Evidence from a cave in eastern Ethiopia has revealed something extraordinary about the origins of symbolic thought among humans. Forty thousand years ago, Porc-Epic Cave was surrounded by lush grassland full of lakes and rivers. It was home to a thriving community of people who devoted considerable time to processing ochre, a reddish powder used for a variety of things including paint. Writing in PLoS One, anthropologists Daniela Eugenia Rosso, Africa Pitarch Martí, and Francesco d’Errico describe how they worked with the National Museum of Ethiopia to analyze these Middle Stone Age people's ochre-making tools. What they found was that this workshop's artisans produced a far more complicated array of substances than anyone had understood before. Some were used for art and decoration, and others were used for engineering better weapons.

Anthropologists often use ochre processing as a proxy for the origins of human symbolic thought. That's partly because ochre is relatively difficult to make, requiring a few steps and at least two kinds of tools. As the researchers write, ochre comes from "rocks containing a high proportion of iron oxides, often mixed with silicates and other mineral substances, which are red or yellow in color, or are streaked with such shades." Ochre itself is made by pulverizing the rock with one kind of tool and then reducing it to a powder between two grindstones.

There are many aesthetic uses for ochre, including as fabric dye, paint for cave walls, or a stain for rocks and other materials. All these artistic or cosmetic uses imply symbolic thought. But early humans used ochre for utilitarian purposes, too. The powder was mixed with other adhesives to keep weapons snugly attached to their hafts. Put simply, ochre was a key ingredient in glue.

The question that has long raged among archaeologists is whether people first began using ochre as a tool for engineering or as a substance for making art. In other words, does symbolism start with science or aesthetics? By examining 23 ochre-processing tools from Porc-Epic Cave, researchers figured out that the answer is that both emerged at the same time, in the same workshops.

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Engineering and art in the Middle Stone Age

When did it all happen? Porc-Epic Cave has been known as a unique archaeological site since 1933, and countless excavations have taken place there over the years. Early efforts to date the finds there were little more than guesswork, but today scientists rely on sophisticated analysis of snail shells and fragments of a human skull and jaw that were found in the same sedimentary layers as the ochre workshop. Rosso and her team report that the skull provided "a date of circa 50,000 years ago through high-resolution low-background gamma-ray spectrometry," a technique related to luminescence dating. Radiocarbon dating on the snail shells suggests they were left in the cave between 41,000- 37,000 years ago. Based on the ochre's proximity to these shells, the researchers speculate that people using the cave started processing ochre about 45,000 years ago, but the most intense activity was 40,000 years ago. That's the era associated with thousands of ochre lumps and a wide variety of grinding tools.

We have evidence of ochre processing in Africa going back more than 100,000 years, but the Porc-Epic is the first site where scientists have found definitive evidence that the ochre was being made for different purposes. Rosso and her colleagues carefully analyzed wear and tear on the rocks, noticing that different kinds of rock were used to produce many granularities of powder. Some of the softer rocks like sandstone would release pale powder during the grinding process and produce a lighter shade of ochre. There was also one unusual limestone rock that was probably dipped in an ochre-tinged liquid to paint or stamp cloth with red spots.

The researchers also used carbon adhesive to remove ochre powders from the surface of tools and analyze them using "Micro-Raman spectroscopy (μ-RS), X-ray diffraction (XRD), and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS)." Essentially, they looked at the ochre with incredibly high-powered microscopes that revealed the shape and chemical composition of what appears to the naked eye to be fine red or yellow powder. Ochre is also made from a wide variety of minerals, reflecting the diversity of rocks it came from: the team catalogued "several types of oxides, aluminosilicates (clays, micas, and feldspars), silicates, carbonates, sulphates, and phosphates."

But what struck Rosso and her team the most were the incredible variations in both the tools used and the kind of ochre produced. Some of the grinding rocks came from far away, which means the workshop crafters went looking for them or traded for them with neighboring groups. Clearly, the ochre makers were looking for distinctive properties in their rock tools, likely because they produced such different results. As the researchers explain:

The coarseness and hardness of grinding tools conditions the granulometry, shade, and composition of the ochre powder. A grindstone made of a relatively soft rock such as limestone or sandstone releases a powder that is incorporated into the ochre powder produced by the grinding process. The color of the powder is generally lighter when using those tools. The same technique using grindstones made of hard rocks such as schist, quartzite, basalt, granitoid, or fine-grained ferruginous rocks, results in little or virtually no release and incorporation of particles derived from the tool. When using these rocks, the granulometry of the powder mostly depends on the roughness of the tool surface, as well as the pressure exerted during the work.

More to the point, different powders were good for different uses. "Fine, clayish sorted ochre powder is more suitable for cosmetic or symbolic activities such as body painting, whereas mixed grain size ochre would be more adapted to utilitarian activities such as hafting," write Rosso and colleagues. This was a workshop devoted to producing ochres that could be used in adhesives and in paints or dyes. Humanity's earliest experiments with symbolism can be traced back to the twin births of art and engineering. Tens of thousands of years ago, our tool-obsessed brains allowed us to convert iron-rich rocks into beautiful adornments—and better hammers, too.

PLoS One, 2016. DOI: 10.1371/journal.pone.0164793

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