Traces of ancient "glue" on a stone tool from 50,000 years ago point to complicated thinking by Neanderthals, experts say.
The glue was made of birch tar in a process that required forward planning and involved several different steps.
It adds increasing evidence that we have underestimated the capacity of our evolutionary cousins.
Only a handful of Neanderthal tools have signs of glue, but experts say the process could have been widespread.
The tool, found in the Netherlands, has spent the last 50,000 years under the North Sea. This may have helped to preserve the tar glue.
Co-author Marcel Niekus, from Stichting STONE / Foundation for Stone Age Research in Groningen, said that the simple stone flake was probably used either to cut plant fibers or to scrape animal skins.
Although bear tar may have been used by Neanderthals to attach stone tools to wooden handles in some cases, this tool probably only had a grip made of tar. Dr Niekus said there was no impression from a wooden or leg shaft in the tar.
It would have allowed the user to apply more pressure to the stone flake without cutting their hands ̵
The tool was made by Neanderthal groups who lived in icy borders for their range, says the study's author. By now, this area would have been part of Doggerland, a land mass that is now being lowered under the North Sea.
These small hunting groups would have inhabited icy tundra, with relatively few trees.
"They had to really plan ahead because the process needs at least 40 kilograms of wood. In steppe tundra conditions it is not easy to collect, because you only have dwarf birch trees," Dr Niekus told BBC News.
"They also had to invest time and energy in building the fire and extracting tar."
Scientists believed that Neanderthals only had ( the action of attaching a handle or strap to a groundbreaking) certain types of special tools, such as points and scrapers.
Dutch discoveries show that "they also had very simple ugly flakes," says Dr Niekus. "It was something we did not expect.
" With the investment in time needed, you would expect that they would only do it with special hunting weapons, but they also did it with special domestic tools. We believe that the use of birch tar was quite widespread. "
There are hundreds of Neanderthal sites in the Netherlands, but birch tar is hardly ever present. Marcel Niekus believes it is because the tar is not preserved under normal conditions. The conditions under the North Sea were perfect for preserving the tar, which gave" a small window to the Neanderthal normality. "
" The important aspect of our finding is that we can show that from the various known methods of distilling pitch from birch bark, Neanderthals used the more complex ones, "said co-author Dr. Gerrit Dusseldorp of the University of Leiden.  "These are more efficient and the distribution of tar impurities that we can see on CT scans is similar to that of complex distillation methods. "
Birch tar is also found in the Neanderthal context in Campitello, Italy, 200,000 years ago and in Königsaue, Germany, where the evidence is 50,000 years old.
The Neanderthals in Italy can also use pine resin to capture 50,000 But this natural substance is not as smooth, which makes scientists believe that birch tar was probably their first choice, and there are also traces of bitumen found in the Neanderthal context between 42,000 and 70,000 years ago.  The stone tool was found on the Zandmotor beach near The Hague, from the same sand beds that have produced a Neanderthal skull fragment. Coal dating of the tar gave an age of about 50,000 years.
"Modern people in South Africa are known to produce glue from about 100 000 years ago, "Dr. Dusseldorp told BBC News.
" This is 100,000 years later than the earliest known Neanderthals. But since such finds are rarely preserved, it does not definitively prove that there are no older modern human glue. We just haven't found them yet. "
The essay describing the finding is published in Proceedings of the journal National Academy of Sciences (PNAS).
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