They begin by identifying three parameters that have to be met for any geological environment to be considered a likely setting for life to have started. First is “Origination,” or the environment’s ability to provide the molecular and mineral components that enhanced the chemical reactions leading to life. Second is “Complexiﬁcation,” or the ability of the environment to sustain continued chemical reactions and build up a suite of diverse organic building blocks. Finally, there is “Plausibility,” which asks whether the specific geological environment was likely to have existed when life originated between roughly 4.4 billion and 3.8 billion years ago.
Using these criteria, the authors then consider various suggested environments for the origin of life such as hydrothermal vents, volcanic-hosted splash pools, and vesicles, or cavities, in volcanic pumice rocks. Finally, they make the case for an environment that hadn’t been suggested in the past: the sedimentary layer between oceanic crust and seawater.
During the Hadean time period, more than four billion years ago, the Earth was much more active than it is today, and hot water percolated through Earth’s crust in many places. The submarine crust, being covered by water, would have protected any primitive organisms from ultraviolet radiation. The porosity and chemical reactivity of the sediments between the crust and the seawater are critical in Westall’s model, as it is thought to have led to miniature “chemical reactors” that enhanced formation of the organic building blocks needed for life.
While I applaud the thorough analysis done by Westall and her co-authors, personally I’m still skeptical of a submarine origin for life. My thinking is that cycles of drying and wetting are much more powerful for accumulating organic building blocks, and these would more likely occur at water-land boundary zones than in a submarine setting. Also, while protection from ultraviolet radiation is certainly an advantage for life today, it may not have been as important for early life. Many organic reactions are actually enhanced by ultraviolet radiation.
Thus, we are still far from solving the riddle of where life originated on Earth. The answer to that question has huge implications. If it occurred in sub-ocean sediments, as Westall and colleagues suggest, then life might only arise in ocean worlds where we also find land areas undergoing erosion. That may rule out water worlds like Enceladus or Europa. But if life started near hydrothermal vents, those two icy moons may have given rise to life after all.