The Informational Theory of Life How complexity and selection shape living systems

Life stands apart in its complexity, far beyond anything seen in non-living systems. The challenge is knowing how to clearly and measurably detect this complexity. Assembly Theory offers a method: look for objects that are both complex and abundant, shaped by the quiet force of selection.

Life is the only known way the universe creates truly complex objects. Compared to living systems, stars, weather, and planets are relatively simple. Even the simplest cell contains intricate molecular machinery that far exceeds the complexity of non-living systems.

Assembly Theory is an informational theory of life. It was first proposed by chemist Leroy Cronin in 2017 at the University of Glasgow. It was later expanded in collaboration with astrobiologist Sara Imari Walker and her team at Arizona State University.

The theory offers a way to quantify complexity by asking how many distinct steps are needed to build an object from scratch. A salt crystal might take just a few steps. A protein could require hundreds or even thousands.

The Life Detection Test

Walker proposes that we can detect life by looking for objects that are both highly complex and found in large quantities. This combination, she argues, is a hallmark of living systems.

Random chemistry might occasionally produce something complex. But it cannot do so consistently or abundantly. Only living systems through replication, selection, and refinement can generate complex structures at scale.

The Protein Paradox

Proteins are highly complex and abundant. But did they appear before life began? How do we tell the difference between prebiotic molecules and those that signal life?

The answer lies in functional complexity. It’s not just that something is complex and widespread; it matters that the complexity is meaningful within a system.

In a lifeless environment, you might get billions of different chains of amino acids. These would be complex and collectively abundant, but not functional as a whole. In life, we see the opposite: far fewer types of proteins, but each one is mass-produced because it plays a vital role.

Selection: The Hidden Engine

This distinction highlights the mechanism underlying  Walker’s theory: the selection for usefulness.

But what counts as useful? And how does selection happen without a designer?

Usefulness emerges from survival and reproduction. A molecule is useful if it helps its system persist, gather resources, or replicate. No guiding intelligence is needed. The environment selects, simply and automatically.

Random processes produce many variations. Some combinations help their systems thrive. Those systems reproduce more. Over time, successful combinations get amplified. The rest disappear.

This leads to what Walker calls “selected complexity”, intricate structures that exist not because they formed by chance, but because they work, and they last.

Beyond Earth: A Universal Framework

Walker’s theory also provides us with a practical tool for searching for life beyond Earth.

Instead of looking for specific chemicals like DNA or oxygen, we can search for patterns of selected complexity: highly organized structures that are also abundant.

This method could help us detect unfamiliar life forms, even if they are based on entirely different chemistries. What matters is the pattern, not the parts.

The Deeper Message

Walker’s theory bridges the gap between physics and biology through the lens of information and natural selection. In this view, life is chemistry shaped by memory and function.

It’s not just that life exists. The universe may require something like life to build and preserve functional information.

If this is true, then intelligence, technology, and culture are not deviations from nature. They are its logical next steps, part of the same process that began with the first replicating molecules and continues in us.

We are not outside the universe’s logic. We are its most complex result so far.

We can start looking at life through the lens of selected complexity. Instead of focusing only on what things are made of, we can ask how they came to be and what they do. By adopting this approach, we open up new avenues for exploring life, both on Earth and beyond.