Our planet is changing before our eyes, and as a result, many species are living on the edge. Yet Earth has been on the edge of habitability from the beginning. New work shows that if Earth had been slightly smaller and less massive, it would not have plate tectonics-the forces that move continents and build mountains. And without plate tectonics, life might never have gained a foothold on our world. picked by AutumnLotus 7 months ago