"The moon's just right upstairs"
But Earth has, in our only satellite, a nice little attic storage space. Asteroids hitting the earth in these early days -- and they did that a lot -- could easily have kicked up material from the infant earth, splashing bits of it all over the lunar surface. In fact, there are potentially millions of tons of Earth rock on the Moon, according to Dr. Peter Ward at the University of Washington. (I'm not sure why they interviewed him, rather than the two graduate students conducting the study, but I guess that's academia for you.) Mind you, we're not talking about easily recognizable, conveniently labeled chunks of material. For one thing, the Earth-to-Moon meteorites were probably crushed to powder in their fall, with no atmosphere to slow them down. For another, it's not immediately obvious how we would tell terran material apart from lunar material, though according to this report there are several possible chemical parameters for differentiating the two. And there's no obvious Earth material in the lunar rocks we've collected so far. But there's evidence that if we knew what to look for, we'd be able to find pristine (if powdered) Earth rock dating nearly to the birth of the planet.
What would that get us? Well, we often assume right now that life began to emerge after the planet was cool and heavy bombardment by asteroids had tapered off. But that's partly because we don't have any reliable records from before then. If signs of life appear in these relics from Earth's first billion years, then life may happen more easily than we thought. It would indicate that the conditions necessary to support life are not as tricky and specific as we might have thought. After all, we knew our ancestors were hardy little buggers -- otherwise they wouldn't have lived long enough for a lineage -- but hardy little buggers who can live on an unfinished planet under a hail of space debris? That presents a different picture. It makes us even bigger badasses, and perhaps more importantly, it lends credence to the idea that life could easily have arisen on other planets, under other -- maybe tougher -- conditions. Statistically that's pretty much a no-brainer, but if life is easier to produce than we'd imagined, alien life may be not only likely but frequent.
And all we need to do to find out is check upstairs.