A huge stash of reddish minerals from a cave in Ethiopia shows how Stone Age people gradually adapted their technologies and practices over a 4500-year period.
“It’s one of the rare sites where we can see a very precise evolution of this cultural feature through thousands of years,” says Daniela Rosso at the University of Valencia in Spain.
Rosso and her colleagues studied materials from Porc-Epic cave in Ethiopia. The cave first became known to scientists in the 1930s, and was thoroughly excavated in the 1970s. It was used by people throughout the Stone Age, but the bulk of archaeological material dates from a 4500-year-long period about 40,000 years ago.
This material included 4213 pieces of “ochre” – an umbrella term for minerals that are rich in iron and consequently have vivid colours: typically red. Prehistoric people often collected these minerals, but the original excavators of Porc-Epic did not study them. “This is the first time there is a systematic study of ochre use at this site,” says Rosso.
Rosso and her colleagues examined what the various pieces of ochre were made of. This changed over time: ochre from the beginning of the 4500-year period was typically high-quality and rich in iron, while ochre from the end of the period was lower-quality and had less iron. The later ochre was also coarse-grained, so instead of grinding it to powder the people tended to chip and cut it.
There are several possible explanations for the shift. One is that the people at Porc-Epic may have been using the ochre for different purposes as time went on, and chose different types accordingly.
The most famous use of ochre is as a pigment for artworks, but Rosso says it was probably sometimes used in utilitarian ways – for making adhesives, or as sunscreen, for example.
However, running counter to the idea that the shift was deliberate is evidence in a 2022 study by Rimtautas Dapschauskas at the University of Tübingen in Germany and his colleagues. They reviewed all known uses of ochre in Africa from 500,000 to 40,000 years ago. Dapschauskas says prehistoric people consistently sought out “fine-grained and blood-red materials”, which were the best for pigment as they could be ground to a very fine powder and produced vivid colours. “People really, really preferred those reddish colours,” he says.
So it may be that, as time passed, the people at Porc-Epic simply found it increasingly difficult to source the best-quality ochre. The team examined local geological deposits and found that the available ochres did not match those in the cave: they were often coarser-grained and had less iron. “Probably they had to go further away” to find the best ochre, Rosso says.
Why it became harder to get the high-quality ochre is unclear, says Dapschauskas, but it may be that the social situation changed: for instance, if the people at Porc-Epic relied on trade to secure good-quality ochre, then conflict with neighbouring groups might have led to shortages.
The study adds nuance to our understanding of technological stasis in the Stone Age, says Dapschauskas. “There’s a form of stability,” he says. “The cultural knowledge is transferred from generation to generation to generation.” But at the same time, the people were flexible and changed their practices over time. “They can really trace several thousands of years of behavioural change.”