Tuesday, October 25, 2011

African cave's ancient ochre lab

Find suggests that Stone Age sophistication extends further back than thought.

Stone Age humans had an elementary knowledge of chemistry. Archaeologists have found evidence that, as long ago as 100,000 years, people used a specific recipe to create a mixture based on the iron-rich ochre pigment.

The findings, published in the journal Science1, "push back by 20,000 or 30,000 years" the evidence for when Homo sapiens evolved complex cognition, says Christopher Henshilwood of the universities of Bergen in Norway and Witwatersrand in Johannesburg, South Africa, who led the work.

"This isn't just a chance mixture, it is early chemistry. It suggests conceptual and probably cognitive abilities which are the equivalent of modern humans," he says.

Lyn Wadley, an archaeologist at the University of the Witwatersrand who was not involved in the research, adds that it implies that people at that time could "think in abstract terms" about the quality and quantity of their ingredients. "Making compounds of any kind implies complex cognition," she says.

The discovery, in 100,000-year-old sediments at the Blombos Cave on South Africa's southern tip, entails two abalone shells lined with a red compound consisting of ochre, bone and charcoal. The raw ingredients, along with hammers, grindstones and a bone stirrer, were found nearby, indicating the existence of an early workshop for producing the mixture.

This is not the earliest evidence that humans used ochre, says Henshilwood, but it is the first evidence for how they combined the pigment after grinding it up. "The components in both of the shells are the same, so they knew what they were doing," he adds.

Painting the cave red

Anatomically modern humans are thought to have lived in Africa from about 200,000 years ago2, but when people acquired advanced mental abilities is a matter of intense debate.

Previous evidence, such as shell beads, ochre engravings and ancient glue from various middle Stone Age sites, indicates that humans had evolved complex cognition by between 80,000 and 70,000 years ago. Henshilwood's finding stretches that further.

It also provides the earliest evidence for the use of containers, pre-dating previous examples3 by 40,000 years, says Henshilwood. The abalone shells' respiratory holes would probably have been plugged to contain the liquid mixture.

Archaeologist Graeme Barker at the University of Cambridge, UK, describes the discovery as a "neat and evocative addition to the gathering information on the behavioural complexity of early Homo sapiens ".

The mixture's purpose is unknown, but the authors speculate that it might have been used as a paint to protect or decorate human skin, artefacts or cave walls. They think that the ochre, which was probably brought to the cave from the nearest source 20–30 kilometres away, was rubbed on quartzite slabs to produce a fine red powder. Next, the authors propose, it was added to the shell containers along with crushed and heated fat-rich mammal bone, charcoal and a liquid, and stirred before use.

Wadley questions whether the recipe was for an art material. Ochre-based glues used to attach stone tools to handles have previously been found4 and the ingredients here seem suitable for a simple adhesive, she says.

But Henshilwood believes paint rather than glue is "much more" likely. No evidence was found of any resin, necessary to make the mixture sticky, he notes.

Corbyn, Zoƫ. 2011. "African cave's ancient ochre lab". Nature. Posted: October 13, 2011. Available online: http://www.nature.com/news/2011/111013/full/news.2011.590.html

Article References:

1. Henshilwood, C. S. et al. Science 334, 219-222 (2011).
2. McDougall, I., Brown, F. H. & Fleagle, J. G. Nature 433, 733-736 (2005).
3. Wadley, L. J. Archaeol. Sci. 37, 2397-2406 (2010).
4. Wadley, L., Hodgskiss, T. & Grant, M. Proc. Natl Acad. Sci. USA 106,
9590-9594 (2009).

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