The researches painstakingly scraped these natural calcite deposits off the top of the paintings, stopping just before they reached the pigment. Then, using a device called a mass spectrometer, they took rice-grain-size samples of this calcite and counted the uranium and thorium atoms. The ratio between the two determines the date of the calcite formation, since the decay happens at a known rate.
Because the calcite came after the paintings, this sets a minimum age for the art. What researchers don't know is how long the initial calcite deposit took, so the paintings could be anywhere from hundreds to thousands of years older than the minimum dates.
The researchers took samples from 11 Spanish caves, including famed spots like Altamira with its painted herds of bison. At Altamira, they found an image of a red horse that dates back at least 22,000 years and a clublike image that is at least 35,600 years old. The club symbol has been painted over with the famous colorful bison herd, which dates to around 18,000 years ago. In other words, Altamira was a popular spot for artists for a very long time. [Images: Altamira & Other Amazing Caves]
At another cave, El Castillo in northern Spain, the researchers found primitive art of mind-boggling age. This cave contained the 40,800-year-old red disk. It also sported a hand stencil, created by an artist spitting red pigment over his or her hand to leave a handprint, that dates back more than 37,300 years.
"We simply did not have any dates this old before," Zilhao said. "Even if this is made by modern humans, we are pushing the age of this stuff by 5,000 years."