Gathering evidence: the case for past life on Mars

Four billion years ago, the surface of Mars may have been conducive to the origin of life. Scientific evidence suggests that Mars had a biologically favorable atmosphere back then, with warm temperatures and abundant water during its formative years. Many scientists contend that life arose on early Mars but was subsequently wiped out. If Mars was a harbor for past life, why did it become extinct? Where should we, on future Mars missions, look for evidence of past life? Are there analogous biological systems on Earth to study and hypothesize the presence of early Martian life?