The reason we know that life on Mars (if it exists) must be similar to life on Earth is because both have been peppered with rocks from the other knocked off by

The reason we know that life on Mars (if it exists) must be similar to life on Earth is because both have been peppered with rocks from the other knocked off by meteorite impacts (from memory something like 100kg from Mars to Earth per year and 1 kg the other way around). Some of these rocks have been ejected under benign enough conditions (and protected during transit) to enable microbes to make the journey alive. I think if we do find life on Mars we might be rather disappointed to find that it is exactly like life on Earth. There is an interesting hypothesis that life arose on Mars and was later transferred to Earth. One of the reasons for thinking this is that life seems to have appeared almost instantly on Earth as soon as it cooled down enough for there to be liquid water. Mars being further out from the sun (and smaller) likely had liquid water well before the Earth.