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Development

Assembly theory is the joint work of Lee Cronin and Sara Imari Walker, developed from 2017 onward with contributions from collaborators at Glasgow, Arizona State, and the Santa Fe Institute. This page holds the history, the people, and the reference list.

Origin of the collaboration

Lee Cronin’s work on chemical complexity and self-assembling chemical systems at the University of Glasgow in the 2000s and early 2010s set the ground — the question of how complex molecules arise from simpler ones, and whether the complexity of a molecule can be quantified through its construction history. Sara Imari Walker’s astrobiology and origin-of-life work at Arizona State University set the question from the other end — what distinguishes biological from abiotic complexity, and whether the distinction can be made measurable.

They met at a NASA astrobiology workshop in 2012 and began collaborating. Joint development of assembly theory proper started from 2017 with the pathway complexity paper.

Development sequence

Marshall, Murray, Cronin (2017), Phil. Trans. R. Soc. A 375: “A probabilistic framework for identifying biosignatures using Pathway Complexity” — introduced the pathway complexity concept for molecular objects.
Mathis, Patarroyo, Cronin (2021): “Understanding assembly indices” — accessible technical introduction to the formal apparatus.
Marshall et al. (2021), Nature Communications: “Identifying molecules as biosignatures with assembly theory and mass spectrometry” — extended to assembly theory proper; demonstrated mass spectrometry measurement.
Marshall et al. (2022), Entropy 24: “Formalising the pathways to life using assembly spaces” — formal extension of the mathematical framework.
Walker and Cronin (2023), Aeon: “Time is an object” — public-facing essay laying out the metaphysical claims.
Sharma, Czégel, Lachmann, Kempes, Walker, Cronin (2023), Nature 622: “Assembly theory explains and quantifies selection and evolution” — the high-profile paper unifying the framework’s empirical and theoretical claims.
Walker (2024), Life As No One Knows It (Riverhead) — Walker’s book-length popular treatment.
“Assembly theory and its relationship with computational complexity” (npj Complexity, 2025) — the theory side’s formal response to the algorithmic information theory critique.

Persons

Lee Cronin. Regius Chair of Chemistry at the University of Glasgow; CEO of Chemify, the chemistry-automation company he founded. Background in inorganic chemistry, particularly polyoxometalates and self-assembling chemical systems. Cronin’s laboratory work on chemical complexity — building and measuring increasingly complex molecular systems — provided the empirical base from which assembly theory’s formal apparatus grew.

Sara Imari Walker. Theoretical physicist and astrobiologist. Professor in the School of Earth and Space Exploration at Arizona State University; Deputy Director of the Beyond Center for Fundamental Concepts in Science; external professor at the Santa Fe Institute; Berggruen Institute fellow. Background in theoretical physics with focus on origin of life. Walker brought the astrobiology framing — the question of what life is and how to detect it — and the broader metaphysical ambition that extended assembly theory from a measurement tool into a proposed account of time and complexity.

Stuart Marshall. Cronin’s PhD student and postdoc; lead author on several foundational papers including the 2017 pathway complexity paper and the 2021 mass spectrometry paper. Carried much of the early formal and experimental development.

Abhishek Sharma. Lead author of the 2023 Nature paper, working on the ASU side. Contributed to the formal unification of the framework’s claims.

Cole Mathis. Chemist working with Cronin at Glasgow. Key explainer and contributor: “Understanding assembly indices” (2021) introduced the formal apparatus accessibly; “On the Salient Misunderstandings of Assembly Theory” (2022) responded directly to early critiques and common misreadings.

The Santa Fe team. Dániel Czégel, Michael Lachmann, and Christopher Kempes contributed to the 2023 Nature paper, connecting assembly theory to the broader complexity-science tradition at the Santa Fe Institute.

Key works

Primary theory papers:

Marshall, Murray, Cronin, “A probabilistic framework for identifying biosignatures using Pathway Complexity,” Phil. Trans. R. Soc. A 375 (2017)
Marshall et al., “Identifying molecules as biosignatures with assembly theory and mass spectrometry,” Nature Communications (2021)
Marshall et al., “Formalising the pathways to life using assembly spaces,” Entropy 24 (2022)
Sharma et al., “Assembly theory explains and quantifies selection and evolution,” Nature 622 (2023)

Defending and explaining:

Mathis, Patarroyo, Cronin, “Understanding assembly indices” (2021)
Mathis, “On the Salient Misunderstandings of Assembly Theory” (2022)
“Assembly theory and its relationship with computational complexity,” npj Complexity (2025)

Accessible and public-facing:

Walker and Cronin, “Time is an object,” Aeon (2023)
Walker, Life As No One Knows It (Riverhead, 2024)
“A New Theory for the Assembly of Life in the Universe,” Quanta Magazine (2023)
Walker, Long Now talk (2025)

Critique side: see The algorithmic information theory debate, The mineral exchange, and Other reception for the critique-side reference lists.