A trove of sophisticated stone tools recently dug up from a South African cliff suggests early modern humans developed complex cognitive ability anywhere from 6,000 to 10,000 years earlier than many scientists believe.

In a study published in the journal Nature, researchers said they had unearthed a large number of small stone blades going back some 71,000 years. The heat-treated blades appear to have been designed for tipping spears or arrows that could be used for hunting game.

Crucially,...