithecines. Measurements of the strength of the humerus and femur show that Au. sediba had a more human-like pattern of locomotion than a fossil attributed to Homo habilis. These features suggest that Au. sediba walked upright on a regular basis and that changes in the pelvis occurred before other changes in the body that are found in later specimens of Homo. The Australopithecus sediba skull has several derived features, such as relatively small premolars and molars, and facial features that are more similar to those in Homo. However, despite these changes in the pelvis and skull, other parts of Au. sediba skeleton shows a body similar to that of other australopithecines with long upper limbs and a small cranial capacity. The fossils also show that changes in the pelvis and the dentition occurred before changes in limb proportions or cranial capacity.  

The combination of primitive and derived traits in Australopithecus sediba shows part of the transition from a form adapted to partial arboreality to one primarily adapted to bipedal walking. but the legs and feet point to a previously unknown way of walking upright. With each step, Australopithecus sediba turned its foot inward with its weight focused on the outer edge of the foot. This odd way of striding may mean that upright walking evolved on more than one path during human evolution.

History of Discovery:

The first specimen of Australopithecus sediba, the right clavicle of MH1, was discovered on the 15^th of August in 2008 by Matthew Berger, son of paleoanthropologist Lee Berger from the University of Witwatersrand, at the site of Malapa, South Africa. It was announced in Science in April 2010. 

How They Survived:

Due to the mixture of derived features in the pelvis and primitive features in other areas of the skeleton, it is unclear to some researchers the extent to which Au. sediba used arboreal habitats or remained on the ground using terrestrial bipedal locomotion. Relatively long arms and a small body may have allowed Au. sediba to utilize arboreal habitats. Derived features in the pelvis and the pattern of diaphyseal strength in the humerus and femur suggest that Au. sediba might have regularly walked upright in a way that was more similar to modern humans than to earlier members of Australopithecus.

The possible increasing emphasis on upright walking is accompanied by differences in the skull and teeth compared with other australopithecines.  The relatively small dentition of Au. sediba may signal a dietary change. As more features of the environment and functional morphology of Australopithecus sediba are discovered, their way of life will become clearer.

Evolutionary Tree Information:

Australopithecus sediba’s mixture of primitive traits found in other australopithecines and derived traits also found in Homo makes the evolutionary position of Au. sediba an interesting question. Similar to other australopithecine species, Au. sediba is small in size, with long arms and small cranial capacity. Its features are more derived than those of Au. anamensis and Au. afarensis. Australopithecus sediba bears a strong resemblance to Au. africanus, a fossil species that is also found in South Africa. They have similar skull, facial and dental features. The species differ in features such as the shape of the cranium and the face, showing that Au. sediba was more derived compared with Au. africanus. The combination of similarities and differences led Berger and his colleagues to conclude that Au. sediba was descended from Au. africanus.

The traits Australopithecus sediba shares with Homo may indicate a closer relationship between this species and Homo than between other australopithecines and Homo. Berger and his colleagues proposed that Au. sediba is ancestral to the genus Homo or is closely related to the ancestral species. However, there are earlier and contemporaneous fossils attributed to Homo, making it difficult to think of Au. sediba as an ancestor to Homo.  The time range for the species Au. sediba is currently unknown. It is not known where in that time span the current sample falls and how it fits with the time ranges of other species. Another possibility is that Au. sediba is closely related to another, still unknown species that was ancestral to the genus Homo. While that species evolved into Homo, Au. sediba may have persisted leading to the overlap in time between Homo and Au. sediba.

Other researchers question the idea that Au. sediba and Homo are closely related at all, citing the possibility that the juvenile MH1 may not reflect the adult post-cranial characteristics of Au. sediba or that the postcranial features of Au. sediba may not be unique to the taxon, but may be found in other australopithecines. Another possibility raised by researchers is that the Malapa finds belong in the genus Homo. The number of different ideas about the placement of the Malapa finds stems from the debate on how early members of the genus Homo should be recognized and which fossils belong in it. There is a question of whether cranial and dental features or the advent of modern postcranial body proportions are most important in defining Homo, since some fossils, such as the Australopithecus sediba remains, contain a combination of features. 

Questions:

We don’t know everything about early humans—but we keep learning more! Paleoanthropologists are constantly in the field, excavating new areas with groundbreaking technology, and continually filling in some of the gaps about our understanding of human evolution.

Since Au. sediba was discovered recently, there are many unanswered questions about it. Below are some of the still unanswered questions about Australopithecus sediba that may be answered with future discoveries:

1. What is the time range and geographic range of Australopithecus sediba? This question can only be answered by the finds of more specimens.
2. Will the close relationship between Au. sediba and Homo be confirmed by future finds? 

References:

The first paper:

Berger, L.R., de Ruiter, D.J., Churchill, S.E., Schmid, P., Carlson, K.J., Dirks, P.H.G.M., Kibii, J.M., 2010. Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa. Science 328, 195-204.

Other recommended readings:

Balter, M., 2010. Candidate human ancestor from South Africa sparks praise and debate. Science 328, 154-155.

Dirks, P.G.H.M, Kibii, J.M., Kuhn, B.F., Steininger, C., Churchill, S.E., Kramers, J.D., Pickering, R., Farber, D.L., Mériaux, A.-S., Herries, A.I.R, King, G.C.P., Berger, L.R., 2010. Geological setting and age of Australopithecus sediba from Southern Africa. Science 328, 205-208.

Wong, K., 2010. Spectacular South African skeletons reveal new species from murky period of human evolution. Scientific American 8 April 2010 (Available a thttp://www.scientificamerican.com/article.cfm?id=south-african-hominin-fossil, 9 April 2010).

Wong, K., 2010. Fossils of our family. Scientific American June 2010.