It could be that all of the skulls were originally mounted on wooden stakes, but the researchers are not sure.
The stone structure upon which the remains were placed must have taken considerable time and effort to build. It measured about 39 feet by 46 feet, was constructed out of large, densely packed stones, and was located at the bottom of the former lake. Today, the site lies at the margin of a small lake close to the river Motala Ström.
The scientists believe that the associated animal bones and artifacts were placed intentionally on the structure and at the same time as the human remains were arranged there. The objects are thought to have been grave gifts or offerings.
Hallgren explained that the depositions of parts of animals were done in a structured way that refers to some kind of symbolism, the meaning of which still eludes interpretation. He explained that while the humans are mostly represented by crania without mandibles, the opposite is true for the animals; they are represented by mandibles and post-cranial bones.
"Furthermore," he said, "the human and animal bones were placed in different spatial zones on the stone-packing according to species and category, with humans in one zone, brown bears in one zone, and big forest game — wild boar, red deer, roe deer, and moose — in one zone."
He continued, "While we cannot decipher the meaning of these patterns, it is clear that the depositions were structured by norms and symbolic intention."
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The image of skulls mounted on stakes is, to modern eyes, an inherently gory one. In the past, some ancient cultures displayed the heads of victims to warn off enemies or to otherwise control people. Europeans did something similar, by letting the public view the remains of individuals beheaded by guillotine. Headhunters in places like South America and Africa would line up heads of victims, placing their remains in unmistakable view of potential enemies.
The unusual Swedish burial, however, does not seem to be connected to these practices.
"The individuals in Kanaljorden appear to have died some time before the crania were mounted or deposited in the lake," Kjellström said. "This separates them from the more 'common' headhunter practice where usually 'fresh' heads were used."
She added that the associated artifacts further strengthen the view that the Swedish burial was a "complex practice with ceremonial features."
The precise meaning of the burial remains unknown, but the researchers pose some possibilities.
Since both men and women experienced head trauma that healed before death, it is possible that the individuals were all slaves. The authors, however, note that slavery is rare among mobile hunter-gatherers. It also would have been a logistical challenge for a group with such a low-density population to have held captives for any length of time.
Raiding and warfare, on the other hand, are common occurrences among hunter-gatherers. Interestingly, the Swedish women victims were hit over the head differently: front-to-top versus back-to-side for men.
These same patterns of head trauma have been observed among prehistoric Native Americans, according to the authors. The patterns have been interpreted as resulting from the different roles and behaviors that men and women have traditionally held in some cultures in combat.
It is possible then that the Swedish individuals were victims of warfare who survived. Gummesson, also of Stockholm University, told Seeker that "the deposition of their skulls could be related to reverence of these individuals or their accomplishments during life."