The Triple Lock: A Model of Sequential Bio-Geochemical Filters on the Path to Complex Life

We propose a multi-layered mechanistic framework explaining the rarity of complex life in the Universe through three sequential, causally linked barriers. Building
on recent work on evolutionary priority effects in phototrophy (Burnetti et al., 2026), Great Oxidation modeling (Horne et al., 2025), and cellular bioenergetics (Lane &
Martin, 2010; Lynch & Marinov, 2015), we identify three filters. Filter 1 (spectral): the host star’s spectrum sets the initial phototrophic system and the biosphere’s en-
ergy budget. Filter 2 (geochemical): oxygenic photosynthesis triggers phosphorus sequestration; overcoming the trap requires tectonic weathering and ocean–mantle
contact. Filter 3 (complexity): high stellar XUV activity forces life into deep refugia, closing the energy window for eukaryogenesis. The filters interact via feedbacks,
forming a network rather than a simple chain (Fig. 1). We apply the framework to eight worlds—Earth, TRAPPIST-1e, Kepler-442b, Proxima Centauri b, ancient
Mars, K2-18b, LHS 1140 b, and Europa—revealing five qualitatively distinct failure patterns. An illustrative calculation yields probabilities spanning five orders of mag-
nitude, from ∼5 % (Kepler-442b) to < 10−7 (K2-18b). The framework is falsifiable and offers predictions for next-generation telescopes (JWST, HWO, LIFE).