Stone Age cave art thought to be the oldest in the world may not have been the work of early modern humans, say scientists.
A new study suggests the symbols, in 11 caves in northern Spain, could have been painted by Neanderthals rather than our ancient ancestors more than 41,000 years ago.
If confirmed, the discovery would have profound implications. It would indicate that advanced, abstract human thinking and possibly language emerged hundreds of thousands of years earlier than has been assumed.
Bison cave drawing discovered in the Altamira cave in northern Spain
Neanderthals were a now extinct strain of human that roamed Europe long before the arrival of early modern humans from Africa.
Both are believed to have evolved from a common African ancestor around half a million years ago.
Once dismissed as primitive and ape-like, Neanderthals are now known to have had a relatively advanced culture.
Evidence shows they engaged in body painting, and wore pendants made of bones, teeth and ivory.
The two kinds of humans co-existed in Europe for around 10,000 years before Neanderthals vanished some 30,000 years ago. They may have been victims of genocide, or more likely, simply lost the competition for scarce resources.
Their genetic identity may also have dwindled through interbreeding with modern humans.
The new research was carried out by British, Spanish and Portuguese scientists who analysed 50 paintings in 11 caves.
They used a new method of dating that measured the radioactive decay of uranium traces in tiny stalactites forming on top of the paintings.
Since the mineral deposits could not pre-date the pictures beneath them, this gave a minimum estimate of the age of the art work.
In the El Castillo cave, the scientists found one example of a red disc symbol dating back no less than 40,800 years.
A stencilled hand outline at the same site may have been painted more than 37,300 years ago, while a large club symbol in the Altamira cave was at least 35,600 years old.
The research, published in the journal Science, suggests European cave art started up to 10,000 years earlier than previously thought.
Lead scientist Dr Alistair Pike, from the University of Bristol, said: "Evidence for modern humans in Northern Spain dates back to 41,500 years ago, and before them were Neanderthals.
"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."
If the first early modern humans in Europe brought artistic skills with them, it is hard to understand why there is no evidence of older painting in Africa.
Earlier expressions of human symbolism in the form of perforated beads and engraved egg shells have been discovered in Africa dating back 70,000 to 100,000 years. But the first cave paintings are in Europe, said Dr Pike.
Co-author Professor Joao Zilhao, from the University of Barcelona, believes the Spanish art works were painted by Neanderthals - although more convincing proof is needed.
"It would not be surprising if the Neanderthals were indeed Europe's first cave artists," he said. "While this may come as a shock, in the context of what we have learned about the Neanderthals over the last decade it really should not be very surprising.
"In probabilistic terms, I would say there is a strong chance that these results imply Neanderthal ownership, but I will not say that we have proven it.
"What we have to do now is go back, sample more and find out whether we can indeed get dates older than 42, 43, 44,000."
The creation of art is considered an important marker in the evolution of modern human thinking, and may be associated with the development of language.
Prof Zilhao said Neanderthal paintings in the Spanish caves would imply that language and advanced thinking emerged "a long time ago" - possibly half a million years.
Cave art expert Dr Paul Pettitt, from the University of Sheffield, said: "Until now our understanding of the age of cave art was sketchy at best; now we have firmly extended the earliest age of European cave art back by several thousand years, to the time of the last Neanderthals and earliest Homo sapiens.
"These earliest images do not represent animals, and suggest that the earliest art was non-figurative, which may have significant implications for how art evolved."