Prehistoric Superglue May Reveal Experiments 200,000 Years Ago

Tar made of birch bark was their best option for gluing handles to axes and tools: coincidence or Neanderthal research?

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Replica blade hafted with a mixture of resin, ochre and beeswax, and a replica spear point hafted with birch bark tar
Replica blade hafted with a mixture of resin, ochre and beeswax, and a replica spear point hafted with birch bark tar, to show how the adhesives could be used for different tools and different purposeCredit: Paul Kozowyk / Lab for Artefact
Ruth Schuster
Ruth Schuster

Glue is a many-splendored thing. Hafting, which means affixing a handle to tools such as stone axes, was a giant step forward for humankind. Imagine trying to chop down a tree or even a bamboo shoot using an ax-head sans handle. Now, Dutch researchers have shown by experimentation that when Neanderthals used birch tar as hafting adhesive in Italy nearly 200,000 years ago, it truly was their best option. That begs the thought that they tried other possibilities too, in a process we would recognize as experimentation.

Resin seeping from pine tree
Resin seeping from pine treeCredit: Paul Kozowyk

Over a range of temperatures, compared with birch tar, pine resin has inferior qualities, say Paul Kozowyk of Leiden University and Johannes A. Poulis of the Adhesion Institute at the Delft University of Technology, writing in the Journal of Human Evolution. Another alternative, bitumen (a naturally occurring sticky substance) isn’t common — though it was used as adhesive in Stone Age Syria. So we may postulate that the Neanderthals deliberately chose to make and use birch tar, which begs the question of how they came to that conclusion and what it means for their capabilities.

Fingerprint of a Neanderthal

The earliest hafting found so far dates to about 300,000 years ago in Germany and France, and apparently involved simple wedging. About 100,000 years later, Neanderthals living in what is today Campitello Quarry, Italy, were making birch tar and using it for hafting.

Additional but later finds of birch-tar hafting were made in Inden-Altdorf, Germany, from possibly about 120,000 years ago; in Crimea, possibly about 80,000 years ago; in Königsaue, also Germany, possibly about 70,000 to 80,000 years ago. Wondrously, a bit of tar from Königsaue has a thumbprint — there is little doubt it's a Neanderthal's.

Turning birch bark into pitch requires processing and begs fresh consideration about Neanderthal smarts 200,000 years ago, at several levels.

First, how did they make pitch from birch bark? Originally, the thinking was that distilling birch bark tar would be technologically challenging, involving heating in anaerobic conditions, which would mean the Neanderthals had advanced mental capacities.

A separate study published this year, though, suggests it ain’t necessarily so. All the Neanderthals would have needed was to put bark on a rock and slowly burn it, argued Patrick Schmidt of the University of Tübingen, Germany, and others, who did that very thing and produced lovely birch tar. That doesn’t mean the Neanderthals weren’t as smart as us marvels of evolution, but it means they didn’t have to be in order to make birch tar.

Conifer resin dripping from natural wounds in a tree.
Resin dripping from natural wounds in a conifer tree.Credit: (c) Paul Kozowyk

Now Kozowyk and Poulis are back with a new paper testing the properties of potential prehistoric glues — and reached the conclusion that birch was best.

The present paper isn’t about the Neanderthal being bright as buttons or bird-brained brutes, or somewhere in the middle. But in the service of resolving the conundrum over Neanderthal smarts, Kozowyk and Poulis point out that little work had been done on the properties and qualities of prehistoric adhesive. And the more we know, the better equipped we are to assess the qualities of the makers.

So the researchers tested the adhesive and physical properties of birch tar made using means available to Neanderthals, and concluded that among the resources available to the ancients it was the most suitable material possible for hafting.

While they don’t go there in their paper, their conclusion could shore up their original contention that Neanderthals were pretty advanced after all, based on their use of superior glue.

Of course, it could be coincidence that the Neanderthals were using birch rather than pine resin. But it is also plausible that they were capable of forethought and planning, and developed expertise and knowledge of the resources available to them — pine resin or birch tar.

Resin extracted from pines was also used as prehistoric adhesive, but it wasn't as versatile or hardy as birch bark tar.
Resin extracted from pines was also used as prehistoric adhesive, but it wasn't as versatile or hardy as birch bark tar.Credit: Susan Montoya Bryan,AP

How is tar made from birch bark superior to pine? It is more versatile, has better working properties and is more reusable than pine resin, they write, based on tests of hardness, rheology (how it flows) and thermogravimetric analysis (how its mass changes when heated and cooled).

It is plausible to speculate that the Neanderthals tried both: “At least by 50,000 years ago they did,” Kozowyk tells Haaretz.

Pine resin proved a trickier substance to handle than birch tar. Kozowyk and Poulis concluded that pine resin is more useful when mixed with beeswax, but in any case it is confined to a “sweet spot” in which it is best usable. Birch bark tar was more versatile and less affected by overheating, or the cold. At one end of the temperature rainbow, rheological tests demonstrated that birch tar glues best in chilly ambient temperatures of 0 to 25 degrees Celsius, while pine resin-based adhesives became brittle in that temperature range.

Birch tar also performed better at higher temperatures — seriously high temperatures. After 30 minutes’ exposure to 70 degrees Celsius (happily not an ambient average temperature anywhere yet), the rheological properties of the tar were pretty much unchanged, while resin-based glue stiffened. Meaning tar could be heated again and again without damaging it, unlike resin.

Bottom line: As Paleolithic glues go, birch tar is more versatile, less delicate and generally more useful than pine resin, though that was used for hafting too — much later, and elsewhere.

Replica spear point hafted with birch bark tar beside a roll of birch bark and a piece of birch bark tar.
Replica spear point hafted with birch bark tar beside a roll of birch bark and a piece of birch bark tar. Credit: Paul Kozowyk / Lab for Artefact

It is plausible that, having tried both, the local Neanderthals living 191,000 years ago in Italy experimented with both and chose to invest in making birch bark tar.

Using birch bark tar still doesn’t prove the Neanderthals possessed advanced cognitive properties, but it would lie on that side of the evidence.

Light my own fire

Other evidence for Neanderthal advancement in the Late Pleistocene remains just as intriguing, if still controversial. A recent paper postulated that Neanderthals knew not only how to use fire, but how to ignite it.

The argument over their pyrotechnical capabilities, as opposed to helping themselves to burning bushes ignited by lightning, is indirect: In Armenia, the researchers report evidence of intensive fire use at a time not characterized by intense wildfires. Also arguing in favor of Neanderthal pyrotechnology, blocks of manganese dioxide — which are thought to be prehistoric fire-starters — have been found at some sites. But in France, in a separate study, intense fire use was correlated with a warmer time in which wildfires were apparently not rare. Anyway, the authors postulate that hominins learned how to light fires multiple times in different places during the Middle Pleistocene.

So, conclusions there are none. But the new study fans the prehistoric fire by distinguishing that the Neanderthals may have been particular in their choice of adhesive. Why settle for lowly pine resin when one can make superglue — the production of which does not necessarily require a prehistoric kiln, as once thought, but was not trivial?

Kozowyk notes that tar could initially have been discovered (and rediscovered) simply by observing a partially burned roll of birch bark, which could have been used for starting fires, or, as Schmidt et al describe, by observing the black smoky residue collecting on rock or cave wall close beside the burning bark.

Maybe that is indeed how it was initially discovered: a Neanderthal noticed sticky black goo on his fire-starter. That, however, wouldn’t have produced much tar, Kozowyk points out: To make the quantities of tar found in Europe, they probably had a more efficient manufacturing method, not to mention the ability to design multicomponent tools — and the capacity for forethought.