So radiocarbon dating, which measures the steady decline of the isotope carbon 14 in organic materials, cannot be used.Our earliest date comes from a Namibian cave, where excavated floors contained painted slabs between 19,000 and 26,000 years old.Others are faint and crumbling, damaged by time, water and the graffiti of unthinking visitors.
With the exception of snakes and bees, the San people rarely painted reptiles and insects.
Rock art research is among the most demanding of archaeology's subdisciplines.
Back in the 1940s, the American chemist Willard Libby used this fact to determine the ages of organisms long dead.
Most carbon atoms have six protons and six neutrons in their nuclei and are called carbon 12. But a tiny percentage of carbon is made of carbon 14, or radiocarbon, which has six protons and eight neutrons and is not stable: half of any sample of it decays into other atoms after 5,700 years.
So by measuring carbon 14 levels in an organism that died long ago, researchers can figure out when it died.