Molecular assembly as a universal biosignature measurable by mass spectrometry

Detecting life beyond Earth requires biosignatures that do not depend on the chemistry of known organisms. Molecular assembly (MA), derived from Assembly Theory, quantifies how difficult it is to build a molecule from basic building blocks, linking complexity directly to selection and evolution. Here, we show that MA can serve as a universal biosignature that is both interpretable and experimentally measurable. Unlike information-theoretic measures, MA can be inferred directly from mass spectrometry data without structural elucidation. We demonstrate this using a machine learning model trained on standardized single-stage (MS¹) spectra, which predicts MA with three-fold lower error than baseline methods. Simulated multistage (MSⁿ) data reveal that small instrumental variations can double prediction error, highlighting the importance of calibration. These findings establish molecular assembly as a physically grounded, quantifiable biosignature measurable by mass spectrometry whose interpretation depends on careful control of instrumental effects, offering a scalable route to life detection on future planetary missions.