Spatial patterning and selection: How the environment shapes molecular complexity

Assembly theory predicts that a distinguishing signature of life is its ability to produce complex molecules in abundance, opening new possibilities for life detection. Experimental validation of this approach has so far relied on abiotic controls like meteoritic material, or simple, well-mixed chemical systems. However, decades of research in self-organization have shown that spatial patterning can foster dynamical complexity. This raises the possibility that systems with spatial patterns might promote abiotic formation of molecules with higher-than-expected assembly indices-a measure defined within assembly theory to assess structural complexity-potentially leading to false positives in life detection approaches based on this framework. To probe this possibility, we compare how assembly indices in a simplified in silico chemistry scale relative to those in molecular space, and then ask how spatial organization shapes their distribution in our model. Our findings reveal that transport factors, such as diffusion, significantly affect the distribution of chemical species within a system. Additionally, spatial topology markedly shapes the distribution of assembly indices: while it does not enable arbitrary complexity, ordered lattices in particular can shift the assembly indices of abiotic chemistry upward. Moreover, we demonstrate that diffusion can impede the formation and detectable upper limit of these high assembly index molecules, bearing important implications for life detection experiments and astrobiological missions.