Assembly theory

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Synthesis of aristolochic acid. Complex molecules require many steps to be synthesized. And the more steps are required to synthesize a particular molecule, the more likely it is of a biological (or technological) origin.

Assembly theory is a hypothesis that characterizes object complexity. When applied to molecule complexity, its authors claim it to be the first technique that is experimentally verifiable, unlike other molecular complexity algorithms that lack experimental measure.[1] The theory was developed as a means to detect evidence of extraterrestrial life from data gathered by astronomical observations or probes.[2]


The hypothesis was proposed by chemist Leroy Cronin in 2017 and developed by the team he leads at the University of Glasgow,[3][4][5] then extended in collaboration with a team at Arizona State University led by astrobiologist Sara Imari Walker, in a paper released in 2021.[6][7] It is difficult to identify chemical signatures that are unique to life.[5][8] For example, the Viking lander biological experiments detected molecules that could be explained by either living or natural non-living processes.[5][9][10]

Assembly theory outputs how complex a given object is as a function of the number of independent parts and their abundances. To calculate how complex an item is, it is recursively divided into its component parts. The 'assembly index' is defined as the shortest path to put the object back together.[1]

For example, the word 'abracadabra' contains 5 unique letters (a, b, c, d and r) and is 11 symbols long. It can be assembled from its constituents as a + b --> ab + r --> abr + a --> abra + c --> abrac + a --> abraca + d --> abracad + abra --> abracadabra, because 'abra' was already constructed at an earlier stage. Because this requires 7 steps, the assembly index is 7. The string ‘abcdefghijk’ has no repeats so has an assembly index of 10.

While other approaches can provide a measure of complexity, the researchers claim that assembly theory's molecular assembly number is the first to be measurable experimentally. They argue that the molecular assembly number can be used to gauge the improbability that a complex molecule was created without life, with a higher number of steps corresponding to a higher improbability.[5] This method could be implemented in a fragmentation tandem mass spectrometry instrument to search for biosignatures.[5] Cronin stated "Our system is the first falsifiable hypothesis for life detection and is based on the idea that only living systems can produce complex molecules that could not form randomly in any abundance, and this allows us to sidestep the problem of defining life."[6]

The theory was extended to map chemical space with molecular assembly trees. These trees were formed by arranging constituent pieces in size order. When two or more molecules have common units, their trees are combined, including the two target molecules and various hybrids.[11][3]

Critical views[edit]

A researcher from Harvard University and a group led by a Cambridge University researcher have criticized assembly theory as being grounded in fallacies and inappropriately hyped by its creators.[12] The group affiliated with Oxford and Cambridge reproduced all the results of assembly theory with traditional statistical algorithms,[13] including Huffman coding that counts "copies" more effectively than assembly theory does. Due to the issues that these researchers identified, critics advocate against a simplistic approach to life based on what they say is an ill-defined measure.[14]

See also[edit]


  1. ^ a b Marshall SM, Mathis C, Carrick E, et al. (24 May 2021). "Identifying molecules as biosignatures with assembly theory and mass spectrometry". Nature Communications. 12 (3033): 3033. Bibcode:2021NatCo..12.3033M. doi:10.1038/s41467-021-23258-x. PMC 8144626. PMID 34031398.
  2. ^ Smith, Adam (25 May 2021). "Scientists create 'life detection' tool help hunt down alien life". The Independent.
  3. ^ a b Welter, Kira (13 October 2021). "Exploiting evolution to explore chemical space shows promise for drug discovery". Chemistry World. Royal Society of Chemistry.
  4. ^ Marshall, Stuart M.; Murray, Alastair R. G.; Cronin, Leroy (2017). "A probabilistic framework for identifying biosignatures using Pathway Complexity". Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 375 (2109). arXiv:1705.03460. Bibcode:2017RSPTA.37560342M. doi:10.1098/rsta.2016.0342. PMC 5686400. PMID 29133442.
  5. ^ a b c d e "Complex molecules could hold the secret to identifying alien life" (Press release). University of Glasgow. 24 May 2021. Retrieved 30 July 2021.
  6. ^ a b Baptista, Kim (28 May 2021). "Scientists develop new molecular tool to detect alien life" (Press release). Arizona State University.
  7. ^ Sara Imari Walker; Leroy Cronin; Alexa Drew; Shawn Domagal-Goldman; Theresa Fisher; Michael Line; Camerian Millsaps (7 April 2019). "Probabilistic Biosignature Frameworks". In Victoria Meadows; Giada Arney; Britney Schmidt; David J. Des Marais (eds.). Planetary Astrobiology. doi:10.2458/azu_uapress_9780816540068-ch018.
  8. ^ Schwieterman, Edward W.; Kiang, Nancy Y.; Parenteau, Mary N.; Harman, Chester E.; Dassarma, Shiladitya; Fisher, Theresa M.; Arney, Giada N.; Hartnett, Hilairy E.; Reinhard, Christopher T.; Olson, Stephanie L.; Meadows, Victoria S.; Cockell, Charles S.; Walker, Sara I.; Grenfell, John Lee; Hegde, Siddharth; Rugheimer, Sarah; Hu, Renyu; Lyons, Timothy W. (2018). "Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life". Astrobiology. 18 (6): 663–708. arXiv:1705.05791. Bibcode:2018AsBio..18..663S. doi:10.1089/ast.2017.1729. PMC 6016574. PMID 29727196.
  9. ^ NASA Astrobiology (2 September 2021). Universal Life Detection: Astrobiology & Assembly Theory. YouTube.
  10. ^ Plaxco KW, Gross M (12 August 2011). Astrobiology: A Brief Introduction. JHU Press. pp. 285–286. ISBN 978-1-4214-0194-2. Retrieved 16 July 2013.
  11. ^ Liu, Yu; Mathis, Cole; Bajczyk, Michał Dariusz; Marshall, Stuart M.; Wilbraham, Liam; Cronin, Leroy (2021). "Exploring and mapping chemical space with molecular assembly trees". Science Advances. 7 (39): eabj2465. Bibcode:2021SciA....7J2465L. doi:10.1126/sciadv.abj2465. PMC 8462901. PMID 34559562.
  12. ^ Zenil, Hector. "The 8 Fallacies of Assembly Theory – Medium". Retrieved 26 September 2023.
  13. ^ Uthamacumaran, Abicumaran; Abrahão, Felipe S.; Kiani, Narsis; Zenil, Hector (2022). "On the Salient Limitations of the Methods of Assembly Theory and their Classification of Molecular Biosignatures". arXiv:2210.00901 [cs.IT].
  14. ^ Benner, Steven A. "Assembly Theory and Agnostic Life Finding – The Primordial Scoop". Retrieved 19 September 2023.

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