Microbial habitability of the Hadean Earth during the late heavy bombardment p419
Diverse Solar System materials indicate that a cataclysmic spike occurred in the number of impacts within the inner Solar System about 3.9 billion years ago. Here, numerical models probe the degree of thermal metamorphism there would have been during this period in the Earth's crust, and thus how habitable the near- and subsurface would have been for microbes; analysis shows no plausible scenario in which the habitable zone was fully sterilized.
Oleg Abramov & Stephen J. Mojzsis
There is also an excellent News and Views comment on this:
Earth science: Life battered but unbowed p335
Early in its history, Earth experienced a pounding from extraterrestrial impacts. But instead of sterilizing the planet, it allowed microbial life to persist, according to numerical models of Earth's crust.
Lynn J. Rothschild