They are made up of dots and lines or abstract shapes and patterns. Nothing in the natural world of the prehistoric artist would have looked like this, or this. And, very often, these patterns are repeated or scattered across more recognizable images in a seemingly random way. Here, the Pech Merel caves in France, the prehistoric artist has painted a splendid pair of horses. But, look. He's covered the image with a series of spots. It really is hard to see what a pattern of spots has got to do with the world of hunting.
Archaeologists realize they were as far away from understanding cave-painting as they had ever been. The obvious explanations, like the paintings having been created to represent things in the world or for hunting, would never unlock the puzzle of these images. Because both of these theories missed the point, instead of explaining why people were painting images in caves, they should have been trying to solve the mystery of how we got the amazing ability to create images in the first place.
To be able to paint a picture or something, you first need to know what a picture is. And how could you know that if you've never seen one before? Henri Breuil himself realized that this was the heart of the problem.
He told the curious story of a Turkish man in the 19th century who was shown a picture, it was a picture of a horse. But the man was mystified. He'd no idea what he was looking at, because never in his life had he seen a picture before. This Turkish man, it seems, was a devout Muslim. Now at its strictest, Islam forbids images of living creatures. So, here was someone apparently who refused to believe that you could create an image of an animal in two-dimensions. He said he didn't recognize it was a horse because he couldn't move around it.
For us, looking at a painting as vivid as this one, it's almost impossible to believe, we can't imagine what it would be like not to understand what a picture was, that it can represent something in the world.
This is what it might look like for people who've never seen any pictures before, a collection of lines, colors, markings without any meaning.