It would not be surprising if ancient life did exist on Mars, since impacts would likely have sent life back and forth between Earth and Mars (and possibly Venus). For the same reason, the existence of ancient life on Mars would not show that life is likely to arise on Earth-like worlds, since one could not conclude it was an independent event.
By what mechanism is an impact giving some debris on Mars enough delta-V to reach Earth, without obliterating Mars and also sterilizing the debris due to heat and plasma?