As a geologist who studies Stone Age cave art, Iñaki Intxaurbe is used to making subterranean treks in a headlamp and boots. But the first time he navigated a cave the way humans thousands of years ago would have — barefoot while holding a torch — he learned two things. “The first sensation is that the ground is very wet and cold,” says Intxaurbe, of the University of the Basque Country in Leioa, Spain. The second: If something chases you, it will be hard to run. “You are not going to see what is in front of you,” he says.
Torches are just one of several light sources Stone Age artists used to navigate caves. Intxaurbe and colleagues are wielding these fiery tools in dark, damp and often cramped caves in an effort to understand how and why humans journeyed beneath the earth and why they created art there (SN: 11/7/18).
In the wide chambers and narrow passageways of Isuntza I Cave in the Basque region of Spain, the researchers tested torches, stone lamps and fireplaces — nooks in cave walls. Juniper branches, animal fat and other materials that Stone Age humans would have had at hand fueled the light sources. The team measured flame intensity and duration, as well as how far away from the source light illuminated the walls.
Each light source comes with its own quirks that make it well suited to specific cave spaces and tasks, the team reports June 16 in PLOS ONE. Stone Age humans would have controlled fire in varying ways to travel through caves and make and view art, the researchers say.
Torches work best on the move, as their flames need motion to stay lit and produce a lot of smoke. Though torches cast a wide glow, they burn for an average of just 41 minutes, the team found. That suggests several torches would have been needed to travel through caves. Concave stone lamps filled with animal fat, on the other hand, are smokeless and can offer more than an hour of focused, candlelike light. That would have made it easy to stay in one spot for a while. And while fireplaces produce a lot of light, they can also produce a lot of smoke. That type of light source is best suited for large spaces that get plenty of airflow, the researchers say.
For Intxaurbe, the experiments confirmed what he has seen himself at Atxurra cave in northern Spain. In a narrow Atxurra passageway, Paleolithic people had used stone lamps. But near high ceilings where smoke can rise, they left signs of fireplaces and torches. “They were very intelligent. They use the better choice for different scenarios,” he says.
While the findings reveal a lot about how Stone Age people used light to navigate caves, they also shed light on 12,500-year-old art that Intxaurbe helped discover deep in the Atxurra cave in 2015. Stone Age artists painted about 50 images of horses, goats and bison on a wall accessible only by climbing up a roughly 7-meter-tall ledge. “The paintings are in a very common cave, but in very uncommon places of the cave,” Intxaurbe says. That may partly explain why previous explorers had failed to notice the art.
A lack of the right lighting also played a part, Intxaurbe and colleagues say. By simulating how torches, lamps and fireplaces lit up a virtual 3-D model of Atxurra, the team saw the cave’s art with fresh eyes. Using just a torch or a lamp from below, the paintings and engravings stay hidden. But lit fireplaces on the ledge illuminate the whole gallery so that anyone on the cave floor can see it. That suggests the artists may have wanted to keep their work hidden, the researchers say.
Cave art wouldn’t exist without harnessing fire. So to unravel the mysteries of subterranean studios, it’s key to understand how prehistoric artists lit their surroundings. “Answering the small questions in an accurate way,” Intxaurbe says, is a path toward answering a main question about Stone Age people, “why they painted these things.”