Panel of hands from El Castillo cave in Spain. Image: Pedro Saura

Alistair Pike and his fellow scientists may be blinking a bit in the sunlight today, having found a surprise in the dark, mysterious caves of northern Spain.

Using a technique called uranium-thorium dating, they say they found that some prehistoric cave paintings are more than 40,000 years old - four or five thousand years older than anyone had believed before. That's a big difference in prehistory, and the findings by Pike's team, reported today in the journal Science, could change what scientists believe about human roots.

Why would 4,000 years matter? The problem is that if cave paintings in Europe are really 40,000 years old, that makes it possible they were done just as the first Homo sapiens were migrating there from Africa - and maybe before.

The researchers say that more than 41,000 years ago, Europe would have been dominated by Neanderthals, who are not direct ancestors of modern human beings. Scientists say Neanderthals were an evolutionary dead end, a branch of the human family tree that died out, presumably because they could not compete for food with the more intelligent Homo sapiens.

"So what does this mean?" asked Pike, an anthropologist at the University of Bristol in England. He said it's possible humans became capable of cave painting well before anyone previously guessed. Or perhaps they started painting as soon as they reached Spain. Or perhaps there's another explanation.

"Alternatively, cave painting started before the arrival of modern humans, and was done by Neanderthals. That would be a fantastic find," Pike said.

Uranium-thorium dating is more precise than anything scientists have had before for dating cave paintings. It actually measures the age of mineral deposits that settled over the paintings in the centuries after they were done - meaning that it gives minimum ages for the artwork.

In the event the Neanderthal theory holds up, it creates a problem. The belief among researchers has been that Neanderthals were just not intelligent enough to create outlines of their hands, or draw pictures. The idea of seeing something and making a two-dimensional image of it would have been beyond their capacity.

But Pike and his colleagues cautioned that they'd like to date the paintings in many more caves before they do anything else. Revising theories of human prehistory is not something done lightly or quickly.