Scientists at the University of Queensland have discovered that colorful bacteria and fungi have colonized the rock paintings, the BBC reported Monday.
Researcher Jack Pettigrew and his colleagues studied 80 rock artworks in 16 locations in Western Australia's Kimberley region.
They found some of the oldest examples 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"'Living pigments' is a metaphorical device to refer to the fact that the pigments of the original paint have been replaced by pigmented micro-organisms," Pettigrew said.
"These organisms are alive and could have replenished themselves over endless millennia to explain the freshness of the paintings' appearance."
These "living pigments" may explain why attempts to date some rock art have shown inconsistent results, Pettigrew says, because although the paintings may be ancient, the life that fills their outlines is quite recent.