Stone age people may have deliberately ventured into oxygen-depleted caves to paint while having out-of-body experiences and hallucinations, according to a new study.
In the 19th century, researchers discovered a series of decorated caves that date back between 40,000 and 14,000 years — to the Upper Paleolithic era or late Stone Age — across Western Europe. The caves, found mainly in Spain and France, were filled with wall paintings, many of them in areas that could be accessed only through narrow passageways. The depictions were painted in black and red and primarily showed animals with some hand stencils, handprints and geometric abstract signs.
But why would people go through the trouble of walking through narrow cave passages to make art? To answer this question, a group of researchers at Tel Aviv University focused on a characteristic of such deep, narrow caves, especially those that require artificial light to navigate: low levels of oxygen.
The researchers ran computer simulations of model caves with different passageway lengths that lead to slightly larger "hall" areas where paintings may be found and analyzed the changes in oxygen concentrations if a person was to stand in the different parts of the cave burning a torch. Fire, such as that from torches, is one of several factors that depletes oxygen inside caves.
They found that oxygen concentration depended on the height of the passageways, with the shorter passageways having less oxygen. In most of the simulations, oxygen concentrations dropped from the natural atmosphere level of 21% to 18% after being inside the caves for only about 15 minutes.
Such low levels of oxygen can induce hypoxia in the body, a condition that can cause headache, shortness of breath, confusion and restlessness; but hypoxia also increases the hormone dopamine in the brain, which can sometimes lead to hallucinations and out-of-body experiences, according to the study. For caves with low ceilings or small halls, the oxygen concentration dipped as low as 11%, which would cause the more severe symptoms of hypoxia.
The researchers hypothesize that ancient people crawled into these deep, dark spaces to induce altered states of consciousness.
"Hypoxia might well be a plausible explanation for many of the depiction locations, which are far from the cave mouth and require passing through low, narrow passages," the authors wrote. "We contend that entering these deep, dark caves was a conscious choice, motivated by an understanding of the transformative nature of an underground, oxygen-depleted space."
Caves had a special significance for these ancient civilizations. They were seen as "portals that connect to the underworld," lead author Yafit Kedar, a doctoral candidate in the Department of Archaeology and Near Eastern Cultures at Tel Aviv University, told Live Science in an email. The findings suggest that the ancient people sought altered states of consciousness and created cave depictions as "a way to maintain their connection with the entities" of the underworld.
There are some parts of the caves that were more ventilated that also contained these depictions. However, altered states of consciousness "could be achieved in these contexts via other agencies than hypoxia," the authors wrote. What's more, the authors only simulated the effect that torches have on oxygen in the caves, but other parameters such as human respiration or natural chemical reactions that take place in caves can even further decrease oxygen concentration, they wrote.
The researchers now hope to analyze how many people could be together at the same time in these caves with limited amounts of oxygen and for how long.
The findings were published on March 31 in The Journal of Archaeology, Consciousness and Culture.
Originally published on Live Science.
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Yasemin is a staff writer at Live Science, covering health, neuroscience and biology. Her work has appeared in Scientific American, Science and the San Jose Mercury News. She has a bachelor's degree in biomedical engineering from the University of Connecticut and a graduate certificate in science communication from the University of California, Santa Cruz.