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One of the biggest challenges in the search for alien life is that Earth remains the only example of life we know. As a result, nearly every attempt to identify biology elsewhere starts, at least partly, by looking for chemistry that resembles life here.
Researchers at the University of Glasgow are exploring a different strategy. Rather than asking whether an alien molecule looks biological by Earth standards, they propose asking a more fundamental question: How difficult was that molecule to make?
In a new study published in the Proceedings of the National Academy of Sciences, researchers demonstrate a method that combines mass spectrometry with machine learning to estimate what researchers call “molecular assembly,” or MA.
The concept comes from Assembly Theory, which attempts to quantify molecular complexity by identifying the minimum number of construction steps needed to build a molecule. Researchers argue that when highly complex molecules appear repeatedly and in abundance, their existence may point to a process that is consistently steering chemistry down particular pathways.
That could make molecular assembly useful as an “agnostic” biosignature—one that does not depend on alien life sharing Earth’s chemistry.
A novel framework suggests life might be revealed not by single clues, but by subtle patterns spanning many worlds, offering a different lens for detecting life in the universe.
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“There is a growing interest in ‘agnostic’ life detection strategies: methods that can identify molecular signs of life without needing an exact understanding of what that life might look like, or how it might interact with a complex, poorly characterized environment,” the researchers write. “The measured assembly index, defined as the number of steps along the shortest construction pathway, is fixed for each molecule and does not depend on external conditions, such as the environment in which the molecule might be detected or the specific ‘alien’ biochemistry from which it might arise.”
The researchers’ proposed “agnostic” approach could become increasingly important as scientists encounter tantalizing, but often ambiguous, chemical signals in the search for alien life.
Methane on Mars, phosphine reported in Venus’s atmosphere, and dimethyl sulfide proposed as a possible biosignature within exoplanet atmospheres have all attracted attention because living organisms can produce those compounds.
However, biology is not the only way to make them. Methane, for example, can emerge from geologic processes. Dimethyl sulfide is strongly associated with life on Earth, yet it has also been detected in cometary material and produced in laboratory experiments simulating primordial chemistry.
The presence of a particular molecule, in other words, does not necessarily tell scientists what produced it. Molecular assembly approaches the problem from a different angle. Under Assembly Theory, researchers calculate the shortest possible pathway for constructing a molecule from simpler building blocks. A molecule with an MA of 40, for example, has a shortest construction pathway twice as long as one with an MA of 20.
The reasoning is rooted in probability. Undirected chemistry has an enormous range of possible molecular structures. As molecules become more complex, the odds of repeatedly producing the same highly elaborate structure shrink unless something constrains the process.
Biological systems store and use information to direct chemical reactions toward particular outcomes, permitting organisms to manufacture the same complicated molecules again and again instead of wandering randomly through the immense space of possible chemistry.
Previous experiments cited in the study found an approximate dividing line between abiotic and biological samples at an assembly index of around 15.
Perhaps more importantly, in the search for alien life, scientists would not necessarily need to know what a molecule is before recognizing that it is unusually complex.
That could be especially useful on worlds such as Mars, Europa, Enceladus, or Titan, where spacecraft may encounter compounds unlike anything commonly found on Earth.
Several proposed planetary missions use, or could use, mass spectrometers. These instruments analyze molecules by measuring ions’ mass-to-charge ratios.
More sophisticated multistage mass spectrometers can repeatedly break molecules apart, giving indications of their structure and assembly history. But spacecraft operate under severe limits on power, mass, computing capacity, and instrument complexity. Many planetary missions, therefore, rely on simpler, single-stage mass spectrometry.
To close that gap, the researchers trained an XGBoost model on standardized mass spectra from the National Institute of Standards and Technology Chemistry WebBook. The model learned to estimate molecular assembly directly from single-stage spectra.
According to the study, it reduced relative mean squared error threefold compared with the best-performing baseline model, cutting the figure from 0.12 to 0.04.
The model also tended to underestimate molecules with especially high MA values. For a life-detection system, that bias could be preferable. It means the model was more likely to be conservative than to classify a molecule as an unusually strong biosignature mistakenly.
Researchers envision future spacecraft using this kind of analysis autonomously. A rover or lander that detects molecules with unusually high assembly scores might flag the surrounding area for closer inspection, prioritize certain samples, or identify material worth returning to Earth.
The study explicitly evaluated conditions designed to resemble instruments proposed for missions to Titan and Europa.
The same framework also raised another, more speculative possibility: when the researchers compared very large molecular databases, they identified several medical compounds with unusually high MA values. Their complexity was not the product of biology alone, but of deliberate human synthesis.
That led the team to suggest that particularly complex artificial molecules might serve as “molecular technosignatures”—chemical evidence oftechnology, rather than life itself.
For now, however, the method remains far from being a ready-to-fly alien-life detector. Instrument settings strongly affected performance. In simulated experiments, changing impact energies could nearly double prediction error, while one comparison increased it by more than 2.3 times. Molecules that break apart unusually easily can also cause the model to overestimate assembly, while molecules that resist fragmentation may be underestimated.
The researchers therefore describe the study as a proof of concept. Turning the approach into an effective tool for planetary investigation would require standardized databases, careful instrument calibration, and chromatographic separation to isolate individual molecules before analysis.
Future versions could also combine mass spectrometry with other techniques, including nuclear magnetic resonance or infrared spectroscopy. Infrared measurements, the researchers suggest, extend aspects of the approach to the remote study of exoplanet atmospheres.
Yet the wider goal is significant. Instead of deciding beforehand what alien biology should produce, scientists could search for evidence that a process has persistently directed chemistry toward highly specific, complex outcomes.
“In this work, we presented a proof-of-concept demonstrating the potential use of ML to predict the MA molecular complexity as a biosignature from in situ mass spectrometers during future space missions,” the researchers write. “Although the application of ML in life detection remains a nascent endeavor, it holds significant promise.”
The recent study, “Molecular Assembly as a Universal Biosignature Measurable by Mass Spectrometry,” appeared in Proceedings of the National Academy of Sciences.
- Tim McMillan is a retired law enforcement executive, investigative reporter and co-founder of The Debrief. His writing typically focuses on defense, national security, the Intelligence Community and topics related to psychology. You can follow Tim on Twitter: @LtTimMcMillan. Tim can be reached by email: tim@thedebrief.org or through encrypted email: LtTimMcMillan@protonmail.com
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