A team from the University of Bern in Switzerland argues that, due to its proximity to the sun, the proto-Earth that existed before this potential collision lost the volatile elements essential to form complex molecules. Any hydrogen, carbon, or sulfur, their analysis suggests, evaporated in just the first 3 million years after proto-Earth's formation. And so if Earth had evolved without external inputs, they say, it would probably be a drier world, more hostile to the development of complex life.