By Paul Braterman
Some time a little less than 2 million years ago, an adult female of the family of apelike creatures to which humans are related by descent was walking through the woodland of southern Africa with her younger companion (child? brother?) when the soft soil gave way beneath their feet. They were plunged down a sinkhole almost 100 feet deep into an underground cavern, and killed (let us hope) instantly. Fragments of their skeletons were first discovered in 2008 and described in the scientific literature in 2010. Burial in the cavern had protected their bodies from predators, and the continuing infall of rubbish into the sinkhole meant that some fragments were unusually well-preserved. It was even possible to examine the teeth in enough detail to study these animals’ (people’s?) diet. Being distinct from but clearly related to Australopithecus Africanus, their species has been given the name Australopithecus sediba. This follows the admirable new fashion of incorporating words from local languages into the names of fossil species; sediba is a seSotho word meaning “fountain”.
Malapa Cave, scene of these events, is one of a group of caves in South Africa and that now form the South African Cradle of Humankind World Heritage Site. The surrounding rock is dolomite, laid down in the late Archaean (a little over 2.5 billion years ago), and fractured much more recently by tectonic forces. Water flowing through these fractures has hollowed out a series of caves, while surface run-off seeping from above has given rise to sinkholes, such as the one involved in this tragedy. Animals including carnivores are often drawn to such sinkholes, attracted by the smell of water or of decaying flesh, with deadly results. Debris falling into the sinkhole forms a cone, burying their remains. Water running through the cave deposits layers of flowstone, calcium carbonate containing traces of uranium, and uranium-lead dating of these layers enable us to bracket the age of the sediments lying in between them. Additional dating information comes from palaeomagnetism (the Earth’s magnetic field changed direction several times during the period of interest, affecting the magnetisation of sediments). Further confirmation comes from other associated fossil remains; the cave also contains bones of horses, which did not arrive in this part of the world until 2.6 million years ago, and of the sabretoothed cat Megantereon, which became extinct roughly one million years later.
Au. sediba is distinct from Au. Africanus in many ways, so much so that the discoverers think it worthwhile to discuss whether it should actually be included in the genus Homo. On the whole, they think not, because the cranial capacity of around 420 cm3 is in the range associated with australopithecines, or even chimpanzees, rather than the more than 600 cm3 for all species classified as Homo. Moreover, Au. sediba has long arms, and ape-like shoulder bones, suggesting that it was partly arboreal. Nonetheless, the shape of the skull, the relatively small size of the back teeth, and the shape of the pelvis and hip region are decidedly more human than those found in Au Africanus.
The biggest surprise about Au. sediba is its diet. Thanks to the beautifully preserved back teeth of the juvenile, we can learn about this in three distinct ways. Firstly, and most obviously, there is the roughness and degree of wear and tear on the tooth surfaces. Secondly, there is the isotopic composition of carbon in the teeth. This requires a little explanation. We are talking about the stable isotopes of carbon, the exact amount of carbon-13 compared with carbon-12; the radioactive carbon-14 that must have originally been present has long since decayed to completely undetectable levels. One of the lies that I used to tell my classes when I was a chemistry teacher, was that different isotopes of the same element have identical chemical properties. This is not true. As a result of what is called zero point vibrational energy, which is itself a consequence of quantum mechanical uncertainty, there is a small but definite tendency for the heavier isotope to concentrate in environments where the carbon atom is more tightly bound, and to react more slowly because reaction almost always involves loosening of bonds. For this reason, plants contain measurably lower concentrations of carbon-13 than the carbon dioxide they use in photosynthesis.
Not all plants are created equal. They can be divided into C3 and C4 types, depending on the pathway by which they fix carbon dioxide. The older C3 pathway involves conversion of carbon dioxide and the five-carbon sugar ribulose into two molecules of the three-carbon molecule glyceric acid (I have missed out some phosphate groups for simplicity). The C4 pathway involves an additional step, and, as the name implies, gives rise to 4-carbon products. C4 material is less depleted in carbon-13 than that formed by the C3 pathway. The C4 pathway is more advantageous than the much older C3 pathway under conditions of drought, or relatively low carbon dioxide abundance, and seems to have arisen independently 40 different times in the past 30 million years or so.
Finally, we can examine the actual dental tartar of these specimens. This contains phytoliths (Greek; phyton, plant; lithos, stone), fragments of non-crystalline silica formed within plants from silica dissolved in groundwater, and different plants give rise to differently shaped phytoliths.
The results were surprising. We know that C4 grasses abounded in the neighbourhood, from the presence of grazers such as the horse, and Megalotragus, a bovid related to the hartebeest, whose skeletons unsurprisingly give the carbon-13 abundances characteristic of animals living on such grasses. However, Au. sediba showed the highest degree of depletion of carbon-13 in any fossil related to humans, indicating something close to a pure C3 diet. The worn surfaces of the teeth (and remember that these teeth were small, compared with Au. Africanus) showed that their owner had been chewing on hard materials. Finally, the phytoliths recovered show that the diet included fruit, bark, and sedges. The importance of fruit and bark is consistent with tree climbing ability, but so far we have no idea why Au. sediba, or at least these two particular individuals, avoided the abundant C4 grasses.
Darwin, famously, lamented the inadequacy of the fossil record in his time, in words that continue to be quoted by Creationists, as if nothing had been discovered in the meantime.
Alfred Russel Wallace, Darwin’s friend and co-discoverer of the principle of Natural Selection, believed human intelligence to be the product of special intervention or purpose. When he wrote, there was a clear gap between humans and apes, the “missing link” of popular imagination.
It was in 1925, Raymond Dart, professor at the University of the Witwatersrand, described the skull of what would become known as the “Taung child”, found in what is now Botswana, the first virtually complete cranium of animal clearly intermediate between apes and humans, which he called Australopithecus Africanus
Since that time, excavations worldwide, but especially in South Africa and in the Horn of Africa, have vastly extended the range of known pre-human species. The problem is no longer one of finding a missing link, but one of tracing an individual branch (the one that led to us) through a densely forking bush. It is always notoriously difficult to distinguish closely related species, because of individual differences. Even when we can, we have no way of being sure which extinct species lie on our direct ancestral line; it is difficult to tell the difference between our great-grandfather and our great-great-uncle, or between one great-great-uncle and another. Nonetheless, the 2007 Yale University Press compendium, The Last Human, lists some 20 different species somewhere on or near the line of descent from non-human ape to human. The fact that neither Australopithecus sediba, nor Denisovans, are among them, is proof of how fast our knowledge is still developing, and, by inference, how much still remains to be discovered.
References: Discovery of Au. sediba and description of cave setting, Science 328, 195 and 205, 2010; diet, Nature doi:10.1038/nature11185, published online 27 June 2012. Discovery of Au. Africanus, Nature 115, 195, 1925.