[pdonis]

Life has been on Earth for at least 3.8 billion years. All of the life on Earth today is descended from that original life. So if life on Earth originally came from Mars, it would have had to do so at least 3.8 billion years ago. But that's too close to the origin of the solar system as a whole for it to be credible that an industrial civilization could have evolved on Mars, caused a climate catastrophe, and then migrated to Earth.

[landryraccoon]

I don't think it's at all likely that life on Earth originated on Mars, but if it did then there could have been cross pollination of microorganisms over billions of years due to meteor impacts on both worlds throwing bits of crust into space. In fact if we do find life on Mars at some point it wouldn't be shocking if they are quite similar to bacteria found on Earth, since they might just be descendants of earth bacteria carried over from the impact that killed the dinosaurs.

[pdonis]

> if it did then there could have been cross pollination of microorganisms over billions of years

I don't think this is consistent with DNA evidence, since such cross pollination would be expected to show up as two sets of genetic lines of descent with very different characteristics, since the environment of Mars is very different from that of Earth; and we don't observer anything like that. In fact, if life was evolving separately on Earth and Mars, it might not take very long before the two lines of descent were not even compatible genetically, meaning that genetic exchange would be impossible even if the organisms were brought back together.

[isostatic]

Or ok the flip side perhaps life originated on mars or europa or even interstellar, with single cells organisms coming from there