Slight nuance. We can most definitely tell if a life source you find there is from mars or not - assuming it has the same dna backbone, sequence comparison can

Slight nuance. We can most definitely tell if a life source you find there is from mars or not - assuming it has the same dna backbone, sequence comparison can tell exactly when life diverged into the two planets (as it likely did if mars ever had life). The real danger is that life forms hitchhiking the first humans can totally overwhelm any existing life and completely condemn it.