It's imaginable. In the distant youth of the solar system, when the faint young sun meant that Venus was only as irradiated as our Earth while Mars' atmosphere and hydrosphere was still uneroded, conditions on these three rocky worlds would have been broadly similar. read more
Life had always thrived on Earth including now, as for Venus and Mars evidence shows that these 2 planets might just be too hot for life to thrive on it, but of course there could have been microorganisms there like bacteria and other stuff like that but I hardly believe that they could have thrived there like here on Earth. read more