Searching for dynamic biosignatures among lifelike chemical systems

A grand challenge in the search for alien life is to establish reliable detection strategies that are agnostic enough to detect life even as we don't know it. Several promising candidates for such agnostic biosignatures have been advanced so far, including pathway complexity metrics and the presence of heteropolymeric assemblies. To complement these strategies, we advocate for the exploration of dynamic biosignatures - features that are expected to change over time as a result of generic lifelike processes. In particular, we propose that research should focus on the ability of a system to sustain itself in the face of perturbation, to grow, and perhaps adapt. Our group is using theoretical and empirical approaches to explore the expected dynamic behavior of lifelike chemical entities, starting with the simplest systems that can show autocatalytic self-propagation. Theoretical models of realistic, complex autocatalytic networks suggest that they can be identified by periods of exponential increase of "seed" chemicals, which are chemicals present at vanishingly small initial concentrations, and exponential decline of "food" chemicals, namely the chemicals that can be converted to "seeds" by autocatalytic processes and are provided in an ongoing flux into the system. We have also developed an experimental approach called chemical ecosystem selection that uses messy chemical inputs and recursive dilution to detect complex, non-linear dynamics. The chemical ecosystem selection framework can be applied to a nearly infinite number of conditions, meaning that this bottom-up strategy has the potential to yield examples of systems that display complex dynamics ranging from simple autocatalysis to adaptive responses, which could provide an excellent yardstick for evaluating putative dynamic biosignatures. Importantly, the ecosystem selection approach of seeding, feeding, and diluting could be incorporated into existing science payloads and integrated into a suite of life detection tools used in future astrobiology missions. By focusing on dynamic patterns and system-level properties rather than specific chemical traces, this strategy will increase the likelihood of detecting life on other worlds, even if it is very different from life as we know it.