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The Biosignature Claim
Below a certain assembly index threshold, objects can plausibly arise through abiotic or random processes. Above the threshold, the combinatorial space of construction paths is too vast for random assembly; selection-like processes are needed to find and retain viable pathways. This is assembly theory’s primary empirical claim and its testable prediction: the assembly index gives a quantitative cut between objects that could form without selection and objects that could not.
The threshold
The experimentally determined threshold is approximately 15 steps, with a sharp transition between 13 and 15 in the molecular systems tested. Below 13, molecules appear in both biotic and abiotic samples. Above 15, they appear only in samples where selection-like processes have operated. The sharpness of the transition is part of what the theory presents as evidence — it is not a gradual fade but a cut.
The threshold is empirically grounded for the specific molecular systems tested: organic, covalently bonded molecules measured via mass spectrometry. Whether it transfers to other molecular families, other chemical regimes (ionic, mineral), and non-molecular objects is being established rather than settled. The universality of the threshold is part of what is contested in the mineral exchange.
Measurement techniques
Three techniques map molecular components to the theoretical assembly index.
Tandem mass spectrometry (MS/MS) is the most developed and most frequently invoked. High-assembly molecules fracture into more complex fragment mixtures than low-assembly molecules, giving a measurable signature. The fragmentation pattern under controlled conditions maps to the construction depth of the molecule.
Infrared spectroscopy provides complementary measurement by detecting the complexity of molecular vibrations, which correlates with structural complexity.
Nuclear magnetic resonance (NMR) offers a third measurement route through the complexity of atomic environments within the molecule.
All three count molecular components, which then map to the theoretical assembly index. Mass spectrometry carries the most published empirical work; the other two extend the measurement base.
Cronin: “A key feature of the theory is that it is experimentally testable.” The experimental measurability is presented as a genuine contribution beyond what abstract complexity measures provide — a point load-bearing in the AIT debate.
Experimental record
Demonstrated detection: high-assembly molecules in biological samples (E. coli cultures, taxol, beer, yeast); low-assembly in abiotic samples (minerals, simple organics). The biosignature threshold gives a quantitative cut across these systems.
The astrobiology application follows directly. If the assembly index is a reliable biosignature, it becomes a tool for life-detection missions — including Mars-sample analysis and broader astrobiological surveys. The appeal is that the measurement is substrate-agnostic in principle: it does not require knowing what kind of life produced the molecule, only that the molecule’s construction depth exceeds what random chemistry can produce.
What the threshold rests on
The threshold rests on the specific molecular systems tested and the specific measurement conditions (covalently bonded organic molecules measured via MS/MS). The transfer to other molecular families, other bonding regimes, and non-molecular objects is an open question — one that the mineral exchange engages directly.