If we had incontrovertible proof that life once existed or perhaps even presently exists on Mars, sufficient analysis would reveal one of two things.
First, it could reveal that life on both planets developed independently. This would suggest much, particularly since such planets are neighbors. It would provide a strong suggestion that life exists all throughout the universe, that the cosmos is fucking teeming with independently-developed life. However baffling it would be that earth is the sole nest for life in the universe, it would be conceivable. What is not conceivable is that life developed only twice — utterly laughable that the only two times it happened it happened on neighboring planets.
There is another possibility, however, and this is that such a discovery could instead reveal that we are relatives — that life developed on one planet and then migrated to the other. This could but would not necessarily require technology allowing for interplanetary travel. In any case, this is a process known as panspermia. When an asteroid impacts a planet it can eject rocks from the impacted planet into space. If the impacted planet is a life-bearing one there is the possibilities that bacteria or perhaps more complex forms of life may hitch a ride on one of these rocks, hurling it towards another planet, effectively seeding it with extraterrestrial life.
Clearly the closer two planets are the more likely it becomes that they will experience cross-contamination. We have found rocks on Earth that originated from Mars, for instance, and impacts such as that which caused the extinction of the dinosaurs could have flung earthen rocks into space on interplanetary or even interstellar journeys that resulted in fertilization. A web of life throughout the universe, all the more likely on neighboring planets, is also bolstered by the process of panspermia — even if the development of life itself turns out to be extremely rare. Once seeded, life would evolve to adapt to its own planetary conditions. Life varying but interrelated in the cosmic Galapagos.
It may even be the case that both aforementioned potentialities manifest: that life independently develops and panspermia takes place as well. We may be an earth-borne species, for instance, but on our planet there may very well be extraterrestrials incognito (Tardigrades, perhaps) roaming undetected among us.