Ancient people may have used human skulls for cups and other body parts for tools, according to a burial site in Spain

• Almost 6,000 years ago, communities used a cave in Spain as a burial place.
• Researchers looked at bone fragments of 12 individuals, some of which had scrape marks.

For thousands of years, prehistoric people used Spain's Cueva de los Marmoles as a burial place for their dead.

Scientists examined the remains of 12 individuals and found people had modified some of the bones, possibly to use as tools and drinking vessels.

Three bones seem to be modified. A cranium looks scraped clean, and two leg bone fragments, a tibia and fibula, appear polished and rounded from use.

"Understanding the funerary practices of a given culture is essential, as it sheds light on a very significant part of its social and cultural structure," Rafael M Martínez Sánchez, a researcher with the University of Cordoba in Spain, told Insider via email.

Martínez Sánchez and his co-authors published a paper about the modified remains in the journal PLOS ONE. The bones date from between about 6,000 and 3,000 years ago, from the Neolithic to the Bronze Age.

"The motivations behind practices like this are actually very hard to get at," said Katina Lillios, a professor of anthropology at the University of Iowa who wasn't involved in the study.

But she believes it signifies that the living had extended relationships to their deceased. "They continued to engage with their dead, and so this tells us that the dead were important for the living," she said.

A skull cup and bone tools

Evidence of bone modification is fairly common for the Neolithic and Bronze Ages in Iberia and Western Europe, and there are many theories about the motivations.

"These range from a desire to honor the remains of close relatives or community members in ritualized actions," to collecting human trophies, Martínez Sánchez said.

But he notes that the latter option is unlikely because they didn't find indications of violence on the remains. The researchers also don't think there are enough cut or chop marks to suggest cannibalism.

Some of the oldest skull cups date back to almost 15,000 BCE. Historical accounts from at least the 5th century BCE note people drinking from human skulls.

"The very globular shape of the human skull makes it unmistakable from that of other animals, as well as allowing it to be transformed into container vessels, such as cups or bowls," Martínez Sánchez said.

Scraping and cutting of these bones left traces "that can only be attributed to human action," Martínez Sánchez said. In contrast, gnawing animals leave their own distinctive marks, Lillios said.

The polishing of the leg bones is clear under a microscope, and their rounded appearance seems to be a result of their use as tools, Martínez Sánchez said. It's not clear what kind of purpose they served, but Lillios suggested they could have been used for basket making or weaving.

The scientists believe other humans modified the bones while the remains were relatively fresh. Yet people may not have put the bodies in the cave immediately. The paper's authors note that the absence of finger and toe bones may indicate the remains were partially decomposed when they entered the cave.

Though the cool, dark cave would help preserve the remains, the researchers put the modifications between a few months and a year after death. "The actions of fragmenting and manipulating the bones may well be related to specific ritual events performed inside the cave," Martínez Sánchez said.

Caves were an obvious choice for burial sites

"The use of natural caves as burial spaces is a universal and transcultural phenomenon, recorded among various pre-industrial human societies across the globe," Martínez Sánchez said.

Caves offer protection from the elements, but Lillios said they're symbolic, too. "They're often viewed as sort of passageways to another world to the ancestors, to other kinds of worlds that are seen as sacred liminal spaces, spaces that are somewhere in between the land and the living and the land of the ancestors," she said.

One of the paper's most interesting findings is how communities used the cave between generations. Using radiocarbon dating for the remains, the scientists learned that people utilized the cave in three different phases between about 3,800 BCE and 1,300 BCE. Sometimes, roughly 1,000 years would pass between uses.

But, the researchers note, they only know the exact age of seven of the 411 remains, so it's possible the time between phases was shorter.

Plus, in the millennia since, the cave has served other purposes, and amateur archaeologists removed stone tools and other artifacts. There's a chance they may have disturbed the human remains as well.

Assuming those gaps don't represent missing or untested remains, the question is how people knew to return to the cave after so long. "I would not entirely discount the possibility that there were some oral histories, myths, or traditions in which people understood that the dead could be or should be buried in those ancestral spaces," Lillios said.