Paris, France | Long thought of as thick-skulled brutes, Neanderthals were already building complex underground structures by firelight, possibly for rituals, 176,500 years ago, said a study that adds to a new, smarter image of our extinct cousins.

These ancient people wrenched fragments of stalagmite from the cave floor and stacked them into walls, some forming rough circles, standing up to knee high, according to research published in the journal Nature.

Deep inside Bruniquel Cave in southwest France, more than 300 metres (984 feet) from the entrance, they built six such structures, one almost seven metres wide -- tens of thousands of years before the first Homo sapiens arrived in Europe.

"Neanderthals were inventive, creative, subtle and complex," study co-author Jacques Jaubert of France's Bordeaux University told AFP.

"They were not mere brutes focused on chipping away at flint tools or killing bison for food."

The dating of these structures pushed back by tens of thousands of years the first known cave exploration by members of the broader human family.

And it ranked the French walls among the oldest-known human constructions.

According to the multi-national research team, Neanderthals broke the stalagmite pillars into about 400 similarly-sized pieces with a total length of 112.4 metres (123 yeards) and a weight of about 2.2 tons (4,400 pounds).

This implied they knew how to work as a group.

- The only ones -Among the fragments of stalagmite -- pillars of mineral deposits growing upward from a cave floor underneath a persistent drip -- the researchers found traces of fire and burnt pieces of bone.

"Early Neanderthals were the only human population living in Europe during this period," they wrote -- and referred to Neanderthals as "the world's first spelunkers".

"Our findings suggest that their society included elements of modernity, which can now be proven to have emerged earlier than previously thought."

Neanderthals lived in parts of Europe, Central Asia and the Middle East for up to 300,000 years but appear to have vanished some 40,000 years ago.

This coincided more or less with the arrival of homo sapiens out of Africa, where modern humans are believed to have emerged some 200,000 years ago.

Neanderthals and homo sapiens interbred, leaving a small contribution of less than two percent to modern human DNA -- except for Africans, as the Neanderthals never lived on the continent.

Several recent studies have found that Neanderthals were much more sophisticated than suggested by the long-standing theory that they disappeared because we outsmarted them.

Reconstructions of Neanderthals often make them out as brawny rather than brainy -- even their name is used to insult someone perceived as uncouth.

Yet they were recently shown to have been making cave etchings some 40,000 years ago, were likely the first to catch, butcher and cook wild pigeons, ate vegetables, cared for their elderly, buried their dead, and may have been the first jewellers.

- Ritual? -The new study contends that "the Neanderthal group responsible for these constructions had a level of social organisation that was more complex than previously thought."

The function of the stalagmite constructions, first discovered in 1992 and recently re-examined, can only be inferred.

Based on other examples of early human cave use, "we could assume" they had a symbolic or ritual use, the authors said, though they may also have been used for "domestic" purposes or as a refuge.

"What surprises us most is the ability of Neanderthals to have explored very deep into caves... far from natural light," Jaubert said.

"We believe we are providing evidence of the capacity of Neanderthals to enter a hostile, underground environment, using fire to light the way, to do things that go beyond mere survival."

The oldest "formally-proven" inhabited cave, according to the team, was Chauvet in southeast France with its more than 30,000-year-old cave paintings left by early humans.

Commenting on the study, archaeologist Marie Soressi of the University of Leiden in the Netherlands, agreed that Neanderthals alone would have built the structures.

"We don't have any other type of humans in Europe at that time," she said in a podcast distributed by Nature.

"It's clearly too big to be a structure made by cave bears which are known to hibernate deep inside caves. It is also completely unknown for cave bears to pile up fragments."