Sara Imari Walker

An Informational Theory of Life

Recorded live on Apr 1, 02025 at Cowell Theater in Fort Mason Center

“What is life?”

In her Long Now Talk, astrobiologist and theoretical physicist Sara Imari Walker explores the many dimensions of that seemingly simple question.

Starting from the simplest precursors, Walker assembled a grand cathedral of meaning, tracing an arc across existence that linked the fundamentals of organic chemistry, the possibility space of lego bricks, and the materialist philosophy of Madonna.

As the leader of one of the largest international theory groups in the origins of life and astrobiology, Walker has worked an interdisciplinary team of researchers to devise assembly theory: a theory of life and its origins that finds that life is the only way to create complex objects, and that the existence of complex objects is fundamentally and quantifiably rare. Assembly theory’s focus on complexity and countability allows astrobiologists like Walker to grapple with the sheer vastness of combinatorial space — the set of all things that could possibly exist.

That set is vaster than the universe itself can hold, which, of course, raises a foundational question: if the universe cannot exhaust all possibilities of what can exist, what determines what actually exists, and what merely could exist?

Assembly theory uses the concept of the "assembly index" to measure the complexity of objects in the universe, quantifying how many steps are required to build something — a molecule like ATP, the primary energy-carrier in cells, for example. The theory finds that items above a given assembly index of 15 cannot be produced repeatedly by any known process save for life itself — a complexity threshold governed by the size of the possibility space. Implied here is that matter itself holds information. Within the physical dimensions and history of any given object is a measure of the information required to construct it. Likewise, historical contingency matters: it determines what gets constructed within the space of all theoretically possible constructions.

In Walker's words: "We are our history."

Life is causal histories — lineages of propagating information. Assembly theory conceptualizes objects as entities defined by their possible formation histories, allowing a unified language for describing selection, evolution and the generation of novelty. Within assembly theory, the fundamental unit of life is not the cell, but the lineage.

watch

primer

Physicist and astrobiologist Sara Imari Walker proposes a new paradigm for using physics to deepen our understanding of what we recognize as life. Assembly theory is a framework that uses the physics of molecular complexity to open a new path to identify where the threshold lies for life to arise from non-life, drawing in insights from new discoveries on the nature of historical contingency and time itself.

Prior frameworks in both physics and biology have failed to explain life as a general phenomenon. While the Darwinian theory of evolution via natural selection governs much of the development of complex life on our planet, and our contemporary understanding of physics has deepened our understanding of what could possibly exist under its laws, neither system can fully explain how life can originate, nor what forms of life are more likely to exist than others.

Assembly theory is an informational theory of life: it fundamentally holds that life requires information to specify its existence — that objects complex enough to require a many-step, informationally-driven process of assembly are evidence of life, even if the forms they take do not necessarily resemble life as we know it.

Why This Talk Matters Now

Walker’s work as an astrobiologist is all about anticipating the alien uncertainties hidden under the regular patterns of life on Earth. Assembly theory allows us to go beyond these patterns, expanding our imagination of what life could possibly be. As our technology develops further and debates rage about artificial intelligence as a potential form of life, a foundational, physics-driven approach is needed to tease out answers to these deep questions.

Key to Walker’s conception of life is the explanatory power of historical contingency and repeatability. Shearing away from purely probabilistic theories that find that complex forms can emerge randomly into existence, assembly theory holds that historical context and the passage of time matter deeply for the formation of the complex objects that it considers life. Assembly theory builds on prior work on complexity science done at institutions like the Santa Fe Institute, as well as David Deutsch and Chiara Marletto’s work on the Constructor Theory of physics.

The Long View

Astrobiology as a discipline has long been of interest to Long Now and long-term thinking more broadly. As a theoretical discipline straddling physics, chemistry, and biology, astrobiology represents an attempt to go beyond the conventional bounds of those fields and take a longer view, leaving aside the assumptions of our present context.

Where to Go Next