As the environmental histories of Earth and Mars have diverged drastically after the first few hundred million years, so would the history of any life on them. While Earth has had liquid water on its surface for billions of years, Mars most likely had long dry and cold environments interspersed with warmer and wetter periods. Life thrived on Earth but may have been severely restricted on Mars. There it could be present today in liquid water in or beneath ice sheets or glaciers, in subterranean aqueous reservoirs, especially in regions of elevated heat flow, or in protected habitats such as lava tubes, caves, or cracks and fissures. The potential for life is enhanced in regions where elevated heat flow may occur, such as in parts of the Tharsis and Elysium volcanic provinces. Possible organisms would be chemoautotrophic psychrophiles adapted to a nutrient-poor environment or photoautotrophic life in selected near-surface habitats. Alternatively, life may have evolved alternating cycles between active and dormant forms, in which case microbes could be present in dormant forms close to the surface and in active forms in protected environments. Periodic liquid water on Mars could have provided opportunities for biologic activity, as well as evolutionary progress, at the surface during the short-lived climatic perturbations. Ancient organisms from any of these environments may also have left a detectable fossil record.