Terraforming Mars is a long ways off. In fact, it’s highly unlikely that it would ever be possible and since there’s no organized magnetic field on Mars, any life that walked outside would be exposed to lethal radiation from the sun with or without an atmosphere.
That being said, life can evolve even with high amounts of radiation under certain conditions. Over time, life finds a way to exist even in extreme conditions. Evolution would follow a random pattern – the fittest of those would survive and the less fit would perish. Survival of the fittest is what drives Evolution, whether on Earth or on Mars, forward.