(1) It was the end of the aptly named Hadean eon: Volcanoes spewed lava across rock baked by ultraviolet radiation; asteroids blasted craters into the landscape.

(2) Most scientists have assumed that the hellish environment lasted for as long as 500 million years, an era thus named the Hadean.

(3) "I think we're in basic agreement on" the habitability of the Hadean, says geophysicist Norman Sleep of Stanford University in Stanford, California, who has done similar calculations.

(4) The history of Earth is divided into four eons — starting with the earliest, these are the Hadean, Archean, Proterozoic, and Phanerozoic.

(5) This material was pummeling the surface of Earth right through the Hadean era. But that doesn't explain why LUCA chose our side of the mirror.