Rounded blade (lower left) and straighter flake (upper right) (Image: Metin Eren)
For most of the Stone Age, Homo sapiens and neanderthalensis both made disc-shaped stone tools called "flakes," says Metin Eren, an experimental archaeologist at Southern Methodist University in Dallas, Texas. But around 40,000 years ago humans in Europe began exclusively producing rectangular blades.
Read the resat on New Scientist.