From the 1930s when Raymond Dart first found evidence of human ancestors in Africa, the vast, vast majority of artefacts have been confined to the eastern branch of the Great Rift Valley, which runs from Ethiopia at its northern extreme, through Kenya and to Olduvai Gorge in Tanzania. There are some hominin remains from the Rift Valley in Malawi while, away from the Rift Valley, a number of sites in South Africa have provided a rich source of hominin fossils (Fig. 1). There is even evidence of hominin presence in the Sahara in Chad.
Figure 1. Outlined in bold are the western and eastern branches of the African Rift. 1: Taung (South Africa); 2: Drimolen, Gladysvale, Gondolin, Kromdraai, Sterkfontein & Swartkrans (South Africa); 3: Malapa (South Africa); 4: Malema, Uraha (Malawi); 5: Olduvai (Tanzania); 6: Chemeron (Kenya); 7: West and East Turkana, Koobi Fora (Kenya); 8: Omo (Ethiopia); 9: Bouri, Hadar (Ethiopia). Crevecoeur et al., 2014.
The latest discovery in the world of human evolution is the first evidence of transitional hominins in the western arm of the Rift Valley, and comes from Ishango on the shores of Lake Edward in the Democratic Republic of Congo.
A transitional hominin is one which lived between about 2.6 and 1.8 million years ago (Mya) and is generally regarded as being one of the species which eventually contributed to the rise of modern humans (Homo sapiens). This narrows the list down to three species: Australopithecus rudolfensis, Australopithecus sediba (although not much is known about this species yet) and Homo habilis (or Australopithecus habilis, as some would have it). There is certainly pressure from some in the world of palaeontology that habilis and rudolfensis are one and the same, and even some who believe they should be grouped within the earlier Australopithecus africanus. As we’ll see, it’s incredibly difficult to make such distinctions with a satisfactory degree of certainty.
In fact, this discovery is not new at all; the single molar on which this new study was carried out was found to be a bit of an oddity during a re-investigation of the artefacts from excavations carried out in the 1950s by Belgian geologist Jean de Heinzelin. Ishango has a strong fossil record of stone tools, some of which date to hundreds of thousands of years ago, whereas all of the hominin remains were thought to date from 19 to 25 thousand years ago, from the NFPr layer (Fig 2). The reason that this single molar had been overlooked is becuase the Ishango deposits have been shifted about over time, meaning many of the fossils were not in their original layers when the trenches were dug.
Figure 2. Stratigraphic section of Ishango 11. Crevecoeur et al., 2014.
The molar was shown to be different from the other teeth it was originally grouped with by Raman spectroscopy, which fires a laser to evaporate some of the surface of the tooth and cause the chemical components to fluoresce, re-emitting the energy they absorbed from the laser as a different colour of light. Different compounds fluoresce slightly differently, and the relative proportions of each compound differ between rock layers, which allowed the molar to be re-assigned from the NFPr layer in which it was found to the Lusso layer (fig. 2), 2.6 to 2.0 Mya.
Now comes the difficulty of determing which species of transitional hominin this molar comes from. Crude shape analysis of the whole tooth suggests it has features more commonly attributed to early members of the genus Homo such as H. habilis or H. rudolfensis / A. rudolfensis. However, its large absolute size would suggest it actually belonged to an Australopithecus species. The tooth is especially large when we consider that it is a first molar which had not fully emerged above the gum line, meaning that the individual it came from can not have been more than true weaning age (6 years old in modern humans, likely around 3 y.o. in a hominin from this long ago). Having said all of this, the relative sizes of different cusps on the tooth are more similar to late members of genus Homo, although this is unlikely to be little more than random chance and individual variation.
Micro CT scans were used to determine the proportion of dentine to enamel since Homo have thinner enamel than australopithecines. By this measure, the Ishango molar was found to be more similar to early members of Homo.
The most sophisticated analyses carried out the the molar involved geometric morphometrics, where 4 different kinds of analysis were carried out either using the shape of the tooth while disregarding size, or taking into account size and shape at the same time. Of these, three suggest it is more similar similar to members of genus Australopithecus, although the authors avoid being any more specific than this, saying it would be necessary to have far more fossils than we actually have discovered from all over Africa in order to confidently assign this single tooth to a particular species.
Importantly, these analyses also show that the molar is similar to those known from the eastern arm of the Rift Valley and those from South Africa. This molar thus provides some early evidence that our ancestors were expanding their ecological range from heavily forested environments to a more open wooded environment.
For more on teeth, see my post The tooth, the whole tooth and nothing but the tooth.
Crevecoeur I, Skinner MM, Bailey SE, Gunz P, Bortoluzzi S, Brooks AS, Burlet C, Cornelissen E, de Clerck N, Maureille B, Semal P, Vanbrbabant Y, Wood B. (2014). First Early Hominin from Central Africa (Ishango, Democratic Republic of Congo). PLoS ONE 9(1): e84652. Available from: http://www.plosone.org/article/info:doi/10.1371/journal.pone.0084652. Freely available.