The wads of gum are made of pieces of birch bark pitch, a tar-like black resin, and are combined with saliva, with teethmarks clearly visible.

They were found 30 years ago next to bones at the 9,700-year-old Huseby Klev archaeological site north of Sweden's western city of Gothenburg, one of the country's oldest sites for human fossils.

The hunter-gatherers most likely chewed the resin "to be used as glue" to assemble tools and weapons, said Anders Gotherstrom, co-author of a study published in the journal Scientific Reports.

"This is a most likely hypothesis -- they could of course have been chewed just because they liked them or because they thought that they had some medicinal purpose," he told AFP.

The gum was typically chewed by both male and female adolescents.

"There were several chewing gum (samples) and both males and females chewed them. Most of them seem to have been chewed by teenagers," Gotherstrom said.

"There was some kind of age to it."

A previous 2019 study of the wads of gum mapped the genetic profile of the individuals who had chewed it.

This time, Gotherstrom and his team of paleontologists at Stockholm University were able to determine, again from the DNA found in the gum, that the teenagers' Stone Age diet included deer, trout and hazelnuts.

Traces of apple, duck and fox were also detected.

"If we do a human bone then we'll get human DNA. We can do teeth and then we'll get a little bit more. But here we'll get DNA from what they had been chewing previously," Gotherstrom said.

"You cannot get that in any other way."