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] This theory suggests that evolution abides to well defined physical laws, conceptualising objects not as point particles, but as entities defined by their possible formation histories. Thus introducing an assembly index (A), capturing the degree of causation required to produce a given ensemble of objects. ^[2] The theory was developed as a means to detect evidence of extraterrestrial life from data gathered by astronomical observations or probes.^[3]

Background

The hypothesis was proposed by chemist Leroy Cronin in 2017 and developed by the team he leads at the University of Glasgow,^[4][5][6] then extended in collaboration with a team at Arizona State University led by astrobiologist Sara Imari Walker, in a paper released in 2021.^[7][8] It is difficult to identify chemical signatures that are unique to life.^[6][9] For example, the Viking lander biological experiments detected molecules that could be explained by either living or natural non-living processes.^[6][10][11]

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]

${\displaystyle A=\mathop {\sum } \limits _{i=1}^{N}{e}^{{a}_{i}}\left({\frac {{n}_{i}-1}{{N}_{\rm {T}}}}\right)}$

Where ${\displaystyle A}$ is the assembly of the ensemble, ${\displaystyle a_{i}}$ is the assembly index of object ${\displaystyle i}$, ${\displaystyle n_{i}}$ is the object's copy number, ${\displaystyle N}$ is the total number of unique objects, ${\displaystyle e}$ is Euler's number, and ${\displaystyle N_{T}}$ is the total number of objects in the ensemble.^[2]

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.^[6] This method could be implemented in a fragmentation tandem mass spectrometry instrument to search for biosignatures.^[6] 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."^[7]

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.^[12][4]

The theory was described in detail as a framework that does not alter the laws of physics, but redefines the concept of an ‘object’ on which these laws act. Assembly Theory (AT) conceptualizes objects not as point particles, but as entities defined by their possible formation histories. This allows objects to show evidence of selection, within well-defined boundaries of individuals or selected units. A measure is introduced called assembly (A), capturing the degree of causation required to produce a given ensemble of objects. This approach means it is possible to incorporate novelty generation and selection into the physics of complex objects. It explains how these objects can be characterized through a forward dynamical process considering their assembly. By reimagining the concept of matter within assembly spaces, AT provides a powerful interface between physics and biology. It discloses a new aspect of physics emerging at the chemical scale, whereby history and causal contingency influence what exists.^[13]

See also

• List of interstellar and circumstellar molecules
• Word problem for groups

References

Further reading

• Cronin, Leroy; Walker, Sara Imari (3 June 2016). "Beyond prebiotic chemistry". Science. 352 (6290): 1174–1175. Bibcode:2016Sci...352.1174C. doi:10.1126/science.aaf6310. ISSN 0036-8075. PMID 27257242. S2CID 206649123.
• Cronin, Leroy; Krasnogor, Natalio; Davis, Benjamin G.; Alexander, Cameron; Robertson, Neil; Steinke, Joachim H. G.; Schroeder, Sven L. M.; Khlobystov, Andrei N.; Cooper, Geoff; Gardner, Paul M.; Siepmann, Peter (2006). "The imitation game—a computational chemical approach to recognizing life". Nature Biotechnology. 24 (10): 1203–1206. doi:10.1038/nbt1006-1203. ISSN 1546-1696. PMID 17033651. S2CID 4664573.
• Ball, Philip (4 May 2023). "A New Theory for the Assembly of Life in the Universe". Quanta Magazine.

External links

• Finding Life by Looking for Complexity: Interview with Leroy Cronin about assembly theory on Planetary Radio broadcast, 30 June 2021.
• Lee Cronin: Origin of Life, Aliens, Complexity, and Consciousness: Interview with Leroy Cronin in which assembly theory is discussed (starting at 1:27:23) on the Lex Fridman Podcast.