The remains of 35 people, many of them teenagers, have been found in a ditch in Slovakia, which is almost 7,000 years old.
Martin Furholt, co-leader of the project, thinks it has something to do with magical ideas about how to fortify the settlement.
"We have a ditch around a site where people are living, and then you put people in the ditch, whether they were killed deliberately, or died of natural causes," he said.
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There is a grave holding a warrior woman in a burial ground full of Stone Age men.
The site at Vrble in Slovakia, near that country's borders with Austria and Hungary, has been the location of work by Furholt and his colleagues at the Christian-Albrecht University of Kiel.
The site is named after the German term "Linearbandkeramik", which means "Linear Band Ceramics", and refers to the type of pottery they made.
The ancestors of the people at Vrble are thought to have crossed into Greece about 1,500 years ago. The progenitors of the first Neolithic settlers in Western Europe may have been the LBK culture, which spread from there to many parts of Europe.
The ancient mass grave is near one of the entrances to the settlement at Vrble, and several aspects suggest the people dumped there were murdered.
Zuzana Hukeov, an anthropologist at the SAV, told Live Science that there were no signs of trauma or pathological changes in the field. The bodies didn't have skulls and we don't know how to remove them.
She said that the decapitation may have been one of the causes of death.
"Like a swimming frog, the bodies lay in various positions, with the arms and legs spread and bent at the knees," Hukeov said. The position indicates that the body was likely thrown into the pit.
The majority of the dead seem to have been young adults between the ages of 18 and 25 years old when they died.
There were no older individuals and only one infant who was buried with their skull. She said that the skeletons would be analyzed.
Each longhouse at Vrble was the home of one or more extended families.
The defensive wall and ditch built around one neighborhood may have been an attempt to keep people out of the other neighborhoods.
The bodies of 15 people who were brutally murdered were buried with care.
Only one of these neighborhoods gets this kind of fortification, and it has six entrances, but none of them connect this neighborhood to the others. It seems like it wasn't to defend against someone from the outside, but to block access to their neighbors.
Christian Meyer has studied mass violence at other LBK sites and isn't involved in the excavations at Vrble.
He told Live Science that there are depictions of massacres, executions, torture, and various cultic practices. The discovery of another mass grave is not surprising given what is already known.
The people buried at Vrble were missing their heads. Meyer said that it was likely of profound significance and that further explanation was needed.
He said that the site would advance knowledge about social interactions and the treatment of the dead.