One of the goals of the Curiosity Rover is to try to find evidence indicating whether life ever existed on Mars. Thanks to its work so far, we know that Mars still retains water in its surface soil. Scientists also know that Mars used to be warmer and wetter than it is now. Examinations of meteorites of Martian origin reveal organic compounds, suggesting not only that conditions there were once favorable, but that the planet did indeed harbor life. Now, Curiosity has found additional evidence supporting both of those hypotheses.
In order to determine whether life has ever existed on Mars, scientists have been looking for evidence of three criteria: water, life-supporting elements (carbon, oxygen, hydrogen, nitrogen, phosphorus), and a long enough period of time in which those conditions were present. It’s that last part that has proven problematic, as previous estimates about the amount of time when those conditions existed was in the neighborhood of hundreds of thousands of years—not very long at all when it comes to creating life out of chemical soup.