The El Castillo Cave art -- the oldest of which is thought to be around 40,800 years old -- could be "the oldest known well-dated cave paintings in the world—a title previously held by France's Chauvet cave paintings, believed to be at least 37,000 years old," according to National Geographic (2012). As far as I know, this record still stands. Very ancient cave paintings have also been discovered in Indonesia, but they seem to date to around the same period as the Chauvet paintings:
"Paintings of wild animals and hand markings left by adults and children on cave walls in Indonesia are at least 35,000 years old, making them some of the oldest artworks known." Source: The Guardian (2014). (See next video.)
What does all of this cave art mean? Why was it created in the first place? No one really knows and archaeologists have tended to focus on technical issues like dating the art rather than speculate about its purpose:
"Ever since the beginning of the 20th century, several attempts have been made to find the meaning(s) of Paleolithic rock art. Art for art’s sake, totemism, the Abbe Breuil’s hunting magic and Leroi-Gourhan’s and Laming-Emperaires’s structuralist theories were proposed and then abandoned one after the other .... Since then, most specialists have made up their minds that it would be hopeless to look for the meanings behind the art. They prefer to spend their time and efforts recording it, describing it and dating it, to endeavour to answer the questions 'what?', 'how?' and 'when?', thus carefully avoiding the fundamental question 'why?'." Source: Bradshaw Foundation.
"Considering the fact that shamanism is so widespread among hunter-gatherers and that Upper Paleolithic people were admittedly hunter-gatherers, looking to shamanism as a likely religion for them should have been the first logical step whenever the question of meaning arose," according to the Bradshaw Foundation.
"... shamanic religions evidence several characteristics which can make us understand cave art better. The first one is their concept of a complex cosmos in which at least two worlds - or more - coexist, be they side by side or one above the other. Those worlds interact with one another and in our own world most events are believed to be the consequence of an influence from the other-world(s). The second one is the belief of the group in the ability for certain persons to have at will a direct controlled relationship with the other-world. This is done for very practical purposes : to cure the sick, to maintain a good relationship with the powers in the other-world, to restore an upset harmony, to reclaim a lost soul, to make good hunting possible, to forecast the future, to cast spells, etc." (Bradshaw foundation.)
Stone Age cave paintings are very similar around the world, a striking fact in itself. If Lewis-William's theory is correct, it means that some form of shamanism was the original religion - the primal religion - of the human race. It also suggests that religion may have a pre-human or pre-Homo Sapiens origin since some experts have conjectured that the art discovered in the El Castillo Cave may have been created by Neanderthals.