But the volcanoes shut down, the atmosphere thinned, and surface water froze solid. Today the surface is more like a very cold desert that is blasted by intense solar ultraviolet radiation.
The awesome power of biological evolution tells us that any Martian microorganisms would have had time to adapt to the global deep-freeze. They could have moved to ecological niches, inches under the soil or deep under the surface where water could be present, along with warmer temperatures.
The discovery of water in the Martian permafrost by NASA's Mars Phoenix Lander, and the suspected detection of methane in the Martian atmosphere from Earth-based observations, points to a potentially active Mars subsurface biosphere.
Whenever we do get around to sending another miniature biolab to the Red Planet, we'll hopefully have a much better understanding of what to test for.
Image credits: University of Padova, NASA