2-Million-Year-Old Creature “Australopithecus Sediba” To Help Explain Evolution Processes

Composite reconstruction of Au. sediba based on recovered material from MH1, MH2, and MH4 and based on the research presented in the accompanying manuscripts. Because all individuals recovered to date are approximately the same size, size correction was not necessary. Femoral length was established by digitally measuring a complete femur of MH1 still encased in rock. For comparison, a small-bodied female modern H. sapiens is shown on the left and a male Pan troglodytes on the right.
“CREDIT: LEE R. BERGER AND THE UNIVERSITY OF THE WITWATERSRAND”

2-Million-Year-Old Creature “Australopithecus Sediba” To Help Explain Evolution. The site of Malapa, South Africa, has yielded perhaps the richest assemblage of early hominin fossils on the continent of Africa. The fossil remains of Au. sediba were discovered in August of 2008, and the species was named in 2010 and given a provisional age of 1.78 to 1.95 Ma. In 2011, detailed studies of four critical areas of anatomy of these remains were published, and a refined date of ?1.977 to 1.98 Ma was proposed. The six articles presented in full in the online edition of Science (www.sciencemag.org/site/extra/sediba), with abstracts in print (pp. 164–165), complete the initial examination of the prepared material attributed to three individuals: the holotype and paratype skeletons, commonly referred to as MH1 and MH2, and the adult isolated tibia referred to as MH4. They, along with the cumulative research published over the past 3 years, provide us with a comprehensive examination of the anatomy of a single species of early hominin.

Irish et al. examine highly heritable nonmetric dental traits in “Australopithecus Sediba” (Au. sediba). The species appears phylogenetically distinct from East African australopiths but close to Au. africanus, forming a southern African australopith clade. The latter shares some derived states with a clade comprising four fossil samples of the genus Homo. This result has implications for our present understanding of hominin phylogeny through the terminal Pliocene and suggests a possibility that Au. sediba, and perhaps Au. africanus, did not descend from the Au. afarensis lineage. De Ruiter et al. examine mandibular material attributable to MH2, including the previously unknown mandibular incisors and premolars of Au. sediba. As seen elsewhere in the cranium and skeleton, these mandibular remains share similarities with those of other australopiths but differ from Au. africanus in both size and shape, as well as in their ontogenetic growth trajectory. These results further support the claim that Au. sediba is taxonomically distinct from Au. africanus. Where the Au. sediba mandibles differ from those of Au. africanus, they appear most similar to those of representatives of early Homo.

Churchill et al. explore the upper limb elements of Au. sediba, describing the most complete and undistorted humerus, radius, ulna, scapula, clavicle, and manubrium yet described from the early hominin record, all associated with one individual. With the exception of the hand skeleton (3), the upper limbs of the Malapa hominins are largely primitive in their morphology. Au. sediba thus shares with other australopiths an upper limb that was well suited for arboreal climbing and possibly suspension, although perhaps more so than has previously been suggested for this genus.

Remains of the rib cage of Au. sediba are described by Schmid et al. and reveal a mediolaterally narrow upper thorax like that of the large-bodied apes and unlike the broad cylindrical chest seen in humans. In conjunction with the largely complete remains of the shoulder girdle, the morphological picture that emerges is one of a conical thorax with a high shoulder joint (producing an ape-like “shrugged” shoulder appearance) and thus a configuration that is perhaps uniquely australopith and would not have been conducive to human-like swinging of the arms during bipedal striding and running. The less well-preserved elements of the lower rib cage suggest a degree of human-like mediolateral narrowing to the lower thorax, indicating a rather unsuspected mosaic anatomy in the chest that is not like that observed in Homo erectus or H. sapiens.

Williams et al. analyze elements of the cervical, thoracic, lumbar, and sacral regions of the vertebral column, showing that Au. sediba had the same number of lumbar vertebrae as modern humans but possessed a functionally longer and more flexible lower back. Morphological indicators of strong lumbar curvature suggest that Au. sediba was derived in this regard relative to Au. africanus and was more similar to the Nariokotome H. erectus skeleton.

Finally, DeSilva et al. describe the lower limb anatomy of Au. sediba and propose a specific biomechanical hypothesis for how this species walked. In isolation, the anatomies of the heel, midfoot, knee, hip, and back are unique and curious, but in combination they are internally consistent for a biped walking with a hyperpronating gait. The implications are that multiple forms of bipedalism were once practiced by our early hominin ancestors.

This examination of a large number of associated, often complete and undistorted elements gives us a glimpse of a hominin species that appears to be mosaic in its anatomy and that presents a suite of functional complexes that are different from both those predicted for other australopiths and those of early Homo. Such clear insight into the anatomy of an early hominin species will clearly have implications for interpreting the evolutionary processes that affected the mode and tempo of hominin evolution and the interpretation of the anatomy of less well-preserved species.

Full article
The Mosaic Nature of Australopithecus sediba
Science 12 April 2013:
Vol. 340 no. 6129 pp. 163-165
DOI: 10.1126/science.340.6129.163

References
– L. R. Berger et al., Australopithecus sediba: a new species of Homo-like australopith from South Africa. Science 328, 195 (2010).

– R. Pickering et al., Australopithecus sediba at 1.977 Ma and implications for the origins of the genus Homo. Science 333, 1421 (2011).