Leang Jarie, or “Cave of Fingers,” in the island of Sulawesi in Indonesia has long been known for the stenciled handprints on the walls. But it was only announced in 2014 that the actual age of the paintings there and in other caves on Sulawesi were determined to be at least 35,400 years. That would make them among the oldest art in the world, and opens up many questions about the evolution and dispersal of humans around the world. The method used to date the paintings is fascinating. Archaeologist and geochemist Maxime Aubert knew that analyzing radioactive decay would give a date to inorganic material that carbon dating can’t.
Instead of analyzing pigment from the paintings directly, he wanted to date the rock they sat on, by measuring radioactive uranium, which is present in many rocks in trace amounts. Uranium decays into thorium at a known rate, so comparing the ratio of these two elements in a sample reveals its age; the greater the proportion of thorium, the older the sample. The technique, known as uranium series dating, was used to determine that zircon crystals from Western Australia were more than four billion years old, proving Earth’s minimum age. But it can also date newer limestone formations, including stalactites and stalagmites, known collectively as speleothems, which form in caves as water seeps or flows through soluble bedrock.
Aubert got a chance to analyze the cave paintings on Sulawesi, which were partially covered by speleothems (called popcorn). As they formed on top of the images, they have to be younger than the paintings themselves.
Aubert spent a week the next summer touring the region by motorbike. He took samples from five paintings partly covered by popcorn, each time using a diamond-tipped drill to cut a small square out of the rock, about 1.5 centimeters across and a few millimeters deep.
Back in Australia, he spent weeks painstakingly grinding the rock samples into thin layers before separating out the uranium and thorium in each one. “You collect the powder, then remove another layer, then collect the powder,” Aubert says. “You’re trying to get as close as possible to the paint layer.” Then he drove from Wollongong to Canberra to analyze his samples using the mass spectrometer, sleeping in his van outside the lab so he could work as many hours as possible, to minimize the number of days he needed on the expensive machine. Unable to get funding for the project, he had to pay for his flight to Sulawesi—and for the analysis—himself. “I was totally broke,“ he says.
The very first age Aubert calculated was for a hand stencil from the Cave of Fingers. “I thought, ‘Oh, shit,’” he says. “So I calculated it again.” Then he called Brumm.
“I couldn’t make sense of what he was saying,” Brumm recalls. “He blurted out, ‘35,000!’ I was stunned. I said, are you sure? I had the feeling immediately that this was going to be big.”
The dating method is just one small part of an article at Smithsonian magazine about the Indonesian cave paintings. Take a tour to the caves themselves, and learn why their age is so important to our understanding of human history.
(Image credit: Cahyo Ramadhani)