Mars today is a dead planet, but in the recent past this was not the case, as evidenced by the weathered structures on Mars reminiscent of the Great Pyramids and the Sphinx. Some life bearing planets have a stronger footing than others, being closer to the warmth of a sun, for instance, and more particularly being a water planet like the Earth. Mars has little of that precious substance, and was a life bearing planet only where the freezing point had not trapped the water. The atmosphere surrounding a water planet can rebuild quickly, particularly in the components that support life. On a dry planet the atmosphere is fragile, and each rebuilding takes away more of the precious water.
In the past, Mars sustained life to a level not unlike our home in Zeta Reticuli - moss and insects and worms. On such worlds there is not enough food in the food chain to support animals above that level, and setbacks occur repeatedly. A bug-eating reptile might get its start, only to die off during lean times, time and again. Thus such planets plateau.
Mars met its demise as a result of visitors from the 12th Planet, who set up mining operations on Mars in preference to Earth where large carnivorous mammals roamed about in great numbers. The 12th Planet has no such carnivores on land, and as large and muscular as these giant hominoid visitors are, they quaked at the thought. Where the atmosphere on Mars was thin, it was ample, so the visitors set about using what water resources they could muster to wash the ore they were after. In so doing they sought to control the run-off on the relatively flat surface of Mars, and did so in a thoughtless manner by directing waste water down a culvert. Thus precious water increasingly was sent underground, and a chain of events was set in motion that could not be reversed. The surface of Mars cooled as the atmosphere thinned, and the freezing surface accelerated this process.
Soon the atmosphere was too thin to breathe, and as the 12th Planet hominoids are used to a perpetual summer they were not all that reluctant to leave a freezing planet. Earth now looked more promising, especially as they had little alternative. They devised ways of dealing with the carnivores, specifically buffering themselves with human slaves trained in defense. Eventually, after being quarantined from Earth, they learned how to create and maintain their own atmosphere in air tight chambers, and thus relocatable they have continued their mining operations within the Solar System, on this spot or that, and are here still.