Are we Martians?
Recent observations on microbes make us to change what we know about life resiliency. Microbes are finally getting respect as Tarter stated. Extremophiles prove that there is life in the most hostile environments. Living microbes were found in high acidity lakes at the bottom of deep oceans, in rocks or in the cooling water of nuclear reactors.
Considering these it is not out of question that some microbes could live under Mars surface. Now if an asteroid hit Mars in the past it might be possible that a dislocated rock escaped Mars' weaker gravitation and was caught by the Earth. It is a hard to believe scenario of Earth's life being "imported" from Mars. This assumption needs to be proven false before concluding on the second genesis that happened on Mars.
Tarter's lecture online
Last year PI lecture Mission to Mars: Still Roving on the Red Planet.