Comment by nephihaha

There are possibilities of invasive species either way. Think of how Australia etc have struggled with various plants and animals that have taken over. We may find that some Earth life likes Mars too much and becomes an uncontrollable pest there (after some mutation), or that some microbe etc from Mars finds the Earth environment more hospitable and starts being invasive here. (Already predicted by HG Wells' "Red Weed".) That would not be unlikely as if Mars used to be more habitable then there could be life hibernating or waiting for a break, and Earth might offer it all the things it needs to reproduce quickly...