Rethinking 'What is Life'—The 'Difficulty of Assembly' of Molecules Becomes a New Map for Searching for Extraterrestrials
What does it mean to be 'alive'?
No one can answer this question properly. Is it having cells? Self-replication? Metabolizing energy? No matter which definition you bring up, an exception eventually appears. Is a virus alive or not? Can organic molecules found on Mars be evidence of life?
In fact, there are scientists now tackling this age-old question from a completely new angle. It is 'Assembly Theory' proposed by Professor Lee Cronin of the University of Glasgow and Professor Sara Walker of Arizona State University.
I have introduced this theory before, so if you are interested, please refer to that as well.
Molecules have 'history' engraved in them
To express the core of Assembly Theory in one word again, it is 'measuring the complexity of a molecule by the minimum number of assembly steps'.
Water molecules (H₂O) are very simple and can be assembled in very few steps. On the other hand, DNA and complex proteins cannot be completed without going through hundreds or thousands of steps. This 'minimum number of steps' is called the 'assembly index'.
The research team subjected a wide variety of samples—such as E. coli, yeast, urine, seawater, meteorites, and Scotch whisky—to mass spectrometry and measured the assembly index of each molecule. The results were surprisingly clear. All biological samples had an assembly index exceeding 15, while all non-biological and inorganic samples were below 15.
The appearance of Scotch whisky is a bit funny, but this is a serious experiment. The products that have gone through the biological process of brewing certainly have traces of 'something that was alive' engraved in their molecules.
Breaking away from 'if it's not carbon, it's not life'
In conventional extraterrestrial life exploration, the mainstream method has been to search for 'biosignature' gases such as oxygen, methane, and ozone in planetary atmospheres. However, this approach has fundamental limitations. Because it is a list created based on Earth life, it can effectively only search for life similar to Earth's. If there were aliens with completely different chemical systems based on silicon or sulfur, they would not show up on this checklist.
Assembly Theory breaks through that. It determines life not by 'what it is made of' but by 'how complexly it is assembled.' Whether it is carbon, silicon, or a chemical composition we cannot even imagine, if it has sufficient complexity, it can be determined to be life. The fact that life detection does not have to assume Earth-type organic matter is a fairly fundamental shift in the context of extraterrestrial life exploration.
'Dragonfly' heading to Saturn's moon Titan
This theory is already being incorporated into actual exploration missions. The Dragonfly probe, which NASA plans to launch in the mid-2030s, will investigate the atmosphere and surface of Saturn's moon Titan. Titan is a celestial body with a thick atmosphere of nitrogen and methane, lakes of liquid hydrocarbons on its surface, and potentially liquid water underground.
The team of Walker and Cronin is considering applying this to the mission using the threshold of an assembly index of 15 as a criterion. Searching for traces of life not with an Earth-based yardstick but with a 'universal scale of complexity'—this is, in a sense, an attempt to rewrite the very philosophy of extraterrestrial life exploration.
The perspective of a 'recursive universe'
Professor Walker says, 'We live in a recursively structured universe.' To build a rocket, first, multicellular organisms must exist, from which humanity is born, and civilization and science must be nurtured. Things have an order of emergence, and complex things always stand on a complex history.
Assembly Theory considers not just the 'current form' of a molecule but the 'history leading up to it' as its essence. It is a radical idea to define life not as an individual, but as a lineage of evolution.
In a previous article, I introduced the latest research stating that 'life is non-equilibrium self-organizing matter that can be derived from physical constants.'
Assembly Theory and this physical approach are approaching the same question from completely different starting points. They are deeply connected in that they both 'strip the definition of life from the framework of Earth biology'.
Also, please recall the discovery I introduced just the other day that the 45-base RNA 'QT45' can self-replicate.
From the perspective of the assembly index, QT45 likely far exceeds that threshold. Whatever the smallest unit of life may be, it must possess 'complexity born as a result of selection, not coincidence'—perhaps life was born under such constraints.
Reading the Past: 'Molecular Fossils'
There is another interesting implication. It concerns Scotch whisky.
Whisky is not alive. But the metabolism of the yeast involved in brewing leaves traces inside the bottle in the form of molecular complexity. If we think about this on a cosmic scale, life on a planet that went extinct billions of years ago might have left records in the molecular composition of that planet's rocks and atmosphere. Conventional space life exploration has searched for 'things that are still alive,' but assembly theory expands the scope to include 'things that were once alive.'
The question of 'what is life' may seem philosophical, but it is actually directly linked to the practical work of space exploration. This is because the definition adopted influences the decision-making regarding which probe to send, to which celestial body, and what to carry on it. I look forward to future observation results to see if assembly theory will become one of the answers.
Reference Articles
〇How a radical new view of life could reveal its origin and aliens (New Scientist)
〇A New Theory for the Assembly of Life in the Universe (May 4, 2023)
〇Radical New Theory Gives a Very Different Perspective on What Life Is (June 23, 2023)
https://www.sciencealert.com/radical-new-theory-gives-a-very-different
-perspective-on-what-life-is
〇Assembly theory explains and quantifies selection and evolution — Nature (October 4, 2023)
〇The Outline of Life Drawn by Physical Laws: What 'Being Alive' Tells Us About the Fundamental Constants of the Universe (October 13, 2025)
〇Life May Have Started from a '45-Character Memo'—The Impact of Ultra-Small RNA That Copies Itself (March 4, 2026)