What is Assembly Theory?

In essence, assembly theory posits that the complexity of a molecular system can be measured empirically with the construction of the molecular assembly index (MA). This theory has multiple applications and is derived from the “theory of assembly pathway.” The more unique pieces a certain molecule can fragment into, the higher its assembly number will be.


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A faction of the scientific community has agreed that biological processes give rise to more complex molecules. It is then assumed that these more intricate systems can act as universal biosignatures to detect where life has been. Asking the question, “what stages of complexity would be needed to create a specified molecule while overlooking the fundamental natural of chemistry?” grants scientists the opportunity to speculate biosignatures of life beyond our world. This measure of molecular complexity is being done today using mass spectrometry.

Assembly Theory Origins

Although assembly theory is often equated with designating chemical structure, the fundamental basis for this theorem is math. With origins in the “Theory of Assembly Pathway”, it postulates that assembly pathways, a sequence of joining operations, can end with a final product. They commence with a basic building block (which are bonds in this case) and end with a large structure of choice. Within these sequences, the sub-units amalgamate, forming larger units and so on, until a final structure is reached. Assembly theory decomposes a molecule into construction steps, yielding the assembly number.

If one were to build a specified molecule algorithmically a set number of times, soft evidence of life could be drawn. This molecule would intrinsically have astronomically low odds of its molecular composition being synthesized naturally. The general scientific populace does not expect to find life in close proximity to earth via assembly theory coupled with fragmentation tandem mass spectroscopy. Yet, we do intend to learn a vast amount about the abiotic chemistry that occurs on the surface of these celestial bodies.

Using remote spectroscopy and assembly theory, elucidations of the chemical composition of exoplanets can theoretically be done, exploring a new planetary frontier that was previously thought to be impossible.

How Assembly Theory is Being Used Today

The astronomers, aerospace engineers, and computer engineers of today have requested to explore the minerals of other celestial bodies using isotopic and atmospheric analysis. These analyses will generate raw data in some numerical form, to which assembly theory can be applied too. According to kinetic and thermodynamic considerations, relative rates of reaction will differ by orders of magnitude. It can then be inferred that the relative likelihood of abiotic events occurring will also differ in the order of magnitude and that proof of abiotic systems can be reached if one were to look in the right place and garner the right numbers.

A relic of assembly theory, one still credited today, can be found in Donald Hebb’s book “The organization of behavior (1949). In it, he asserts that repeated and assiduous stimulation of pre-synaptic and post-synaptic cells can increase synaptic efficacy. This was founded within “Cell Assembly Theory,” derived from the mathematical model of Harry Klopf.

In layman’s terms, cells that fire near each other and excite other neurons in a repetitive process will strengthen their signal with one another. One will develop a learned behavior/habit because these neurons fire in a probable and constant pattern. Though Pavlov avoided this notion while approaching his studies on memory, the theory is grounded in math and holds credibility. This is just another example of how this mathematical theorem can be applied to science on earth, not just within the depths of space.

Limitations of Assembly Theory

One shortcoming theory brings to the extraterrestrial front is that life has no universal definition as of now. This is because life on earth is our only data point. To characterize living and non-living entities, we look towards carbon and nitrogen fixation, chiral enrichment, photosynthesis, and morphogenesis. However, this may not be a fundamental truth of life. Perhaps not all forms of life were derived from a common ancestor, as Darwinian evolution stipulates.

Presuming that living systems of other worlds can be defined using earthly criteria would make for a nebulous experiment, though no other methodology can be found. Therefore, unearthing complex molecules that defy nature as we know it may be a fluke. This outlook deems that a complex molecule that might embody an enormous assembly number would prove obsolete.

Similar limitations of expounding upon the unknown are also present in cell assembly theory. The axioms in Donald Hebb’s theorem do not include all types of long-term plasticity. In it, he leaves out rules for inhibitory synapses and anti-causal spike sequences. Derivatives of assembly theory are often used in fields where proofs are difficult to access, and although the veracity they bring is somewhat grey, they still hold a great deal of value within the scientific community.

Future Applications of the Technology

Though assembly theory is often used in the context of designating complexity to justify extraterrestrial life, it is simply a method of measuring quantities. Code is written to put assembly theory into practice and can be used to distinguish the complexity of any entity, not just chemical bonds. Yet, scientists continuously endeavor to find patterns and truth behind these intricate bonds. One day, current astronomers hope to allocate premium mass spectrometers to Mars, Titan, and especially Venus, the closest analog we have to earth in terms of proximity.


  • Langille, J. J., & Brown, R. E. (2018). The Synaptic Theory of Memory: A Historical Survey and Reconciliation of Recent Opposition. Frontiers in systems neuroscience, 12, 52.
  • Hebb, D. O. (1949). The organization of behavior: A neuropsychological theory. New York: Wiley. ISBN: 0-8058-4300-0
  • Marshall stuart, Mathis C, Carrick E, Keenan G, Cooper G, Graham H, et al. Identifying Molecules as Biosignatures with Assembly Theory and Mass Spectrometry. ChemRxiv. Cambridge: Cambridge Open Engage; 2020; This content is a preprint and has not been peer-reviewed.
  • Marshall SM, Mathis C, Carrick E, Keenan G, Cooper GJT, Graham H, Craven M, Gromski PS, Moore DG, Walker SI, Cronin L. Identifying molecules as biosignatures with assembly theory and mass spectrometry. Nat Commun. 2021 May 24;12(1):3033.
  • Domagal-Goldman, S.D.; Wright, K.E.; Adamala, K.; de la Rubia, L.A.; Bond, J.; Dartnell, L.R.; Goldman, A.D. Lynch, K.; Naud, M.E.; Paulino-Lima, I.G.; et al. (2016) The Astrobiology Primer v2.0. Astrobiology, 16, 561–653

Further Reading

Last Updated: Jan 25, 2023

Vasco Medeiros

Written by

Vasco Medeiros

Obtaining an International Baccalaureate Degree at Oeiras International School, with higher levels in Chemistry, Biology, and Portuguese, Vasco Medeiros has just graduated from the University of Providence College with a Bachelor of Science. Before his work as an undergraduate, he first began his vocational training at the HIKMA Pharmaceuticals PLC plant in Ribeiro Novo. Here he worked as a validation specialist, tasked with monitoring the gauging and pressure equipment of the plant, as well as the inspection of weights and products.


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