As we send more missions to Mars, it's becoming clear that the planet was once a wet world with features that were very Earth-like. For example, NASA's recently landed Mars Science Laboratory rover Curiosity touched down on an ancient riverbed inside Gale Crater where water, perhaps two feet deep, used to flow. Evidence of clays near Mars Exploration Rover Opportunity are also evidence that minerals have interacted with surface water some time in the past.
There's also evidence for huge gullies and river deltas that can be seen from orbit. Ancient coastlines have also been spotted, forming the outline of a vast ocean that likely filled the deep Vastitas Borealis basin in the northern hemisphere.
It is also known that the Martian atmosphere was a lot thicker than it is now. Over the eons, the solar wind has been eroding the upper atmosphere - as there's no global magnetic field to deflect the solar wind efficiently. So far, though, there is little evidence that the world used to support any alien flora or fauna, let alone an entire biosphere.