12 Jan 07

Oxford researchers have used new dating techniques on a human skull to help find out where our most recent common ancestor came from. The skull, which was discovered more than 50 years ago near the town of Hofmeyr in the Eastern Cape Province of South Africa, is thought to be 36,000 years old, according to the study published in the Science journal. The finding by researchers from Oxford University in collaboration with Stony Brook University, New York, supports a growing body of genetic evidence, which suggests that modern humans originated in sub-Saharan Africa and migrated about this time to colonise the Old World.

The international team of scientists relied on a new application of dating methods developed by Dr Richard Bailey and his colleagues from the School of Geography and the Environment, the Research Laboratory for Archaeology and the Department of Earth Science at Oxford University. Traditional radiocarbon dating of the Hofmeyr skull was not possible because so much carbon had been leached from the bone while it lay buried in sediment. Instead, the researchers measured the amount of radiation that had been absorbed by sand grains that filled the inside of the skull's braincase. Measurements of radioactive isotopes in the sediment, combined with a sophisticated radiation transport model using data from a CT scan of the skull, allowed them to calculate the yearly rate at which radiation had been delivered to the sand grains. From this, the researchers were able to determine that the Hofmeyr skull had been buried for 36,000 years.

Dr Richard Bailey, from Oxford's School of Geography and the Environment, said: 'Grains of sand have the ability to record the amount of radiation they have absorbed. It is this remarkable property of crystals that makes this kind of dating possible.

' Many problems we face in understanding the evolution of humans and the evolution of the wider natural environment can be expressed in terms of hypotheses related to the timing of key events. This is why the range of dating techniques available to us is so important in so many areas of science.'

This discovery is key to knowing more about a critical period in human evolutionary history, given the lack of human fossils in sub-Saharan Africa between 70,000 and 15,000 years ago. During the middle of this period, sophisticated stone and bone tools and artwork first appeared in sub-Saharan Africa, and anatomically modern people are seen for the first time in Europe and western Asia in what archaeologists refer to as the 'Upper Paleolithic' period.

Research conducted in the Max Planck Institute in Leipzig, Germany, established similarities between the Hofmeyr skull and contemporaneous Upper Paleolithic skulls from Europe. These findings, combined with the new dating evidence, provides strong support for the genetics-based 'Out of Africa' theory, which predicts that modern humans inhabiting western Asia in the 'Upper Paleolithic' period should also be found in sub-Saharan Africa around 36,000 years ago. The skull from South Africa provides the first fossil evidence in support of this prediction.

Lead author, Professor Frederick Grine of the Departments of Anthropology and Anatomical Sciences at Stony Brook University in New York, said: 'The Hofmeyr skull gives us insights into the morphology of such a sub-Saharan Africa, which means the most recent common ancestor of all of us - wherever we come from.'