In 1976 NASA ran the only experiment built to test Mars for life, and it came back positive.
Viking fed Martian soil a radioactive soup, and the soil breathed out gas, just as microbes would. Then a salt found in 2008 offered a chemical explanation, and the case was closed. A discovery in Chile's Atacama Desert led one astrobiologist to a darker reading of the same data. It is a hypothesis, not a finding; most scientists still favour chemistry.
Sources: Levin & Straat, "Viking labeled release
