Sara Imari Walker
Sara Imari Walker is a theoretical physicist and astrobiologist at Arizona State University, where her research investigates the origins of life and how biological information emerges from planetary processes. She is the deputy director of the Beyond Center for Fundamental Concepts in Science.
Essay
Sara Imari Walker, Assembly
Assembly is a material property.
Assembly is the action of fitting together the component parts of a machine or other object.
Assembly is the conversion of instructions in low-level code to machine code by an assembler.
Assembly is the gathering of a group for a common purpose.
Over the billions of years of evolution in the history of life, atoms assemble into molecules, molecules into cells, cells into multicellular organisms like humans. This layering of interacting objects over time can be described by a concept that might amount to a new branch of physics: Assembly Theory (AT). AT quantifies the structure of complex material objects over time. That is, it analyzes an object’s entire formation history, unfolded sometimes over periods of time as long as billions of years, rather than analyzing that object as an atemporal thing that simply appears ready-made.
Assembly Theory doesn’t just describe scientific objects like protons, or cells, or blackholes. It also describes objects like political assemblies. Because it examines all phenomena in the universe based on their historical complexity, AT incorporates objects that need not look material to us; an object’s materiality as coherent, assembled entities with causal power in the universe depends on their relation to other objects in time. For instance, AT tracks how organisms assemble societies, whose multitude of bodies includes the body politic. For AT, a “political assembly” is a complex object composed of constituent complex objects, like “humans” (who themselves are composed of complex constituent objects like “cells,” etc.). Because AT describes the intertwined assembled histories of all objects on Earth, from ancient cell lineages to the entire planetary whole, it is a powerful tool that should empower us to contemplate new possibilities for future political assemblies of human and more-than-human Others.
In AT, an object’s history (its temporality) and materiality (the expression of recursive interactions of matter) cannot be disentangled from that object’s existence. AT is interested in the fact that the interior structure of the ribosome has changed less than most rocks on this planet in the last four billion years, and that the lineage of sharks has been around longer than Saturn’s rings. Assembled objects can operate beneath or above humans’ immediate perceptibility and be deeper than us in time. At the molecular scale, the complex coevolution of proteins, cells, and organs produces life like us. Entire biospheres (if they are a general phenomenon and not unique to our own Earth) self-organize from assembly over eons as the embedded structures become more deeply embedded in time. The Technosphere, the skeleton of infrastructure enmeshing our planet’s biosphere, arose through layer upon layer of human innovation and selection on previous innovations (geological, chemical, biological, multispecies, planetary). That is, each novel innovation opened possibilities for those that followed while foreclosing assembly paths not taken. Life, then, is a structure that carries the past into the present, and that history constrains possible future histories.
AT analyzes every object in the universe by quantifying its underlying structure. We measure that structure through what we call the assembly index, the minimum number of recursive steps required to build that structure from simple constituent parts. The assembly index measures an object's depth in time and suggests time is a tangible attribute of matter. A high assembly index implies profound selectivity at each step, as the vast combinatorics of past alternative possibilities were discarded in favor of whatever was actually assembled. In this way, the existence of any object is always contingent on its assembly history. The accumulation of deep complexity requires information—a memory of the past—to preserve specific assembly pathways that cascade into the future. In other words, the structures that get assembled in an evolutionary process are always recursive. Recursivity of objects increases over time. Complex, highly assembled objects must be embedded as structures in time that are recursive stacks of their past history. This claim—that objects are deeply historical clusters of interrelated recursively assembled objects—goes against the grain of standard physics that merely describes objects as-is.
An object’s complexity directly corresponds to its depth in time. Some objects are too complex, or possess too high an assembly index, to have existed in the early universe due to insufficient assembly time. The objects we see today required a universe old enough to accumulate the requisite steps. Planets could not form before stars, and the complex chemistry that gave rise to life on Earth could not have occurred in the absence of the planet that possessed the sufficient chemical diversity to explore the space of possibilities to transition to life. Our own biosphere has evolved over billions of years to evolve a technosphere, a more complex object because it only could have proceeded the biosphere as time unfolded. The technosphere’s structure is much deeper in assembled time. AT thus proposes that time is fundamental and physical for any object—including the universe itself. The material is temporal. The assemblage of ancient objects formed the basic substrates from which complex objects are constructed through selection.
You may look upon a living entity such as a redwood tree and know it to be ancient, its assembly necessitating billions of years of embryonic time within stars and roiling clouds of gas to gather the elements from which its tissues spawned. Yet the redwood trees arose not as a discrete entity but through a nested hierarchy of assemblies, molecule into cell into organism into ecosystem. Each layer prefigured the next. Complexity through assembly initiates, through recursive actions and shaped via selection, in concepts like cognition and intelligence that are not simply emergent, immaterial properties of objects like “the mind,” but objects themselves that are both the underlying and overlying structures of other objects. Intelligence is a planetary process because this recursivity emerges at a planetary scale.
Political assemblies—like cellular assemblies, planetary assemblies, the Technosphere, etc.—cannot be disaggregated from their material counterparts. Individual delegates coordinate behaviors, perspectives, and knowledge to shape collective decisions. Along the way, these delegates discard the multitude of possible policy alternatives to enact their chosen course. An object’s historical assembly is the very feature that creates powerful possibilities for future assemblies. Social contracts can be renegotiated, and so can some features of the material assemblies they navigate.
As our political assemblies steer the fate of deeply historical material assemblies, from ecosystems to the technosphere, their resolutions resound at vast scales spanning all of time—the abiotic past, the living present, and possibilities yet unrealized. A more expansive political body will represent the interests of the ultimate material assembly in which we are embedded: the biosphere itself. Given our deep, historical embeddedness with the biosphere, and our innovations within it from which the technosphere evolved, AT discloses humans’ agency to recursively shape the next complex object— hopefully, one that more generously accounts for more-than-human Others. We must recognize that political assemblies are evolving and constructive entities and should incorporate other complex objects inextricable from our own assembled lineage. Only through such recognition can we thus construct even more possibilities for the universe to explore.
