A 3,000 year-old village discovered in central California has been found to contain an unusual set of burials — with more than a half dozen individuals buried without their heads, and nearly as many others buried intact, with an extra skull by their side.

In two of the graves, the crowns of the severed skulls had even been fashioned into smooth, polished bowls.

But despite this eerie tableau, these decapitations are not signs of violence or war, archaeologists say.

Instead, analysis of the remains shows that they may be evidence of an ancient method of paying tribute to ancestors — possibly even a way of re-uniting family members after death.

“In our own society, we sometimes keep the ashes of our ancestors in a vase on our mantle, or keep photographs of them to remind us,” said Dr. Jelmer Eerkens, an anthropologist at the University of California at Davis, who led the study of the remains.

“But in some societies, they kept the bones of their ancestors.”

The village was first uncovered in 2002, during the construction of a development project near the town of Brentwood, about 70 kilometers [45 miles] east of Oakland on the banks of a small stream known as Marsh Creek.

A large-scale archaeological excavation followed, revealing the remnants of cooking fires, sites for making food and tools, and nearly 500 human burials.

Some artifacts dated back more than 7,000 years, but radiocarbon dates from more than 130 of the burials returned dates ranging from 3,000 to 4,000 years ago.

The Marsh Creek site provided a rare view into the lives of California’s sedentary hunter-gatherers, people who lived in small settlements and subsisted on acorns, seeds, fish, and small game.

But the most salient feature of the village was its unusual graves.

In seven of the burials — two of men, two of women, and three of indeterminate sex — the remains had no heads.

And eight other graves — four of men, two of women, and two indeterminate — contained a complete skeleton, with an additional skull buried alongside it.

And among those burials, two of men in their late 30s or early 40s, also included an unusual artifact: the top of a human skull that had been meticulously polished into a small, smooth bowl shape. One of these skull crowns — called calottes — was made from the head of an adolescent, and had been daubed with red ochre.

What purpose these artifacts served is unclear, Eerkens said, but the removal of skulls found in the Marsh Creek graves is not entirely unique.

“Such burials are not common, but are certainly not unheard of in other parts of California,” Eerkens said.

“Extra skulls and individuals missing their skull are found once in a while. There are also many examples of people buried missing arms and/or legs.” [See a 5,000-year history of violence in California: “From Stone Darts to Dismembered Bodies, New Study Reveals 5,000 Years of Violence in Central California“]

“It’s the context that is important for figuring out if such activities are the byproduct of violence, or are part of ritual or other activities,” he said.

Previous research in California has turned up evidence of head-taking and scalping, and ethnographic accounts have shown that, in some native societies, trophy heads were displayed as warnings to would-be interlopers.

As a result, when the Marsh Creek burials were first uncovered, some archaeologists chalked up the severed heads to prehistoric warfare, with the victors having taken the heads of the vanquished as a kind of grim spoils of war. [Learn more about the nature of prehistoric warfare: “Grisly Mass Grave in Utah Cave Is Evidence of ‘Prehistoric Warfare,’ Study Says“]

“Interpretations were being made about these items, either skull cap bowls or extra skulls that were buried with people, that they were trophies from victims of warfare,” Eerkens said.

“This would imply that the skulls were from people who were not living at the site, but lived elsewhere.”

So Eerkens and his colleagues set out to test this theory, by analyzing the chemistry of the Marsh Creek remains.

They sampled teeth and bone from nearly 200 individuals, in search of isotopes of strontium — which can tell scientists where a person was born and raised — as well as nitrogen, which can provide clues about the person’s diet.

Strontium is an element that occurs naturally in groundwater and has different forms, or isotopes. But those isotopes show up in different concentrations, depending on the local geology.

When we drink water, or we eat animals that drink the same water, those specific concentrations become fixed in our teeth and bones. So anthropologists can study the strontium in human remains to determine where a subject lived — especially during childhood, as their teeth were forming.

Eerkens’s team measured the strontium in the teeth of those who had been buried with extra skulls, as well as in the bones of those who had been decapitated, and in one of the calottes.

They then compared those figures with the levels found in the rest of the burial population.

Their results showed that all of the burials contained the same strontium signature, which was specific to the Marsh Creek area.

“The stable isotope context is the key to determining if the people who had their heads removed, and the people buried with an extra skull, were from there or not,” Eerkens explained.

