Dating cave art is notoriously difficult. But a team of researchers has taken advantage of serendipitous conditions in caves on the Indonesian island of Sulawesi to establish that images there rival any known from Western Europe in terms of age. A stencil created as the artist blew pigment around a hand is at least 39,900 years old, they report, and a painting of a piglike animal was laid down at least 35,700 years ago.

The researchers established the designs’ minimum ages by calculating the dates of deposits that had built up on top of the pigment. They had observed that, as mineral-laden water percolates through the caves’ limestone walls, calcite gradually accumulates on their surfaces. These deposits contain uranium, which decays to thorium at a known rate, so their age can be ascertained from the ratio of the two elements.

The discovery raises a new question: Did people in Southeast Asia and Western Europe develop artistic expression independently, or was it pioneered by early humans before they left Africa? “We don’t know,” says Maxime Aubert of Griffith University in Australia, “but my opinion is it probably developed a long time ago, in Africa, and then it just spread out.”