I hate to sound like a panspermist, but could some microbe-bearing fragments have been flung into orbit and the life in them survived long enough to "re-infect" the Earth?
Which leads to the question, how long would it take for a global magma ocean to produce enough solid crust and liquid water for a returning extremophile to survive there?
Edit: How much radiation the meteor was exposed to would depend on whether it orbited below the planet's magnetosphere or into the Van Allen belts.
STARGAZING: All I see are the lights of a billion places I'll never go. --Howard Tayler, Schlock Mercenary