The well-known cave paintings at El Castillo in Northern Spain possibly weren't produced by modern humans at all, say scientists, but by Neanderthals.
New dating techniques show that the images date back at least 40,800 years, making them Europe's oldest known cave art, and probably predate the arrival of modern humans. The finding indicates that the practice of cave art in Europe began up to 10,000 years earlier than previously thought.
Such paintings can't be dated using standard radiocarbon dating, as they contain no organic pigment. But an international team used a new method to examine 50 paintings in 11 caves in Northern Spain, including the UNESCO World Heritage sites of Altamira, El Castillo and Tito Bustillo.
They dated the formation of tiny stalactites on top of the paintings using the radioactive decay of uranium, giving a minimum age for the pictures.
And the hand stencils and disks made by blowing paint onto the wall in El Castillo cave were found to date back to at least 40,800 years, making them the oldest known cave art in Europe, as much as 10,000 years older than previous examples from France.
A large club-shaped symbol in the famous polychrome chamber at Altamira was found to be at least 35,600 years old, indicating that painting started there 10,000 years earlier than previously thought, and that the cave was revisited and painted a number of times over a period spanning more than 20,000 years.
"Our results show that either modern humans arrived with painting already part of their cultural activity or it developed very shortly after, perhaps in response to competition with Neanderthals - or perhaps the art is Neanderthal art," says Dr Alistair Pike of the University of Bristol.
"That would be a fantastic find, as it would mean the hand stencils on the walls of the caves are outlines of Neanderthals' hands, but we will need to date more examples to see if this is the case."