Washington, D.C. ? More than a million years ago an ancient human ancestor walked across a sandy plain in eastern Africa, leaving footprints that scientists are now hailing as the earliest evidence of modern upright walking.

The footprints, dated to between 1.51 million and 1.53 million years ago, were discovered at Ileret, Kenya, researchers report in today’s edition of the journal Science.

With a large toe parallel to the other toes, the prints indicate a modern upright stride, the researchers said. They are likely to have been made by the early hominid Homo ergaster or early Homo erectus.