An opposable thumb is supposed to be a sign of a sophisticated species. But apes called bonobos make little use of their thumbs when they hang from tree branches – even though we use ours to keep a tight grip.

Bonobos are our closest living relatives , along with chimpanzees. We are all descended from the same common ancestor , so studying bonobos can give us clues to what that common ancestor was like.

Tracy Kivell at the University of Kent, UK and her colleagues studied bonobos living at Planckendael , a …