The indicated region (white box) shows black fungi at a sharp boundary
While some rock art fades in hundreds of years, the "Bradshaw art" remains colourful after at least 40,000 years.
Jack Pettigrew of the University of Queensland in Australia has shown that the paintings have been colonised by colourful bacteria and fungi.
These "biofilms" may explain previous difficulties in dating such rock art.
Professor Pettigrew and his colleagues studied 80 of these Bradshaw rock artworks - named for the 19th-Century naturalist who first identified them - in 16 locations within Western Australia's Kimberley region.
They concentrated on two of the oldest known styles of Bradshaw art - Tassel and Sash - and found that a vast majority of them showed signs of life, but no paint.
The team dubbed the phenomenon "Living pigments".
"'Living pigments' is a metaphorical device to refer to the fact that the pigments of the original paint have been replaced by pigmented micro-organisms," Professor Pettigrew told BBC News.
"These organisms are alive and could have replenished themselves over endless millennia to explain the freshness of the paintings' appearance."
Among the most frequent inhabitants of the boundaries of the artwork was a black fungus, thought to be of the group of fungi known as Chaetothyriales.
Successive generations of these fungi grow by cannibalising their predecessors. That means that if the initial paint layer - from tens of thousands of years ago - had spores of the fungus within it, the current fungal inhabitants may be direct descendants.
Black fungi with yellow "fruiting bodies" (left), alongside red bacteria, give one work its colours
The team also noted that the original paint may have had nutrients in it that "kick-started" a mutual relationship between the black fungi and red bacteria that often appear together. The fungi can provide water to the bacteria, while the bacteria provide carbohydrates to the fungi.
The exact species involved in these colourations have yet to be identified, and Professor Pettigrew said that the harsh conditions in the Kimberley region may hamper future research.
However, even the suggestion of these "living pigments" may explain why attempts to date some rock art has shown inconsistent results: although the paintings may be ancient, the life that fills their outlines is quite recent.
"Dating individual Bradshaw art is crucial to any further understanding of its meaning and development," Professor Pettigrew said.
"That possibility is presently far away, but the biofilm offers a possible avenue using DNA sequence evolution. We have begun work on that but this will be a long project."
Didier Bouakaze-Khan, a rock art expert from University College London, said that "there's a general consensus that what we're looking at might not purely be pigment as it was applied when the depictions were made", but that studies like this one would help archaeologists worldwide to take into account what effects life itself may be having on the art.
"It's very interesting and very exciting what they're showing - that there's some microorganisms going into the pigments and not destroying them, which is usually what's associated with the effect," he told BBC News.
Speaking about African rock artists, he said that "they had an intimate knowledge of ingredients theye were using and knew how long they would last, the rate of decay and how dark they would go and so on - not necessarily them controlling it, but they were definitely aware."
As such, Dr Bouakaze-Khan said it would be interesting to investigate whether the Bradshaw artists knew about the long-term effects of the specific pigments they used in their works.
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.
We surveyed 80 figures in 49 panels of Bradshaw art from 16 different locations in the Kimberley region, on a rough transect from its East to West Coast at around 14-15°S, using portable, digital and measuring microscopes. We concentrated on Tassel and Sash styles of Bradshaw figures because they are easily recognised, there is little controversy about their classification, and because it is widely accepted that they are derived from the oldest epoch of Bradshaw art (Walsh 2000). As expected for a living biofilm, we were able to obtain DNA from 80 per cent of the paintings non-invasively using a cotton swab. The DNA will be used for later microbial identification and metagenomic sequencing.
The vast majority of paintings, independent of location and overall colour, was occupied by colonies of micro-organisms, with no sign of paint (Figures 1-3). There appeared to be a great variety of micro-organisms when all samples were considered, but a black pigmented fungus, identified as such by yellow fruiting bodies (Figure 1), was prominent. Its black pigment made a major contribution to the famous 'mulberry' coloured paintings. A reddish organism that could not be resolved into single cells in the field with our portable microscopes, probably a species of Cynaobacteria, was usually found along with the black fungi. When the black fungi had a minor presence and the red 'cyanobacteria' dominated a particular painting, the overall colour was the well-recognised 'cherry' (or 'terracotta') shade (Figure 2) rather than the 'mulberry' shade (Figure 3).
The black fungi probably belong to the Chaetothyriales, an extremely conservative group of rock-adapted fungi that replicate without hyphae by cannibalising their predecessors in situ (Gorbushina et al. 2005). Their suite of conservative traits could explain why the sharp contours of Bradshaw art have not been corrupted by fungi growing beyond the edges of the image. We saw rare examples where 'rogue' fungi appeared to have destroyed most of a painting, but in 98 per cent of cases the fungi stayed strictly within the art's boundaries. The possible mechanisms by which the micro-organisms are confined within the edges of the painting are testable, and include:
Biofilms ('patina' or 'desert varnish') are thought to be contributing to the deterioration of other Australian rock art, the petroglyphs of the Burrup peninsula in Western Australia (Bednarik 2002). This is just the opposite of the case reported here, where the tolerance of the biofilm organisms for extremes of temperature, radiation and osmotic pressure (Gorbushina et al. 2005) would permit indefinite survival and replenishment of the paintings. DNA sequence studies of the organisms could reveal their phylogenetic history and so yield more information on the age of the enigmatic paintings that they delineate with 'living pigments'.
We are grateful to Steve Macintosh, who showed us his rock art discoveries at Faraway Bay, to Joc Schmiechen, who shared with us many sites he had discovered in Drysdale River National Park. Access to Drysdale River National Park was under a permit from the Kimberley Land Council, and we acknowledge the Wanjina Wunggurr Wilinggi traditional owners.