For nearly a century, skeptics dismissed a Paleolithic cave painting as merely rust-red iron oxides, the kind of natural staining one might see from mineral runoff on cave walls practically anywhere in the world. The painting’s almost modernist aesthetic didn’t help either: a column of parallel lines beside speckled splotches of red like a Jackson Pollock original.

But new research, including radioactive isotope dating and advanced image processing, has now confirmed that these unusual markings found inside a south Wales cave in 1912 really are ancient abstract art.…