Is interplanetary transport of microbes plausible?
Yes. Earth is often hit by meteorites from Mars – rocks that have been blasted off their home world by a violent collision, then wandered through space before finally being swept up by our planet. Bacteria and their spores are so hardy that they could easily survive such a journey. So meteorites might have carried primitive life around the early solar system. As Mars may have been a hospitable environment even before the Earth, it has been suggested that life arose there first and spread to Earth, in which case we are all descended from Martians.
