Earth is some 4.5 billion years old. When it formed from colliding rocks around a dim, young sun, it was presumably lifeless, and geologists long thought that life didn’t emerge for a billion years or more. This idea came from analysis of moon rocks brought back from the Apollo landings, which indicated Earth was pummelled by space rocks between 4 billion and 3.8 billion years ago – an event called the Late Heavy Bombardment. The implication was that the origin of life as we know it must have begun after that, since any earlier organisms would have been blitzed.
