Related comics: https://www.badspacecomics.com/post/apostles-of-mercy

If we eventually find martian microbes, or at least their fossils, my bet is that we'll find them to be related to us.

I'm always amused when sci-fi writers think they're being deep by having ancient humans be aliens from another planet. They just completely forget about the 10 million other species we're so clearly related to? Ancient bacteria on meteorites might explain why DNA looks the same, but it doesn't explain the steady, continuously increasing genetic difference between us and our increasingly distant cousin species. It doesn't explain the dozens of "missing link" fossils between modern humans and ancient primates. If ancient humans came from another planet, then they must have brought billions of other lifeforms from millions of different species with them as well, including millions of fossils that they then buried at the appropriate strata as some sort of prank on future paleontologists, on a giant ark that has completely disappeared. It's such a weak take. The natural record is far too clear for that to make any sense.

Now, Mars / Earth cross-seeding proto-bacteria, billions of years ago, sure. Of course the energy required to kick up a rock to a trajectory where it will hit Mars (or vice versa) is orders of magnitude more than the energy required to vaporize all nearby life, so we've got a pretty big problem already. More likely, they both got seeded when our solar system passed through a cloud of primitive organic proto-life (this is my favorite theory). But eukaryotic transfer? Multi-cellular life transferring between the two worlds? No.