The colonization of Mars was a failure. Humans weren't suited to the climate or gravity. And when heated up, the water in the icecaps flooded the planet, resulting in a relatively unpleasant world of deep, cold wetlands and towering forests. When mankind totally abandoned Earth, and the Solar system as a whole, the colonized worlds were left to themselves. Titan's humans, while promising, eventually died off after a devastating invasion enacted by the Lok, the only true non-terran sapients in the Solar system. Venus is, to date, much like Earth. It is still flourishing, with unique fauna and flora, but nobody to see it through intelligent eyes.
Mars, however, pursued a totally different path. The surviving Martian colonists were given no warning before the migration, when humans travelled far into the Milky Way, and were never heard of again. This stunted the colonists' development, and within the next few hundred years, advanced technology was all but a legend. They started degrading into a tribal culture, they grew taller, and developed strange skintones, unlike most terran mammals. The tribal people traversed Mars' primary continent before ocean travel was discovered, and they spread to the northern and southern continent, while the western continent remained totally free of human influence.
Through the passing millennia, the Dwarves died out on Venus, mankind was long gone from the Solar system, and the Lok continued milling about in their cities, at this point having colonized every moon of Saturn. The tribal culture grew more and more alien, until there was no culture, and the last humans in the Solar system were animals again.
Some developed into immense flying animals, others into running creatures, bounding along on stong legs. Many, at this stage were unrecognizable as humans. Quadrupeds, aquatic forms, flying creatures reminiscent of the sharovipteryx of the Triassic, soaring the thick atmosphere on huge, outstretched hindlimbs. Amongst all these aberrant forms, a small species developed on the western continent, only touched by humans long after humanity was a distant memory. On the western continent, a marshy wetland spanning an area half the size of Australia (the second largest continent on Mars) many flying creatures, be they descendants of birds, men, lizards or fish were drawn to the land by its seemingly infinite source of food.
One species of flying hominid repurposed its gangly flying legs into stilts, used to wade through the marshes in search of food. The species was odd, and disturbingly human. They had long, curved, noses with narrow nostrils, greenish skin and jet-black tufts of fur on parts of the bodies. They stood a meter tall, and had lower jaws that jutted out, alluding to a connection with Mars' goblins.
It was from these small wading fishers, rose the Martians. A staggering four meters tall, most of that height being leg. Evolving a culture based around honour and intellect, they ended up the (second) most successful posthumans in the Milky Way. The Martians spread throughout the galaxy, until they too, simply left the Solar system entirely. And for a long time, the planets lay dormant, there were no intelligent beings on Mercury, Venus, Earth or Mars. All were silent.
But nobody mentioned any moons.