Deciphering Ancient Minds: The Mystery of San Bushman Rock Art, By David Lewis-Williams & Sam Challis

When I came back to this book after finishing it, I was looking forward to further reflections, but they were dispelled by a shimmering ring that appeared floating above the pages. It seemed to be made of prisms arranged in a glassy parquet; as usual it was incomplete, and as always it was mercurial. It's a familiar kind of apparition; not just in my field of vision but in those of people suffering from migraine, people under the influence of hallucinogens and, according to David Lewis-Williams, the San people who painted on rocks in southern Africa over thousands of years until the end of the 19th century.

If you're trying to read a book, such neural phenomena are obstacles to cognition. But if you believe you are embarking on a shamanic journey into a spirit world, they're the fairy lights round the doors of perception. Lewis-Williams, now emeritus professor at the University of Witwatersrand's Rock Art Research Institute, has identified images of these inner mirages both in San art and that of Palaeolithic Europeans. In each he sees the signatures of altered states of consciousness. Across the world and across millennia, he argues, the quirks of the human nervous system shape spiritual experience. Most powerful of all are the sensations of flying and entering tunnels that seem to be a common feature of altered states. The result, Lewis-Williams and his colleague Sam Challis argue, is the familiar cosmological arrangement in which ordinary life on the earth's surface is the material sandwich between two spiritual layers, the heavens above and the underworld below. As with European mythology, so with the San. They also have a dominant deity, though he is a trickster or ur-shaman rather than an almighty god.

Unsurprisingly, European settlers failed to see symbolically rich traditions based on an architecture similar to that of their own spiritual culture. Instead they saw primitive daubs, and saw the San themselves as benighted savages who stole cattle. They hunted them down and shot them. Others they imprisoned; and comments made by a number of these convicts, recorded in the 19th century, form a key to the authors' interpretation of what they consider to be the "most exquisite rock art in the world". San communities and culture have since gone extinct in the southern African areas where the art is found, though the San who still live in the Kalahari to the north share similar beliefs and rituals.

At first the San were simply "Bushmen" as far as Europeans were concerned, easily pronounced and easily dismissed. Nowadays, the orthography asserts the complexity and unfamiliarity of San culture: the distinctive click-sounds in their words are represented by slashes and exclamation marks that make them look like the kind of passwords your online bank approves but you will inevitably forget. The meaning of a word like !gi:ten demands an attentive exploration of San beliefs, Lewis-Williams and Challis insist, rather than the ill-fitting label "sorcerers" applied to it in a 19th century translation.

As anthropologists began to appreciate how difficult the cultures they studied were to read, they made their own ruminations ever more impenetrable. This may help to explain why Lewis-Williams and Challis feel that general audiences still need to be told that cultures can be technologically simple but symbolically rich. Their own writing is direct and clear - though there are moments, such as one in which they detect the image of a tusked serpent emerging from a rock face, where one feels one just has to take their word for it. Most San images, however, are recognisable human figures and animals, especially the eland antelopes that figure large in their symbolic landscape.

Many of the paintings depict elements of the trance dance around which San ritual revolves. This "nest of concentric circles" is the never-ending channel into the spirit world, producing the required altered state through the hypnotic effects of collective circular motion rather than hallucinogens. How long has this been going on? The paintings are hard to date, but some that seem to show trance dance postures are a couple of thousand years old. Some depicting eland are a thousand years older still, and people painted on southern African rocks more than 20,000 years before that. One of the reasons it's hard to appreciate a culture like this is that our own culture is inclined to regard continuity as a failure to progress rather than as a triumph of information transmission.

Lewis-Williams and Challis, whose scientific analysis is illuminated by sympathetic intuition, suggest that the San themselves thought like scientists, making hypotheses and testing them, when tracking animals. This rationality ran in parallel with their supernatural imagination, which remained impervious to reason. In that respect, humankind at large shows scant evidence of progress.

Marek Kohn's latest book is 'Turned Out Nice' (Faber)