Deep in a dark cave in southwestern France lie half a dozen mysterious structures that scientists believe were built by Neanderthals 176,000 years ago -- about 140,000 years before the first modern humans arrived in Europe.
The structures, described Wednesday in the journal Nature, are located in what is known as the Bruniquel Cave. They are made of roughly 400 pieces of stalagmites, all roughly, almost eerily, the same size.
Archaeologists say these mineral formations were probably broken off the cave floor by ancient hands and then deliberately arranged into two large rings and a series of four round piles up to 15 inches high.
If all the pieces were gathered up and placed on a scale, they would weigh 2.4 tons.
Red and black soot smudges and other evidence of fires can be found inside the structures, but not outside them. That suggests they may have been used to contain fire, perhaps to light the cave.
Experts say the assemblages are unlike anything else in the historical record. It is still unclear exactly what purpose they served -- whether some type of domestic use or a ritual or symbolic behavior.
Regardless, they represent some of the earliest known constructions made by hominids, as well as the earliest known evidence of cave use by early humans. They also suggest that early Neanderthals may have had better control over fire than was previously thought.
And perhaps most intriguingly, the study authors say that the group responsible for the mysterious construction must have had a fairly sophisticated social structure.
"Building these structures was a project. It required an objective which has been discussed among several people and enough social organization to assign tasks," said Jacques Jaubert of the University of Bordeaux in France and the first author on the study. "Certainly, it was a collective work."
William Rendu, an archaeologist with the French National Center for Scientific Research, who was not involved in the research, said the peculiar arrangement of broken stalagmites, as well as their association with fire, makes it clear that the structures could only have been made by early humans, and not by bears or other animals that probably also used the cave.
In addition, he said, they must have been made by Neanderthals because they were built during a time when only Neanderthals were present in Europe. Modern humans would not arrive in these parts for another 140,000 years.
Finally, because the structures are located in a dark and difficult-to-access cavern 300 yards from the entrance of the cave, this early Neanderthal group must have already mastered the underground world.
This was a big surprise to archaeologists because until now, there has been no evidence of Neanderthals making their way so deep into a cave. Indeed, before this study, the earliest known evidence of hominids occupying deep caverns came from the Chauvet Cave paintings that date back a mere 36,000 years.
"It is, in all aspects, a truly extraordinary discovery," said Emmanuel Discamps, an archaeologist at the University of Bergen in Norway who was not involved in the work. "Comparable behavior is known for Upper Paleolithic modern humans -- younger than 40,000 years old -- so it bridges the gap between 'them' and us."
Archaeologists know that Neanderthals lived in Eurasia from around 400,000 to 40,000 years ago, but evidence of how they lived has been difficult to find. In a commentary accompanying the study, Marie Soressi, an archaeologist at Leiden University in the Netherlands, explained that part of what makes the finding so thrilling is how the chemistry of the cave ensured that the assemblages were preserved over so many millenniums.
"These structures are among the best-preserved constructions known for the whole of the Pleistocene epoch, probably because they were sealed by calcite very soon after they were erected," she wrote. "When the best evidence is found in the best-preserved context, it serves as a reminder for archaeologists of how much we depend on preservation."
The site is not new. It was discovered in 1990 by local cavers who had dug through its entrance, which scientists say collapsed sometime in the Pleistocene, a geological epoch that stretched from about 2.6 million to 11,700 years ago. Before those '90s cavers made their way into the darkness, no person had set foot in it for many thousands of years.
The first academic study on the cave was published in 1996 and included a detailed plan of the structures, as well as a single carbon date taken from a burned piece of bone found in the larger of the two ring structures. That particular dating technology suggested it was at least 47,600 years old, but was not capable of looking deeper into the past.
Nearly two decades later, in 2013, a new team of archaeologists including Jaubert, Sophie Verheyden from the Royal Belgian Institute of Natural Sciences and Dominique Genty of France's National Center for Scientific Research went back to the cave for a closer look.
This time around, they dated seven stalagmites from the two ring structures using a method called uranium series dating. By sampling the calcite that had grown both before and after the stalagmite fragments were broken, the researchers could constrain the date when the structures were built to roughly 176,000 years ago, give or take 2,000 years.
Now that the authors can say with more certainty that these structures were indeed built by Neanderthals, and that they were constructed in the Middle Paleolithic era, they say their next step is to determine how the structures might have been used.
Other experts say the newly published findings bolster the idea that Neanderthals may not have been that different from modern humans. This notion, once considered unthinkable, has been steadily gaining acceptance in archaeological circles.
"For me, this, as well as other evidence, seems to support the idea that Neanderthals developed symbolic behavior way before they encountered us," Discamps said. "We have to acknowledge the fact that these long-lost 'cousins' did not simply 'copy' us, as some argue. In many aspects, they had a mind shaped just like ours."
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6:07 p.m.: The story was updated with comments from Jacques Jaubert of the University of Bordeaux in France, the lead author of the study.