A little-known fact is that each year Earth is hit by by half a dozen or so one-pound or larger rocks that were blasted off the surface of Mars by large impacts and found their way into Earth-crossing orbits. Nearly 10% of all rocks blasted off into space from the Red Planet end up crashing into Earth.
This natural "interplanetary transportation system" begs a fascinating question: If primitive and nearly indestructible micro-organisms exist on a given planet, must they by definition as a natural act or nature, travel to their immediate solar-system neighbors?
Scientists think it's possible that one or more of those rocks might have contained tiny microbes that successfully made the journey from one planet to the other. In 2018, they plan to test this hypothesis by searching for Earthling-like DNA under Mars' surface.