A team of researchers from NASA's Mars Science Laboratory Curiosity rover mission reported the discovery of evidence that a lake existed on Mars 3.6 billion years ago that may have sustained primitive microbial life in the Dec. 9, 2013, issue of the journal Science.
The scientists based their conclusion that life may have existed on Mars on the chemical analysis of a set of sedimentary rock outcrops at a site named Yellowknife Bay in Gale Crater, near the Martian equator. Gale Crater is a 90 mile wide impact crater with a mountain at its center.
The scientists determined the lake had fresh water and may have lasted for hundreds of thousands of years. The lake may have dried up and been refilled numerous times based on the present evidence from the geological strata.
The presence of water and the direct evidence that water did exist on Mars in the ancient past as well as the presence of carbon, hydrogen, oxygen, nitrogen, and sulfur in the same location are sufficient evidence to conclude that microbial life of some nature may have existed on Mars at one time.
The chemical and geological evidence are not sufficient in themselves to support the claim that life did exist on Mars in some primitive form 3.6 billion years ago or at any other time.
The presence of an impact crater with all the necessities to produce a simple life form like life on Earth is highly possible. If that life form did exist, it could have been deposited by the asteroid or comet that created Gale Crater.