“[Neanderthals] distilled tar in an intentionally created underground environment that restricted oxygen flow and remained invisible during the process,” the researchers wrote
. “This degree of complexity is unlikely to have been invented spontaneously.”
There was one piece of evidence that made the underground methods stand out. Only the tar produced underground contained a significant amount of suberin, a polymer found in birch bark that was also prominent in the ancient tool residue. There was hardly any suberin in the tar created by burning bark above ground.
“Our results suggest that Neanderthals invented or developed this process based on previous simpler methods and constitute one of the clearest indicators of cumulative cultural evolution in the European Middle Palaeolithic,” the researchers also said in the
study.