About 1.5 million years ago, human ancestors walked upright with a spring in their steps just as modern humans do today, suggests an analysis of ancient footprints found in northern Kenya.

The prints are the oldest known to show modern foot anatomy.

The discovery also helps round out the picture of a cooling and drying episode in Africa that compelled tree-dwelling human ancestors to venture into the open landscape for food, said John Harris, a paleoanthropologist at Rutgers University in New Brunswick, New Jersey.