Spaceflightnow is reporting on a recent-rexamination of moon rock samples which has lead to the discovery of a significant "meteorite impact period" which ocurred about 4 billion years ago:

The idea that meteorites have hammered the moon's surface isn't news to scientists. The lunar surface is pock-marked with large craters carved out by the impact of crashing asteroids and meteorites, said Robert Duncan, a professor and associate dean in the College of Oceanic and Atmospheric Sciences at Oregon State University.

But the narrow range of the impact dates suggests to researchers that a large spike in meteorite activity took place during a 100-million year interval - possibly the result of collisions in the asteroid belt with comets coming from just beyond our solar system.

It's thought this might have had something to do with the formation of life on Earth. Or not. Regardless, it's yet another indication that when the Earth was young it wasn't even a nice place to visit, let alone a place you'd want to live.