[TASMANIANRED]

I thought most meteors burned up in the earths atmosphere.

[LibWhacker]

Yes, many of them do, particularly the smaller ones. But some do make it to the surface.

Note this "panspermia" theory is all very hypothetical. Nothing's been proven yet. Except that bacteria can be revived after spending extended periods in the rigors of space. But exactly how long, I don't think anyone knows. Check out Blam's link above. Lots of good stuff there about it.