It may have taken Michelangelo four long years to paint his fresco on the ceiling of the Sistine Chapel,but his earliest predecessors spent considerably longer perfecting their own masterpieces. Scientists have discovered that prehistoric cave paintings took up to 20,000 years to complete. Rather than being created in one session, as archaeologists previously thought, many of the works discovered across Europe were produced over hundreds of generations who added to, refreshed and painted over the original pieces of art. Until now it has been extremely difficult to pinpoint when prehistoric cave paintings and carvings were created, but a pioneering technique...