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The Mineral Exchange
The surface disagreement is technical: can inorganic minerals exceed the biosignature threshold? Beneath it sits a deeper conceptual question that makes this exchange productive rather than merely adversarial. Both Hazen and Walker/Cronin hold that construction history is materially load-bearing for complex objects; they disagree on where the biosignature cut falls and what counts as the right kind of measurement.
Hazen’s broader programme
Robert Hazen (Carnegie Science, Smithsonian) and collaborators have developed an account of mineral evolution: the evolutionary system of mineralogy. Earth’s 5,000+ mineral species could not have arisen from chondrite-style chemistry alone. Minerals diversified through stages over Earth’s history, driven by progressively richer geological conditions — initial cooling, water cycles, plate tectonics, mantle activity, oxygenation (driven by life), and eventually biology acting back on mineralogy.
The diversification required enabling conditions to accumulate over geological time. This is a path-matters claim about minerals, structurally similar to assembly theory’s path-matters claim about molecules. The intellectual sympathy runs deeper than the specific dispute about threshold values.
The critique
Hazen, Burns, Cleaves, Downs, Krivovichev, Wong (2024), J. R. Soc. Interface 21, 20230632: “Molecular assembly indices of mineral heteropolyanions: some abiotic molecules are as complex as large biomolecules.”
Certain inorganic mineral subunits — ewingite, ilmajokite, and others — have calculated assembly indices up to 21, well above the biosignature threshold of 15. These are abiotic: produced by geological processes without biological involvement. If the assembly index is supposed to distinguish biological from abiotic complexity, minerals that score above the threshold are direct counterexamples.
The response
Walker, Sharma, Cronin et al. (2024), same journal, 20240367: “Experimentally measured assembly indices are required to determine the threshold for life.”
Two main points. First, the Hazen calculations are theoretical — derived from crystal structures of charged units not isolable in solution. The assembly theory authors’ assembly indices are experimentally measured for organic, covalently bonded molecules. The distinction between calculated and measured assembly indices is methodologically significant.
Second, the mineral heteropolyanions in question are charged subunits within crystal structures, not free-standing molecules. Comparing their theoretical assembly indices with experimentally measured covalent-bond assembly indices is comparing across different chemical regimes — the comparison is of limited utility because the objects are only superficially alike.
The reply
Hazen et al. (2024), same journal, reply to Walker et al.’s response.
The reply maintains the substantive challenge. Mineral subunits’ assembly pathways of up to approximately 25 steps inform a central premise of assembly theory — that only life produces numerous copies of molecules above a threshold value. What that threshold value is, and whether it applies across different chemical regimes (organic and inorganic, covalent and ionic), are questions deserving additional study.
The reply does not concede that the theoretical-vs-measured distinction resolves the issue; it notes that theoretical assembly index calculations are a legitimate way to explore the framework’s scope and that the restriction to experimentally measured covalent-bond assembly indices may be too narrow a base for the claims being made.
The deeper question
What about geological selection — processes operating at the geological and mineralogical layer (tectonics, mantle activity, water cycles, oxidation chemistry) — producing complexity without biology? If minerals can accumulate construction depth through purely geological processes, the biosignature claim needs either a sharper threshold or a different kind of boundary.
This is a productive question for assembly theory to engage, not just an attack to deflect. Hazen’s evolutionary mineralogy gives it substance: the mineral kingdom has its own history of diversification, driven by accumulating geological conditions, and the question of whether that history produces complexity on the same scale as biological selection is open.
The shape of the exchange
Both camps share the underlying intuition that construction history is materially significant. They differ on the sharpness of the biosignature threshold and on whether the threshold transfers across chemical regimes. The exchange has run through three rounds in J. R. Soc. Interface and continues. The threshold value and its applicability across organic and inorganic domains remains an open scientific question.