>Microbial life fossils
But since the RNA world is still poorly understood, and the nature of the lifeforms is highly speculative, there are many possible explanations for this consistent with an Earth origin. For instance it could be that RNA life didn't form microbial mats, or was geographically much more limited(perhaps only existing in deep sea vents, much more dependant on chemistry from the environment) until colonising the rest of the ocean later, leading to a much stronger signal in the fossil record. The RNA world might have been much shorter lived too for reason I'll get back to.
>popping into existence and only complexifying after hundreds of My
One very plausible explanation for this apparent lag is that the evolution of eukaryotes was a very unlikely event like endosymbiosis. Couple with the fact that RNA based life would be much more genetically unstable, allowing to evolve faster, and might have been much more fragile, representing a stronger selective pressure. Whereas DNA based prokaryotes are extremely versatile, which is clear from the fact that they've colonised the entire planet and lived through more drastic changes in climate than any other life form on earth.
There's also no reason that I know of to think Mars was a more(or less) likely location for abiogenesis to have occured than Earth was. More and more evidence is suggesting that the building blocks for RNA are quite abundant even sans planets, and that abiogenesis itself might actually be very likely, but later complexification might not be.
I think both hypotheses have a lot of merit. Although I will say if we find life on Mars and it turns out we are Martians I will be disappointed, because it would mean we still only have one sample of life.
There's also a possibility that there was life on mars seeded from Earth, some of which came back to us fossilised, though I'm still undecided on whether ALH-84001 contains any fossils or not. There doesn't seem to be much consensus there.