This is what happened: at some point in the distant past, when the ancestors of the Martian Huylee were still only grub-like creatures scurrying in the shadows of the sporangiae forests, the real masters of Mars were the viruses. For them the red planet was then an ideal environment : slime molds were covering nearly the entire surface of this still-wet world, trying desperately to trap as much water as possible against the deadly rays of the Sun.
The molds provided the shelter the viruses needed, the humidity and, in the slow way of their microbian metabolism, the relative warmth. They were food also, the herbivores of this world on which the viruses fed : organised masses of independent cells, they were not a true organism, they had no specialised defence, no immune system. For a long, long while the viruses prospered, more parasites than predators after all, and, as always when life thrives, it looks for new territories to conquer.
And up there, in the unprotected air above the slime mat, there existed these small creatures, running around and grazing on the nutrient rich substrate.
Now, these were properly organised beings in their own right, not a loosely assembled collection of cells; they had an outside and an inside, and they defended the latter against the intrusions of the former. But the quick-lived viruses had time on their side; they accumulated generations i the blink of an eye after all. Sooner or later, changing, evolving, trying blindly a myriad of new combinations, it was always clear they would find a way.
And when they did, it was with deadly consequences.
Remember: Evolution does not care about you. It only cares about your genes, and that only inasmuch as they are able to transmit themselves to the next generation. Like a giddy socialite, Evolution is pretty much a creature of the here and now. It does not toll to create the perfect being, it does not think of a century hence; it tries little to improve your well -being. It doesn’t care if you suffer.