“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.