How the World Builds Itself
The Origins of Us
The beauty of life is something to marvel at, but the process of its creation is even more spectacular to explore. We’re surrounded by complexity: our minds, bodies, cities and ecosystems are breathtakingly intricate. All of them trace back to a single common ancestor, and if we went back further, to the creation of the universe, it ultimately started with a Big Bang, the life and death of millions of stars and the slow, improbable creation of the planet we all call home. This leaves us with the big question: how on Earth did we, as humans, get here?
Assembly Theory [1] approaches this mystery from a unique angle. Instead of asking only what life is made of, it asks how much history is required to build it and how many layers of selection must accumulate before matter is unequivocally produced from life.
Figure 1: A representation of increasing complexity through selection
Assembly Theory
Assembly Theory, developed by Sara Walker and Lee Cronin, posits that life has been built up layer by layer to create the complex world we see around us. They go further and have determined a way to measure this complexity by identifying the number of steps needed and the pathway chosen for an object to move from basic building blocks into a complex form.
At its core is the assembly index, the minimum number of steps required to construct an object from basic building blocks [2].
Consider baking a single-layer cake. The assembly index is low because you are only mixing flour, eggs, sugar, butter and adding frosting. If you are baking a two-tier dark chocolate cake with raspberry coulis filling and a mirror glaze, the assembly index is much higher.
The Four Assembly Spaces
Assembly Theory frames complexity within four nested “spaces”, a way of moving from everything imaginable to what we observe:
Assembly Universe: everything that could ever exist in principle. Every possible arrangement of building blocks, whether realistic or not. Imagine every LEGO structure that could be built, including one piece of LEGO passing through another, unconstrained by the laws of physical matter.
Assembly Possible: the subset that exists by the rules that bind our Universe, such as physics, chemistry and grammar. These constraints prune the universe down into what is possible. It takes the universe of all imaginable LEGO structures and identifies the LEGO building constraints that allow bricks to click together.
Assembly Contingent: the subset shaped by history and resources. It shows what gets built, given available materials and constraints created by the knowledge of that historical period. If you’ve only got red LEGO bricks and two hours, your castle will be different from what you can build with six hours and eight different colours of bricks.
Assembly Observed: what survives long enough to be seen and measured. Not everything makes it out of the contingent space. The Assembly Observed shows the stable structures that are selected and persist. These are the LEGO spaceships that didn’t get smashed.
These spaces show how complexity moves from imagination into reality, filtered first by physical law, then by history and survival.
Universe → Possible → Contingent → Observed
Imagination → Rules → History → Reality
The four spaces aren’t extra abstractions of each other; they are scaffolding. They remind us that nothing complex just “happens.” Every object created is the survivor of all possible forms, through physical rules and historical accidents, into the actual things in front of us.
Assembly Theory’s role in complexity.
Complex objects require accumulated instructions, whether encoded in DNA, culture, chemistry or technology. The more steps required, the more stored knowledge and selection must exist to make that object possible. Assembly Theory gives us a framework to detect complexity in the real world [3]. By quantifying how much selection is necessary to construct an object, it distinguishes between things that could plausibly arise by chance and things that demand directed, information-rich processes like life or technology.
One of the most powerful implications of Assembly Theory is that it may help us detect life itself. Traditionally, scientists search for life by looking for specific molecules or environments associated with biology. Assembly Theory proposes a different approach: life may be recognisable by the depth of construction history embedded in an object. An observed molecule with an extremely high assembly index is unlikely to arise through random chemistry alone. Their research revealed an unequivocal threshold for life and determined that an assembly index of 15 or more cannot arise through chemistry alone and therefore is a signature of life [1]. In this sense, complexity becomes evidence of accumulated selection and information, a possible biosignature (signature of biology) written directly into matter.
Assembly Theory also reveals a continuity between chemistry, biology, intelligence, and technology. Viewed through this lens, a protein, a poem, and a microchip can all be analysed under the same principle: nothing new comes from nowhere. Chemistry creates the conditions for biology, biology creates systems capable of memory and adaptation, intelligence enables symbolic thought and both culture and technology externalise knowledge into the material world. Each layer inherits and builds upon the history of the layer beneath it. DNA stores biological memory, minds store cognitive memory, technology stores cultural and engineering memory. Together, they demonstrate complexity preserved through history that can be traced.
Assembly Theory shows us not just how the world has been built, but how new forms of complexity might emerge in the future. Molecules, minds, and machines become part of the same unfolding process: the accumulation of complexity through time. In this view, every complex object becomes a fossil of history. Cells, languages, cities, and computers are not random arrangements, but records of complexity written through time.
References
Sharma, A. C. (2023). Retrieved August 2025, from Assembly theory explains and quantifies selection and evolution. : https://doi.org/10.1038/s41586-023-06600-9
Jaeger, J. (2024). Assembly Theory: What It Does and What It Does Not Do . Retrieved August 2025, from https://doi.org/10.1007/s00239-024-10163-2
Abel, D. L. (2025, January). ‘Assembly Theory’ in Life-Origin Models: A Critical Review. (BioSystems 247 ) Retrieved from https://doi.org/10.1016/j.biosystems.2024.105378
