By Sid Perkins
Imagine our planet unmarred by humans: no buildings, no highways, no farms, no dams, no open-pit mines. Now, imagine the world suddenly swept clean of all life whatsoever: no plants, no animals, not a single microbe. Would the newly vacant landscape retain unmistakable evidence that life had ever existed?
And would that topography be noticeably different if life had never evolved at all? For instance, would Earth’s mountains be craggier, or the courses of its rivers less sinuous, if life had always been absent?