Resolving the History of Life on Earth by Seeking Life As We Know It on Mars

Abstract

An origin of Earth life on Mars would resolve significant inconsistencies between the inferred history of life and Earth's geologic history. Life as we know it utilizes amino acids, nucleic acids, and lipids for the metabolic, informational, and compartment-forming subsystems of a cell. Such building blocks may have formed simultaneously from cyanosulfidic chemical precursors in a planetary surface scenario involving ultraviolet light, wet-dry cycling, and volcanism. On the inferred water world of early Earth, such an origin would have been limited to volcanic island hotspots. A cyanosulfidic origin of life could have taken place on Mars via photoredox chemistry, facilitated by orders-of-magnitude more sub-aerial crust than early Earth, and an earlier transition to oxidative conditions that could have been involved in final fixation of the genetic code. Meteoritic bombardment may have generated transient habitable environments and ejected and transferred life to Earth. Ongoing and future missions to Mars offer an unprecedented opportunity to confirm or refute evidence consistent with a cyanosulfidic origin of life on Mars, search for evidence of ancient life, and constrain the evolution of Mars' oxidation state over time. We should seek to prove or refute a martian origin for life on Earth alongside other possibilities.

Keywords: Cyanosulfidic origin of life; Lithopanspermia; RNA world.

FIG. 1.

Planetary context for a hypothesized cyanosulfidic origin of life on Mars and its transport to Earth via lithopanspermia. The horizonal axis for all panels is billions of years ago (Ga). (A) Early Mars had orders-of-magnitude more land area available to support a cyanosulfidic origin of life. (B) Life arising on early Mars would have been protected by a magnetic field and thicker atmosphere (∼0.5 bar) than at present. The lack of a rock record prevents similar knowledge of early Earth, although it is likely Earth's dynamo had started by 3.4–3.45 Ga (Tarduno et al., 2010). (C) Impact cratering on early Mars, especially from ∼4.2 to ∼3.8 Ga, would have provided both habitable environments for life to arise as well as facilitate its transport to Earth via meteoritic ejecta. Volcanism would have played a role in facilitating a cyanosulfidic origin of life (Sasselov et al., 2020) in combination with hydrothermal subsurface activity and ephemeral surface waters. The aqueous record of this time and the geochemical transformation of Mars is recorded by the presence of hydrated silicates and salts, with sulfates as a particularly important record of early oxidation on Mars in comparison to Earth, where widespread oxidation occurred over a billion years later. (D) If a cyanosulfidic origin of life occurred on early Mars, such life could have been transferred to Earth around the time of, but not necessarily coincident with, the last universal common ancestor (LUCA). For example, LUCA may have existed on Mars, and life on Earth could have arrived via multiple transfer events. Estimated timing of Mars' processes are as reported by Ehlmann et al. (2011).