The solar wind has been carrying biological organisms through space, some hitting Mars, for a billion years. Also long ago Mars had seas. So evidence of life on Mars is not impossible.
Unless living organisms were blasted from Earth into space by an ancient asteroid strike (which might have happened), life on Mars or any other planet is impossible, because abiogenesis (the evolution of life from inorganic materials) is impossible. No matter how many billions of us are here on Earth, we didn’t get here by random chance combined with undirected energy such as lightning. And hypothesizing billions of years for an impossibility to happen doesn’t make it any more likely.