[Sluggo]

The solar system has a non-zero interchange of material. I think the big buzz about the modern prospect of life on Mars started from a meteorite of Martian origin 10 or so years ago. Evidentially, the "fossils" found turned out to be explained by geologic processes however there are areas identified by satellite that show ancient signs of water on the surface and one of these areas is where Perseverance landed.

In addition, the chances on sample return that Earth life would contaminate the samples is high so they are going through extreme measures to keep them isolated as a side effect that would greatly hinder the chances of a Martian bug getting out. If unfortunately a Mars bug did get out, odds are our more diverse and therefore better evolved immune systems/microbial life would most likely destroy or eat anything evolved/survived on the small niche that is the current Martian environment.

One of the theories going around is that of panspermia and if life is found on Mars, it could equally be as likely as being of Earth, Martian, another planet, or other origin. So we could actually be decedents of Martian, another Planet, Moon, or other celestial body. If they find life on Mars that is not like anything found on Earth it its likely any other solar system with minimal survival conditions would be almost guaranteed to harbor some form of life.