Monday 3 October 2011

A re-evaluation of the Iwo Eleru skull.

Iwo Eleru is an archaeological site in southeast Nigeria, which comprised a rock shelter containing a number of stone tools and a single human skeleton. It was excavated in 1965 by a team lead by Thorstan Shaw of the University of Ibadan. The findings were published in 1971 in a paper in the journal Man (since 1995 replaced by The Journal of the Royal Anthropological Institute) by Don Brothwell of the Natural History Museum and Shaw. The initial study made morphometric analysis of the skull which suggested that Iwo Eleru man was similar to modern West Africans, but with some archaic features. Morphometric analysis is a method by which a large number of measurements are taken of a fossil, then compared to similar fossils within the same group in order to determine affinities. A radio-carbon date was established for charcoal associated with the skeleton, which gave a age of approximately 13 000 years.

The location of Iwo Eleru and the reconstructed skull.

This month a team lead by Katerina Harvati of the Senckenberg Center for Human Evolution and Paleoecology, Eberhard Karls Universität Tübingen published a paper in the journal PLoS One in which they re-examine the Iwo Eleru skull and provide a new morphometric analysis based upon digital measurements of the skull (the original study used calipers, the best technology available at the time) and a uranium-thorium date for the site.

Despite the extensive studies that have been made of the origins of humanity in Africa, comparatively little is known about human populations there during the Late Pleistocene, and even less for West Africa. For the purpose of the study the Iwo Eleru skull has been compared to a number of neanderthal, Homo heidelbergensis, Homo erectus, modern and other Pleistocene African and Near Eastern skulls. The other African and Near Eastern Pleistocene skulls were the two skulls from Jebel Irhoud in Morocco, currently dated at 160 000 years old, the Ngaloba skull from Laetoli in Tanzania, currently dated at 120 000 years old, two skulls from Qafzeh in Israel, currently dated at between 80 000 and 120 000 years old, the Singa skull from Jebel Irhoud in eastern Sudan, currently dated at between 87 000 and 127 000 and the Skhul skull from Mount Carmel in Israel, currently dated at 100 000 years old.

The morphometric analysis produced by Harvati et al. produced to distinct groups of skulls, one containing the archaic humans (neanderthals, Homo heidelbergensis, Homo erectus) and one containing the modern humans, with some of the Pleistocene African and Near Eastern skulls falling into these groups and some intermediate.

One of the Qafzeh skulls fell consistently within the archaic group, and the Skhul skull was consistently either in or close to this group (more than one analysis was run, since none of these skulls is complete). The other skulls were, depending on the analysis, either in or close to the modern set, or intermediate between the two - with the exception of Iwo Eleru, which was consistently intermediate between the two groups.

This suggests that Iwo Eleru is not a fully modern human, and that therefore more archaic humans can be thought to have persisted in West Africa later than was previously thought. However Harvati et al. do not consider Iwo Eleru to be distinctive enough to classify it as a separate species, preferring to consider it to be an archaic form of modern human (Homo sapiens).

Until comparatively recently the idea of archaic humans surviving as recently as the Late Pleistocene would have been highly controversial. However the recent discoveries of Homo floresiensis in 2003, a distinct species of humans that survived as recently as 13 000 years ago on Flores Island in Indonesia and the Denisovan people in 2010, a species of human living in Siberia around 40 000 years ago, make this rather less surprising. Instead this extends our knowledge of archaic human forms that lived alongside us until quite recently into a new area.

The skull of Homo floresiensis, a 1.1 m hominid from Indonesia.

The Uranium-thorium date established for the bones gave an age of between 11 700 and 16 300 years old, consistent with the original dating.