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Writing in the Antiquity Journal, Jack Pettigrew et al report that –
“The age of Bradshaw rock art (also called gwion gwion) is uncertain but estimated by indirect methods at between 46 000 years ago, based on the time of extinction of depicted live megafauna (Roberts & Brook 2010), and 70 000 years ago, which is the age of the boab tree in Australia. In view of this antiquity, it is remarkable that Bradshaw paintings, often exposed to sun and rain, can be vivid and with high contrast, even though they have never been repainted. Conversely, other rock art in the same region, such as the Wandjina paintings, fades at a rate measured in hundreds of years and is often repainted. Here we report on cases where the original paint is no longer present in Bradshaw art, but has been replaced by a biofilm of living, pigmented micro-organisms whose natural replenishment may account for the longevity and vividness of these ancient paintings.”
More here – http://antiquity.ac.uk/projgall/pettigrew326/