/Neanderthal glue was a bigger deal than we thought

Neanderthal glue was a bigger deal than we thought


Color photo of a roll of birch bark, a puddle of tar, and a spear with birch tar securing the point.

This replica shows how Neanderthals might have used birch tar to haft a projectile point.

Paul R. B. Kozowyk

Fifty-thousand years ago, a Neanderthal living in Northwestern Europe put sticky birch tar on the back side of a sharp flint flake to make the tool easier to grip. Eventually, that tool washed down the Rhine or Meuse Rivers and out into the North Sea. In the 21st century, dredging ships scooped it up along with tons of sand, other stone tools, and fossilized bones, then dumped the whole pile on Zandmotor Beach in the Netherlands.

Despite all of that, the birch tar still clung to the flake, and it provides evidence that Neanderthals used a complex set of technology to make elaborate tools.

Living on the edge

Making birch tar at all is a fairly complex process. It takes multiple steps, lots of planning, and detailed knowledge of the materials and the process. So the fact that archaeologists have found a handful of tools hafted using birch tar tells us that Neanderthals were (pardon the pun) pretty sharp.

But the Zandmotor Beach flake tells us more than that. Making birch tar adhesive for tools was so routine that Neanderthals would do it even for a simple domestic tool like a small flake—even in the extreme environment of Ice Age Northwestern Europe, in the shadow of glaciers at the very northern edge of where Neanderthals could survive. And all the while, they were using fairly advanced methods for more efficient production.

“Despite [the] mounting evidence, the degree of Neanderthal technological expression is still under debate,” notes the new paper by archaeologist Marcel Niekus and his colleagues (Niekus is at the Netherlands-based Foundation for Stone Age Research). “The Neanderthal tar finds provide evidence of a complex technology so ingrained in their behavior that it was maintained at the limits of their ecological tolerance: glacial northwestern Europe.”

There’s not much room left to debate Neanderthals’ intelligence in the face of evidence that they used fire and created art. But a technology like producing tar adhesive—only one component of a complex, multi-piece tool—requires more than brains. Anthropologists usually assume that such technologies require a larger, relatively sedentary population; hunter-gatherers could still pull it off, but they’d need to live in larger groups, and move around less, than the archaeological record suggests for Neanderthals.

As far as we know, Neanderthals lived in relatively small groups, with a sparse population scattered across the Eurasian landscape. Based on the shape of their femurs, they walked much more than modern hunter-gatherers. Most anthropologists wouldn’t expect them to be able to develop, much less routinely practice, a technology that’s every bit as complicated as pottery or metallurgy. But it now appears that they did.

High-tech and efficient

When Niekus and his colleagues examined the thick black tar with a micro-CT scan, they noticed fine grains of charcoal, sand, and iron oxide mixed in with the tar. Those contaminants were mixed in very evenly, as if they’d been worked into the tar while it was molten and flowing. To manage that kind of thorough mixing, birch tar would have to reach temperatures of 350ºC or more. The amounts of chemical compounds like botulin and lupeol in the tar also suggest a temperature in that range. To get the tar that hot, Neanderthals must have produced it in a relatively high-tech way.

As a study earlier this year pointed out, it’s really not very hard to make birch bark tar; burning a roll of birch bark next to a flat rock will do the trick. But that’s also a super inefficient way of making tar; Niekus and his colleagues—who tried their hands at tar production for the sake of science—estimate that it would have taken ten hours to make enough tar just to haft a single flake. If Neanderthals were going to the trouble of putting tar on a small, everyday domestic tool like a flake (whether to attach it to a haft or just to make a simple grip), then producing tar in usable amounts must have been routine. And that means they probably found a more efficient way to go about it.

The most efficient way to get tar from birch bark is to heat the roll of bark in a clay vessel buried inside an earthen mound. It’s a more complicated process, which requires more steps, more planning, and more detailed technical knowledge, but it also makes more tar more quickly, and with about 40 times less bark required for the same amount of tar. It’s also the only method that produced temperatures hot enough to explain the fine grains of sand and charcoal mixed with the tar (360ºC inside the vessel and 310ºC inside the bark roll).

The mother of invention

So the Zandmotor Beach flake suggests that Neanderthals were using Stone Age high-tech to make adhesives for their multi-part tools (which were pretty high-tech in their own right). It involved a complex process of gathering birch bark and heating it to extract the tar, then using the tar to haft a tool or shape a grip. That would have taken a lot of time and energy, yet “the technological investment must be worth the trouble,” wrote Niekus and his colleagues.

That’s especially true in an extreme environment like glacial Northwestern Europe 50,000 years ago, where resources where scarce and uncertain, and just surviving at a basic level must have been a challenge. But Niekus and his colleagues suggest that the cold, inhospitable environment may actually have pushed the Neanderthals to develop more complex tools, and more efficient ways of producing them, in order to make a living.

PNAS, 2019. DOI: 10.1073/pnas.1907828116.  (About DOIs).