Heat-treated, Stone Age style ( Image: Science/AAAS)

If you want to make really sharp stone spearheads, do like Stone Age cave dwellers did and cook them first.

Palaeoanthropologists have discovered that this two-step trick was invented 50,000 years earlier than they previously thought. The findings add to evidence that Africa’s southern tip was a centre of technological and cultural development during the middle Stone Age.

The key technique, known as pressure flaking, was used to make finishing touches to stone spearheads. Pushing a narrow tool against one side of the spearhead releases a thin flake of material from the other side. This allows for very fine control, producing narrower and sharper tips. It was previously thought that pressure flaking was invented around 20,000 years ago.

Paola Villa of the University of Colorado at Boulder and her colleagues have spent years analysing artefacts found in the Blombos cave near Still Bay in South Africa, including some 75,000-year-old spearheads made of a cement-like substance called silcrete they had found there. The team looked for evidence of pressure flaking on the spearheads, but concluded that they could not have been pressure-flaked as the material would have come away in chunks rather than flakes.


Then last year another team discovered that some of the artefacts had been heat-treated. This makes silcrete much easier to work with.

Make your own

Villa went back to the spearheads and noticed that the scars where layers had come off were smooth and glossy, which is characteristic of heat-treated silcrete. If heat-treating had not been used, the scars would have been rough and dull, which lead them to believe that the cave dwellers must have cooked their spearheads before flaking them.

To check that pressure flaking really was used, the team tried to make spearheads themselves. They collected silcrete from the local area and heat-treated some pieces while leaving others untouched. When they tried to shape the pieces, the heat-treated silcrete could be pressure-flaked, producing spearheads exactly like the ones found in Blombos cave, but untreated silcrete could not.

The finding is yet more evidence that the Blombos cave humans were modern in their behaviour, says Bruce Bradley of the University of Exeter, UK. They also produced bone tools and carved jewellery from mollusc shells. It is thought that other groups of humans may have learned the techniques from them.

Journal reference: Science, DOI: 10.1126/science.1195550