“And in this case the original interpretation” — that the extra skulls were taken as trophies from outsiders — “did not hold up.”

Moreover, Eerkens pointed out that the general profile and condition of the burials gave no suggestion that anyone buried at Marsh Creek had been killed or injured in battle.

Based on other mass graves that have been found in central California, signs of warfare are relatively easy to spot, he said.

“In those mass burial cases — two of which you previously covered in Western Digs — the individuals are all males, and they show clear evidence of violence as a probable cause of death: arrows embedded in bone, cranial fractures, et cetera,” he said.

“As well, the isotopic data show the men in those mass graves were not local to the site where they were buried — they were from somewhere else.” [See two recent studies of California mass graves: “Mass Grave Found in California Reveals Prehistoric Violence Against ‘Outsiders’” and “Mass Grave of ‘Prodigal Sons’ in California Poses Prehistoric Mystery“]

“In this case, at Marsh Creek, we have both males and females represented, who were buried with an extra skull. There is no clear evidence of violence on anyone with such unusual burial treatment, and the isotope data show they were all locals — both the people buried with extra skulls, and the people lacking a skull.”

“The Marsh Creek pattern is inconsistent with warfare as an explanation for the presence of extra skulls and headless burials.”

Rather than being signs of conflict, Eerkens said, the skulls and calottes may have represented an opposite kind of relationship: one of kinship.

“The data are much more in line with ancestor worship, where sometimes mementos of people were kept and turned into artifacts — bowls, in this case, but we have examples of flutes and whistles [made from human bone] in other cases.”

“These items were probably used in rituals to remember the dead. Some individuals were later buried with these items, or other remains of one of their ancestors.”

To explore this theory, the researchers returned to the subjects’ teeth, where another surprising clue turned up, this time about their diet.

The team analyzed teeth from two of the people who were buried with extra skulls — one male and one female — and focused on their levels of nitrogen, which can provide insights into the quality of nutrition that they received in childhood.

High levels of nitrogen found in the deepest, oldest layers of teeth — which form during infancy — correlate with a diet rich in breast milk, Eerkens explained. Low nitrogen levels, meanwhile, suggest a diet consisting of solid foods, such as acorns and roots.

The results showed that both of the people experienced a sudden drop in nitrogen in their diet when they were between 12 months to 21 months old.

To Eerkens, this means that both subjects had been weaned from their mothers, suddenly and at an unusually young age.

“We were surprised to learn that of the two we tested who were buried with an extra skull, both showed that they had been weaned at a very early age, earlier than anyone else we studied at the site,” he said.

This made the team wonder if there was a connection between being separated from a mother at a young age, and being buried with someone else’s head.

“One hypothesis we came up with is that their mothers may have died early, while they were still breastfeeding,” Eerkens said.

“As a result, they may have been adopted into another family.”

By this thinking, removing the skull from one of the person’s parents — whether biological or adopted — and burying it with them, may have been a way to re-unite child and parent after death.

“When the people who had been weaned early died themselves, there may have been conflict about where they should be buried: with their biological kin or their adopted families,” Eerkens explained.

“Burial with an extra skull from either their genetic or adopted families may have been a way to resolve this.”

Of course, such a scenario is purely speculative. But it could be tested, with the help of DNA evidence, Eerkens noted.

In the meantime, he said, the burials at Marsh Creek bear witness to the many forms that veneration can take, some of which may be difficult to recognize across cultures.

“I think it shows the strong connection native Californians had to their ancestors, and how they invested time and ritual effort in expressing that connection,” he said.

“That they kept mementos, like calottes fashioned into skull bowls, speaks to the importance of ancestors and the strong ties people had to their past.

“This is a theme that continues, of course, to the present. Native Californians today continue to have intimate and emotional ties to their ancestors and places that their ancestors lived.”

And the fact that Marsh Creek’s macabre artifacts were once thought to be war trophies is a reminder that, when studying a culture across the millennia, things are not always as they first appear.

“This is how archaeological science works,” Eerkens said.

“We have hypotheses about past human behavior, but we are always trying to falsify those hypotheses using independent lines of data.

“That’s how we advance our knowledge and understanding of the past.”

Eerkens and his colleagues report their findings in the journal American Antiquity.

