Dating ancient paintings in the caves of Borneo.

The matchstick figures and images of hands lining the Gua Saleh Cave in southeast Borneo were made at least 9,900 years ago, a team of French archaeologists has determined. That date suggests that people inhabited the Asian island, the third largest in the world, some 4,000 to 5,000 years earlier than scientists had previously believed.

"It's difficult to know exactly how old the paintings are," says Jean-Michel Chazine of the National Center for Scientific Research in Marseille, France. He and his team established the artworks' minimum age by estimating when a mineral coating on the paintings had begun to form.

Paul Tacon of the Australian Museum in Sydney comments that the study is "extremely important, [providing] the first significantly old and reliable date for rock art of the region." He's not surprised by the early date for the artwork, however. It's "consistent with recent research by a range of scholars in nearby Timor and other parts of southeast Asia," he says.

Usually, archaeologists date rock art by evaluating the carbon content of organic pigments. However, the Gua Saleh Cave artists used pigments containing no carbon; they're made of pure hematite, an iron-ore mineral. So, Chazine and his colleagues used carbon dating on the calcite coating the paintings. Calcite, or calcium carbonate, is the main constituent of limestone and of cave formations such as stalagmites and stalactites.

Chazine and his team dated the calcite that had deposited over two hand shapes applied to the cave walls by the ancient artists. The researchers used a method known as uranium-thorium dating to cross-check the carbon-dating estimate of the calcite layer's age. Both methods rely on measuring the extent of radioactive decay of elements normally interspersed within the calcite.

In their study, published in the September Quaternary Research, the researchers use dates from both methods to estimate that the calcite veil is about 9,900 years old. Therefore, the paintings must be at least that old. Still unknown is how much time passed between the creation of the paintings and the formation of the calcite covering.

Because the Gua Saleh Cave and others like it are in limestone outcrops--some as high as 1,000 meters above Borneo's forests--the island's prehistoric inhabitants probably didn't live in the caves, but only visited them for important rites, Chazine says. Making images of hands was probably part of rituals such as healing ceremonies, he says.