Australopithecus sediba contains several archaic traits present in other Australopithecines such as a small cranial capacity, small body and relatively long upper limbs. However, it also contains several derived traits more characteristic of Homo including smaller teeth, reduced prognathism and cheek bones as well as a more human hip. Given this combination of archaic and derived traits Au. sediba is believed to either be the immediate precursor to our genus or a close relative of that precursor.
However, physical traits are not the only thing which has evolved in the human lineage. Hominin behaviours have also changed, in particular our diet. For example, meat is infrequently consumed by chimpanzees yet makes up a large proportion of the modern diet (of normal people). These dietary changes can have a strong influence on other traits, selecting for new adaptations/behaviours suited to the diet or creating the right conditions for other traits/behaviours to arise (like co-operation developing from food sharing).
Recognising the important of identifying the diet of hominins some researchers have recently examined Australopithecus sediba teeth from an adult and a juvenile in an effort to learn about what the species ate. They utilised three techniques to investigate the creature’s diet: microwear analysis, stable isotope analysis of carbon and examination of phytolyths preserved in calculus (as in hardened plaque, not the field of mathematics).
Microwear analysis, as the name suggests, involves studying tooth wear on the microscopic scale. A lifetime of food consumption will cause damage to teeth and high powered microscopes can spot that damage. This can then be compared to the damage seen on animals whose diet we know, revealing what kind of diet causes what kind of damage. Such comparisons have revealed that “anistropy” is produced by tough foods (like leaves) produce scratches on the tooth whilst hard, brittle foods (like seeds) produce more “complex” damage.
The Australopithecus sediba teeth analysed showed a level of anistropy that is similar to most other hominins, including members of Homo, Australopithecus and Paranthropus. The adults teeth had complexity measurements that were also consistent with these other hominins but the juvenile Au. sediba had complexity that was significantly higher than members of Homo and Australopithecus. Instead, their teeth had complex wear that was more consistent with Paranthropus. The first Paranthropine to be discovered was nicknamed “nutcracker man” as its powerful jaws looked like they were designed for that very purpose. Anatomically, Au. sediba is very different making this dietary resemblance mysterious.
Does this mean that the juvenile ate nuts? Well unfortunately microwear can’t reveal the specific type of food being eaten, only the nature of it. In this case it shows that the juvenile ate tough foods and hard, brittle foods whilst the adult mostly ate the former. Might this difference be variety in the Australopithecus sediba diet or could it be a difference in the diet of age groups? The latter is somewhat suspect since the juvenile appears to be very close to adulthood so would probably not have behaved that differently to the adult. Instead, it would seem Au. sediba had a very varied diet.
This interpretation is also supported by the analysis of the calculus. They examined the teeth of the juvenile and found nearly 40 phytoliths (small plant “skeletons”) in the hardened plaque which was encrusted on its teeth. These phytoliths came from a range of different sources including leaves, fruits, bark, sedges and grasses. This broad spectrum of resources is certainly consistent with a varied diet. Bark hasn’t been identified in a hominin diet before! Unfortunately we can’t work out how much bark they ate since different plants produce different numbers of phytoliths, preventing a comparison.
However, the savannah grasses – common in the sediment from which the adult was recovered – are absent from the calculus. This may be a preservation bias although it is unlikely given how many are preserved in the sediment. Instead it would seem Au. sediba did actually avoid these savannah grasses, although this is not that surprising given the low nutritional value they have. It would seem that every resources Australopithecus sediba was able to exploit, they did.
The stable isotope analysis also testified to the fact they did not consume these savannah grasses. I love stable isotope analysis become it seems to border on witchcraft (“you worked out diet from bones! What black magic is this?”). However, rather than being the result of any wizardry SIA is a product of three facts. First, that elements come in different “flavours” known as isotopes. Secondly, that the ratio of isotopes is different in different types of plants (based on how they photosynthesise energy). Finally, that bone will absorb elements from the diet as it maintains itself. Thus the isotope ratio present in bone will be a reflection of the isotope ratio present in the plants eaten.
In particular we are interested in learning whether Australopithecus sediba consumed any C4 plants since the savannah grasses absent from the calculus are C4 plants. The answer is a resounding “no” with the stable isotope data for both the adult and juvenile being consistent with an animal that avoided C4 plants. This is in stark contrast to many of the grazing animals found with the adult which had ratios consistent with C4 consumptions since they were grazing on the C4 savannah grasses. Au. sediba avoided those grasses.
All 3 lines of evidence converge on the same conclusion. Australopithecus sediba had a varied diet consuming a range of resources from several different plant types. However it seems to have avoided the C4 grasses which were common in its environment, presumably because they offered little nutrition to the species.
This diet is seemingly unique, with no other known hominin eating bark or focusing exclusively on C3 plants. Understanding the evolutionary implications of this has the potential to be fascinating. For example, could the fact that the resources they liked were seperated by large quantities of C4 plants have selected for more efficient bipedalism? This could explain why Au. sediba has a more “human” pelvis. Could its diet indicate it was arboreal? The anatomy certainly seems to imply it spent at least part of its time in the trees.
Only time will tell whether any of this speculation will be confirmed. As it stands we’re left with an interesting revelation about diet which opens the door for even more fascinating research.
|Berger, Lee R., Darryl J. de Ruiter, Steven E. Churchill, Peter Schmid, Kristian J. Carlson, Paul H. G. M. Dirks, and Job M. Kibii. 2010. ‘Australopithecus Sediba: A New Species of Homo-Like Australopith from South Africa’. Science 328(5975): 195–204.|
|Henry AG, Ungar PS, Passey BH, Sponheimer M, Rossouw L, Bamford M, Sandberg P, de Ruiter DJ, & Berger L (2012). The diet of Australopithecus sediba. Nature PMID: 22763449|
|Scott, Robert S., Peter S. Ungar, Torbjorn S. Bergstrom, Christopher A. Brown, Frederick E. Grine, Mark F. Teaford, and Alan Walker. 2005. ‘Dental Microwear Texture Analysis Shows Within-species Diet Variability in Fossil Hominins’. Nature 436 (7051): 693–695.|