Analyses of samples collected by the Apollo astronauts, augmented with those of lunar meteorites, support an epoch of intense bombardment of the Earth-Moon system about 4 billion years ago. That data, plus a survey of the size distribution of impact craters in the ancient highlands of the Moon, suggest the source of the debris was the asteroid belt. The asteroids appear to have been dynamically excited by shifting orbits among the giant planets. This extraordinary event in the history of the solar system reshaped the surfaces of the terrestrial planets and, intriguingly, immediately preceded the earliest isotopic evidence of life on Earth.