Neanderthals were venturing deep into caves and building fires in them much earlier than previously thought, according to a new study in France.

Archaeologists with the French National Center for Scientific Research found 176,500-year-old evidence of fire construction in Bruniquel Cave in southwestern France.

Prior to the discovery, the oldest evidence of human use of a cave was 38,000 years old -- found in Chauvet Cave, famous for the Paleolithic cave art that adorns its walls.

Deep inside Bruniquel Cave, researchers found 400 stalagmites broken off from the cave floor and arranged in circles. Many of the the stalagmites are held in place by other debris. Many of the stalagmite artifacts -- which researchers dubbed "speleofacts" -- are scorched and darkened by soot, suggesting they were arranged around the outside of fire pits.

Researchers dated the stalagmites by measuring the thorium and remaining uranium in the calcite. New calcite growth has begun on the stalagmites since the site was abandoned by Neanderthals thousands of years ago. By comparing the age of the old calcite growth -- prior to stalagmites being broken from the floor -- and the new growth, researchers were able to estimate the age of the fire structures at approximately 176,500 years.

"We now know that, some 140 millennia before the arrival of modern man, Europe's first Neanderthals were occupying deep caves, building complex structures and maintaining fires in them," scientists wrote in a news release.

Because researchers didn't find other remains or artifacts, the believe it's unlikely early humans would have used spaces so deep in the cave as shelter. Why exactly they were building light sources so far from the cave's entrance isn't clear.

Researchers published their latest findings this week in the journal Nature.

"Their presence at 336 metres from the entrance of the cave indicates that humans from this period had already mastered the underground environment, which can be considered a major step in human modernity," the researchers concluded.