Genetics of Earth and Moon

"The giant impact theory echoes what we know about the formation of the entire Solar System, which coalesced from a disk of gas and dust around the young Sun. The Earth and the other planets grew in a series of collisions, starting from dust particles and culminating in massive impacts between planet-sized bodies. The last giant impact involving the growing Earth may have spawned the Moon by creating a disk of rocky debris in orbit around the planet. The giant impact model has become so popular that the two bodies involved in the impact even have names: the proto-Earth (the progenitor of Earth) and Theia (the impacting body, named for the Greek goddess who was the mother of the Moon)....


In the giant impact theory, the proto-Earth and Theia are generally assumed to have had different oxygen isotope signatures. That’s because each planet’s signature is determined by its orbit; presumably, the only way to get nearly identical signatures is for two planets to form on nearly the same orbit, which does not fit with how we think planets grow. So if Earth and Theia had different fingerprints, why do the Earth and Moon share the same one?" (source)