Clay Impression in 175,000-Year-Old French Cave May Be Neanderthal Kneeprint, Researchers Hypothesize

An impression in clay in Bruniquel cave in south-west France could be a kneeprint left around 175,000 years ago by a Neanderthal who helped build circular structures from broken stalagmites. Bears later moved into the cave and obliterated almost all footprints and traces on the cave floor except for this one clay impression, which has been covered and preserved by a thin layer of calcium carbonate.

Sophie Verheyden at the Royal Belgian Institute of Natural Sciences presented her findings on 4 May at a meeting of the European Geosciences Union in Vienna.

“It’s just a hypothesis. To be sure of that, we need a lot of imprints of knees to compare it to,” she stated. Verheyden and colleagues plan to ask people to kneel in a variety of clays to see what impressions are left.

A bear specialist examined the impression and concluded it isn’t a bear print. @NewScientist reported that forensic studies of kneeprints have shown they can contain DNA from skin cells, hair or blood. Mareike Stahlschmidt at the University of Vienna has shown that DNA can diffuse into calcite and be preserved by it, and stated that for Bruniquel cave it is worth a shot to look for DNA.

Homo sapiens was not in Europe around 175,000 years ago. “We don’t know of any other humans present at that period,” Verheyden stated. Researchers are confident all artefacts within Bruniquel cave are Neanderthal in origin.

The entrance to Bruniquel cave collapsed at least 140,000 years ago and remained closed until its rediscovery in 1990. The stalagmite circles were discovered in the 1990s and described in a 2016 paper by a team including Verheyden. Earlier this year her team reported the collapse date, reinforcing that the artefacts are Neanderthal.

Neanderthals broke off stalagmites and used them to build circular structures in Bruniquel cave around 175,000 years ago. Some of the broken stalagmites are more than 20 centimetres thick at the base. Sophie Verheyden’s team located the bases of a handful of the broken-off stalagmites and dated the breakage to around the same time that the stalagmite circles were built.

Both the structures and the broken stalagmites in Bruniquel cave were dated by measuring radioactive isotopes in the calcite growths. “The age is a strong argument because it’s simultaneous with the construction of these structures,” Verheyden stated. The largest stalagmite circle is around 7 metres in diameter and the circles are found more than 300 metres into the cave.

“You need to be sure of your light when you go 300 meters underground,” Verheyden stated. Traces of fire suggest the circles in Bruniquel cave were lit. The location deep underground has prompted questions about whether the structures served a cultural or religious purpose rather than a practical one.

Sophie Verheyden’s team is developing ways to fingerprint the stalagmites based on their mineral composition to determine where in the cave the broken ones came from. The team is trying to find ways to look through the layers of calcium carbonate covering parts of the Bruniquel cave floor. “Every time we go in the cave, we discover new stuff, even after 10 years,” Verheyden stated.