We know Neanderthals used fire because we have burned animal bones from several archaeological sites along with evidence that Neanderthals collected manganese dioxide, a black mineral powder the helps lower the combustion temperature of wood. But archaeologists haven't been convinced that Neanderthals kindled the fires themselves.

It's been suggested that the first humans to use fire happened to make use of convenient natural fires, such as those sparked by lightning strikes, and only learned how to set things on fire themselves later. Without clear evidence of firestarting, there's no way to demonstrate that Neanderthal technology had progressed to that stage by the time the last of them died off.

"While it is generally assumed that modern humans were proficient fire-markers, some researchers doubt Neanderthals knew how to make fire despite evidence that they used fire regularly," wrote Leiden University archaeologist Andrew Sorensen and his colleagues in a new paper. "Only by identifying the tools used to make fire can we know if Neanderthals possessed this skill."

Paleolithic forensics

A new study suggests that we haven't been giving Neanderthals quite enough credit, arguing that their firestarting tools happened to be ubiquitous, multipurpose flint tools called bifaces. A biface, sometimes also called a hand axe, is a flint tool with two worked faces, sharp edges, and one pointed or sharpened end. They were multipurpose tools, handy for butchering animals, preparing hides, cutting up edible plants, and (it turns out) striking against a mineral like pyrite to make sparks. Think of the biface as a Paleolithic Swiss Army knife.

Archaeological evidence points to the flint-and-pyrite method as a precursor to flint and steel in cultures around the world, and the method dates back to the Mesolithic period at sites in Greenland, Alaska, and throughout Eurasia. Even as recently as 5,000 years ago, Ötzi the Iceman seems to have carried pyrite as part of his kit, and some of his stone tools showed evidence of being used to strike a light when needed. Instead of carrying flint just to strike sparks, he made expedient use of tools he carried for other purposes. It seems Neanderthals did the same thing.

Sorensen and his colleagues studied a sample of bifaces from Neanderthal sites around the Dordogne Valley in France. They're examples of the Mousterian Culture, which is defined by hand axes and small points made with the relatively advanced Levallois technique, a way of chipping flakes off a prepared stone core.

Different types of use leave distinctive marks on flint tools. If archaeologists could find evidence of the kind of wear associated with striking against pyrite to make a spark, that would be fairly solid evidence that Neanderthals did, in fact, start the fire (whether or not they also tried to fight it).

Hands on

To better understand exactly what that kind of mark looked like, the researchers made their own set of replica Neanderthal-style flint bifaces, struck them against pyrite and other hard minerals, and then compared the marks left on the flint to those on the actual middle Paleolithic blades from Neanderthal sites.

Some of those marks were visible with the naked eye. There’s a knack to striking sparks from flint and pyrite, as any sufficiently old-school backpacker can tell you. You can’t just bang the two together and hope for the best; you’ve got to hit the pyrite with the flat surface of the flint at an oblique angle, something flintknappers and archaeologists call directional percussion. That action often leaves C-shaped marks on the flint, with the open end pointing in the direction the impact was moving. Straight-on hammering leaves a more circular mark.

And directional percussion, along with rubbing stone and pyrite together to produce friction (which is a workable but less effective way to get sparks), tends to round and wear down the sharp ridges (called flake scars) left by flintknapping or retouching the tool. In fact, the wear marks from fire-striking often cut across those flake scars at nearly a right angle, probably because the ridges create a rough, abrasive surface that produces more sparks, a little like the rough side of a matchbox.

Other signs showed up only under a microscope, which revealed roughly polished areas and parallel striations or scratches on the surface of the flint, etched by pyrite. For comparison, using a biface to grind up hematite or manganese dioxide to make powdered pigments leaves the flint surface polished to a bright gloss without scratches, with much shallower C-shaped marks and much less wear on the tool's sharp ridges.

Twenty of the bifaces showed distinctive strike-a-light marks that matched the ones produced by Sorensen and his colleagues' experiment. Many of them had been found in the same sediment layers as burned bone, hearth features, manganese dioxide, and stones that showed signs of being heated.

"While we cannot know the motivations behind many activities performed by Neanderthals using stone tools, the gestures required to produce the traces present on the late Middle Paleolithic bifaces appear to fit well within a fire-making framework," wrote Sorensen and his colleagues.

Multitool

Eight more bifaces in the sample showed evidence of retouching that had partially covered old fire-striking wear, and some showed microscopic traces of wear from other tasks, like butchering animals or cutting up edible plants. That emphasizes the point that bifaces were multipurpose tools that Neanderthals kept for a long time, carried with them from place to place, and relied on for most of the basic tasks of survival, from lighting fires to making clothes to preparing food.

"This promotes the idea that using a biface to make fire is not an expedient task but was instead a known use for these tools, just as the purpose of the can opener on a Swiss Army Knife is clear despite this not being the primary function of the tool," Sorensen and his colleagues wrote.

Now, Sorensen and his colleagues want to study the tools of earlier Neanderthals and possibly even slightly earlier hominin relatives elsewhere in Eurasia and Africa. They'll look for similar traces of flint tools being used to strike a light. Those findings, they wrote, "could provide valuable insight into when and where in our deep past fire production became a fixed part of the hominin technological repertoire."

And finding signs of past fire-lighting on tools used for other purposes may offer the best window into the origins of humanity's collective pyromania. The pyrite half of the Paleolithic firestarting kit often doesn't last as long as flint, because pyrite tends to oxidize and corrode away over time. That's probably also why Sorensen and his colleagues didn't see any traces of pyrite residue in the scratches and scars on the bifaces in their sample, although some has turned up on tools at more recent sites. Just in case there are microscopic traces still left on the tools after all these millennia, however, Sorensen and his colleagues say they're looking into using techniques like X-ray fluorescence or Raman spectroscopy to look for microscopic residue.

Meanwhile, this study is the latest of several to support the idea that Neanderthals were cognitively more-or-less a match for modern humans, with the capacity to use symbols, create art, and make and control fire.

"This has significant implications for our understanding of Neanderthal cognitive abilities, including increased planning depth and the use of multi-component tools, and further highlights the intimate relationship these people had with fire," wrote Sorensen and his colleagues.

Nature Scientific Reports, 2018. DOI: 10.1038/s41598-018-28342-9 (About DOIs).