A venture to Mars and bringing back material and the travelers also could destroy all life on earth. Below is on relevant link, but not the only claim that microscopic life and indications of it otherwise has been found in space.
Life on Mars Found by NASA's Viking Mission?
NOTHING more threatens human (and all) life than disease. Anyone who understand the environment in Florida understands the massive destructive damage an invasive species can cause. Nothing of earth and certainly humans shows evolved protection against alien microbes.
So what if a mission to Mars and asteroids destroys every life form on earth? "Oh well, nothing ventured nothing gained?"
IN FACT, since everyone keeps mentioning Columbus, IN FACT that exploration lead to taking diseases to where there was no evolved protection, literally genociding entire races - such as 100% of the human race in the entire Caribbean and much of the entire populations of South, Central and North America - and those were earth origins micro-organisms.
Space is not a nice, forgiving place. Nor are invasive life forms.