Weathering, plate movements, and volcanoes destroyed many of the craters. The factors contribute to the lack of evidence of heavy bombardment of earth. This evidence was subjected or, if not subjected, weathered; since Earth was and still is a "living" planet, meaning a planet with active plate tectonics and an active hydro/atmosphere. This activity is in contrast to the Moon, where plate tectonics stopped shortly after its formation and where a hydro-atmosphere could never form. Both because the Moon was simply too small to sustain plate tectonics and to keep volatility with him/her.
It compares the sequences of shared genes or proteins in different species to determine how closely they match.