A New Ancestor?

In the past few weeks there has been a great amount of publicity about a new discovery from caves in Malapa, South Africa - a hominin named Australapithecus sediba represented by two fossils excitingly named MH1 and MH2.  In fact its been so well publicized that a fair amount of work colleagues have mentioned it to me, knowing my area of interest.   My automatic reaction has been to bristle and growl a bit, especially when the phrase 'missing link' is bandied about in the press so much.  The fossil description was first published back in April 2010 when I was doing my Masters.  We discussed it at some length at uni and finally dismissed it as a variant of A. africanus, rather than as a new species ancestral to Homo.  Now, after seeing the more detailed papers, I'm not so sure...

Artist's impression of Australopithecus sediba (source: Science)

Dating
One of the original reasons for rejecting the Australopithecine fossils as a Homo ancestor was an age considerably younger than the oldest known Homo fossils.The two fossils discovered were originally dated to between 1.78 and 1.95 million years ago, based on fauna present and uranium and geomagnetic dating of the strata (Dirks et al 2010).  QED, they could not be ancestral to a genus that was older than them.  However the more recent paper gives a case for a much tighter date of 1.977 +/- 0.002 million years ago.  As the authors point out, this new dating conveniently places A. sediba much earlier than the earliest uncontested evidence of Homo in Africa (Pickering et al 2011).  I know nothing about the finer points of dating, so I can't offer much comment on the reliability of the methods used.  However this is a very, very opportune turn of events for those arguing for A. sediba to be recognized as a transitional species between Australopithecus and Homo.


Anatomy
There are several papers, each focusing on a specific area of A. sediba's anatomy including the hand, foot, pelvis and cranium.  I'm going to start with the foot, because I like feet.


The Foot and Ankle
Well preserved foot and ankle bones are fairly rare in the hominin record so the remains associated with A. sediba are very exciting.  The question palaeoanthropologists want to answer are: what form of bipedalism did A. sediba possess and how does the structure of the foot compare to Homo and other Australopithecine species?  Calcaneus, talus and tibia were all preserved in association with the adult fossil MH2 (Zipfel et al 2011).  A right talus and calcaneus show an interesting mix of mosaic features, including a cuboid facet angled like that of a modern human foot and suggesting an arched foot (apes have flat feet).    Yet, the calcaneus itself is very ape-like as are the thickened malleoli on both tibia, suggesting some form of arboreal locomotion was still important.

Remains of the foot and ankle. A=tibia,talus and calcaneus cemented together, B=distal tibia, C= fourth metatarsal (right), D=fifth metatarsal (right) (source: Science)

Overall the mosaic foot and ankle morphology suggests a bipedal gait unique to the species.


Pelvis
The pelvis is wider than that that of other Australopiths and similar to the shape of Homo erectus (Kibii et al 2011). However, A. sediba only has a cranial capacity of 420cc - that's over a hundred cubic centimeters smaller than the smallest cranial capacities measured in Homo habilis.  Until now it has always been assumed that widening pelvises were related to an increase in brain size.  In A. sediba this is obviously not the case and raises questions about how important the relationship between brain size and pelvis shape actually is.


Hand

A. sediba hand - very human-like (source: Science)

The diagram above gives an excellent idea of the features of the hand. As well as being amazingly complete for a fossil 2 million years old it also shows some clear human-like features such as shorter fingers and a longer thumb than apes, allowing for greater manipulation of objects and perhaps tool-making (Kivell et al 2011). A. sediba may not have been spending as much time in the trees as its other Australopith relatives.

Cranium

As mentioned before, the cranial capacity (420cc) is typical of an Australopithicine. However, Carlson et al (2011) produced a virtual endocast for the MH1 skull and observed some very interesting features of the hominin's brain. This includes a more human-like frontal lobe, intermediate between A. africanus and early Homo. The authors suggest that this is brain reorganization that occurred just before brain size expanded. The video below, featuring a very excited Lee Berger, shows how the skull was scanned to create this view.

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I am certainly prepared to accept A. sediba as a species in its own right after reviewing this excellent and comprehensive selection of papers. There are some clear unique traits, such as the very modern shape of the talus and the overall structure of the hand, that do not fit into the range of A. africanus. Yet, I still withhold my judgement on its status as an ancestor to species such as Homo erectus. More fossils are needed and more comparative studies should be carried out before such conclusions are made. Nevertheless, there is no doubt that these are extremely important specimens dated to just before the emergence of Homo.

To Close...
A final fascinating titbit is that there may be soft tissue preservation from Malapa! Astonishing and brilliant. A whole project has been devised to investigate this - the site is here, if you wish to take a look. It certainly looks like an exciting time for Palaeoanthropology. I only hope that I can get PhD funding for next year and get back in the game!

Carlson K, Stout C, Jashashvili T, de Ruiter DJ, Tafforeau P, Carlson K and Berger LR (2011) 'The Endocast of MH1, Australopithecus sediba' Science 333: 1402-1407.

Dirks PHGM, Kibii JM, Kuhn BF, Steininger C, Churchill SE, Kramers JD, Pickering R, Farber DL, Mériaux A-S, Herries AIR, King GCP and Berger LR (2010) 'Geological Setting and Age of Australopithecus sediba from Southern Africa', Science 328:205.

Kibii J, Churchill SE, Schmid P, Carlson KJ, Reed ND, de Ruiter DJ, and Berger LR (2011), 'A Partial Pelvis of Australopithecus sediba' Science 333: 1407-1411.

Kivell TL, Kibii J, Churchill SE, Schmid P, and Berger LR (2011) 'Australopithecus sediba Hand Demonstrates Mosaic Evolution of Locomotor and Manipulative Abilities' Science 333: 1411-1417.

Pickering R, Dirks PHGM, Jinnah Z, de Ruiter DJ, Churchill SE,Herries AIR, Woodhead JD, Hellstrom JC, and Berger LR (2011), 'Australopithecus sediba at 1.977 Ma and Implications for the Origins of the Genus Homo', Science 333: 1421-23.

Zipfel B, DeSilva JM, Kidd RS, Carlson KJ, Churchill SE, and Berger LR (2011) 'The Foot and Ankle of Australopithecus sediba', Science 333: 1417-1420.