We live in a highly complex world, one that requires many different skills and abilities to make our way through it. One of these skills is to look at lines and shapes that we see around us and give meaning to them. This ability to read images is an essential part of our lives. If we didn't have it, life, as we know it, would be impossible; our world would be unrecognizable. But at some point in our ancient past, that's what the world was like — imageless. And then something changed. At some point in our human history, probably about 35,000 years ago, we began to create pictures and to understand what they meant. Archeologist call this period the "creative explosion." But why did people suddenly decide to start creating images of the world around them?
The discovery of the prehistoric cave paintings of Altamira gave 19th century experts a clue to this question—they first theorized the obvious, prehistoric humans painted simply to represent the world around them. But that was not a real answer, for these early artists only seemed to paint one thing—animals. And they painted their pictures in dark caves, too, well away from the eyes of admirers.
Scientists who study altered states of consciousness suggest the answer lies in the hard-wiring of the brain. People didn't just one day decide to invent making pictures. Rather, prehistoric artists where experiencing sensory deprivation deep within their caves—in a sort of trance state—resulting in powerful hallucinations. These hallucinations were of such powerful emotional importance they felt compelled to paint them on the walls. According to this theory, these artists were simply nailing down their visions.
Source : http://www.pbs.org/howartmadetheworld/episodes/pictures/