[aruanan]

I wonder, myself. Mars represents the bottom of a very deep gravity well, and the Earth is at the bottom of an even deeper one. How could a rock get from one place to the other, and how could it get there in such a manner that evidence of life on or in the rock was not obliterated?

A big asteroid impact will cause some ejecta to be kicked out into space.

[Oberon]

A big asteroid impact will cause some ejecta to be kicked out into space.

Okay, let's say I grant you, for the sake of argument, that a meteor strike on Mars could cause some rocks to achieve escape velocity. Then there's two more issues:

1. How could evidence of life survive the passage of the rock into the Earth's atmosphere? It will be heated to incandescence during the process. I suppose it could be a piece of a larger chunk that survived re-entry.
2. Such rocks don't come with return addresses. Isn't it really a WAG that it came from Mars originally?