Alkanes, from the possible breakdown of fatty-acids, in distant past..
To me, the question then becomes .. is there any nonbiological-method for Mars to have produced those molecules?
IF so, THEN .. probably that's how they got there..
but without knowing the answer to that question..
it isn't decided, is it?
I'd read, years ago, in the science news-headlines sites, that someone had cracked Colony Collapse Disorder:
They'd said that in ALL the hives which didn't have CCD, there was never the coincidence of both a specific virus and a specific fungus, infecting them.
They'd said that in all the hives which did have CCD, both that specific virus & that specific fungus were in-play.
Apparently, to them, the infection of both of those specific-pathogens, meant that their immune-systems were defeated, & everything else got them, then.
Since I can't find that article, here's a different article which identifies some specific pathogen, which I hope includes both the specific-virus & specific-fungus that the other researchers had found to be the culprit:
https://beeprofessor.com/colony-collapse-disorder/