FIG. 1.

Framework for a coevolution on Mars—uncertainties about the origin(s) of life on Earth and the early martian environment, and the use of Earth's chronology for early life in the context of Mars, limit any notional coevolution model. Here, the variables are the plausible environments of origin(s) for life on Mars in view of terrestrial theories and what is known of the evolution of habitability on Mars. The fix parameter is life as we know it, with the chronology provided by discoveries made on Earth to date. We assume a timing for the earliest evidence of life consistent with Earth at 4.28 Ga (references in the text) and focus on early life colonization and dispersal pathways from these environments of origin.