Neandertals really knew their way around a fire. In a sea cave sheltered from the blustery winds of Gibraltar, our ancient cousins created a hearth capable of making tar from nearby plants, a new study suggests.

Previous research has shown Neandertals used tar as an adhesive for crafting weapons, and that they used fires for heat and cooking (SN: 8/31/17). Yet the new finding, reported November 12 in Quaternary Science Reviews, suggests Neandertals designed specialized fire structures to make tar itself.

“We did not expect to find it,” says Juan Ochando Tomás, a botanist at the University of Murcia in Spain. But after collecting evidence from various scientific disciplines, the researchers were able to identify the structure as a hearth and realize it was indeed made by Neandertals.

Ochando Tomás’ team found a small pit in a layer of sand, clay and silt dating to around 65,000 years ago in a cave known to have been inhabited by Neandertals (SN: 9/22/08). The researchers carried out a series of chemical and visual analyses on the contents of the pit and surrounding sediments.

Their results suggest the pit contained charcoal, pollen and chemical compounds that point to the burning of resinous plant material, along with yellow crystals they interpret as likely tar. The tar was seemingly made from the resin of gum rockrose (Cistus ladanifer), a plant that grows in nearby shrubland.

Based on their findings, the researchers re-created the structure by filling a hole with rockroses, sealing it with sand and soil, and setting a fire on top to steam the plants. After several attempts, the team was able to make enough tar to make two spears from flint and olive wood.

The structure is an “oddball” for this period, says archaeologist Andrew Sorensen, who was not involved with the study. Most Neandertal hearths were much simpler, he says, and the ancient hominids’ use of fire appears to have been less frequent during this time period.

Initially skeptical of the study’s conclusions, Sorensen says he was won over by the analysis of the pit contents and the re-creation. The find indicates that Neandertals could have used one of the more complex and efficient methods for making tar, he says, and adds another possible adhesive to the short list of those they used to make tools.

“My main curiosity now is whether they are able to find residues on stone tools at the site matching the physical and chemical signatures of rockrose tar,” says Sorensen, of Leiden University in the Netherlands. “I think this would do much to appease any remaining skeptics.”