Sara Imari Walker is an astrobiologist and theoretical physicist at Arizona State University whose collaboration with Paul Davies has produced some of the most rigorous contemporary work on the boundary between physics, information theory, and biology. Her research focuses on the question of what makes matter alive—not as a philosophical puzzle but as a problem in physics with testable consequences. The 2013 paper 'The Algorithmic Origins of Life,' co-authored with Davies, argued that life is distinguished by its informational architecture rather than its chemistry, and that the transition from non-living to living is marked by the emergence of top-down causation. Walker's subsequent development of assembly theory—a quantitative framework for measuring complexity by the minimum number of steps required to construct an object—has provided empirical tools for detecting life and intelligence in systems where traditional biochemical markers may not apply.
Walker's trajectory into astrobiology was unconventional. She studied physics and chemistry at Florida State University, completed a PhD in chemistry at Dartmouth, and spent her early career investigating the thermodynamics of far-from-equilibrium systems before turning to the question of life's origins. Her collaboration with Davies began at Arizona State University's Beyond Center, where the institutional commitment to cross-disciplinary investigation allowed her to pursue questions that did not fit neatly into any established field. Assembly theory emerged from this work—a framework that measures an object's complexity not by its material composition but by the minimum number of distinct operations required to construct it. A random molecule has low assembly index. A biological molecule—protein, nucleic acid—has high assembly index, because its construction requires a specific, ordered sequence of operations.
The framework has direct implications for the search for extraterrestrial life and for the classification of artificial systems. Traditional biosignatures—oxygen in an atmosphere, methane alongside oxygen, phosphorus in specific compounds—assume life will be chemically similar to Earth life. Assembly theory provides a signature that does not depend on chemistry: high assembly index in abundant quantities indicates a generative process, because random chemistry does not produce complex objects repeatedly. The same framework applies to artificial intelligence. A large language model's outputs have high assembly index—the construction of a coherent paragraph requires a long sequence of specific word choices—but the model itself does not: it is generated through a training algorithm that is computationally expensive but conceptually simple. The distinction reveals that AI processes information at high sophistication but does not (yet) exhibit the recursive self-construction that characterizes living systems.
Walker's most provocative recent claim, developed in her 2024 book Life as No One Knows It, is that life and intelligence are not biological phenomena that happen to have physical instantiation but physical phenomena that happen to have, so far, taken biological form. This reframing dissolves the boundary between 'natural' and 'artificial' intelligence—both are expressions of the same underlying physics, and the question is not which is real but what each reveals about the space of possible minds.
Walker's intellectual formation combined rigorous training in physical chemistry with an outsider's willingness to question disciplinary boundaries. Her doctoral work on thermodynamics prepared her for Davies's framework, but her turn toward astrobiology and the origins of life was driven by a conviction that the biggest questions in science lie at the intersections where traditional disciplines meet and fail to provide answers. The Beyond Center at ASU, founded in 2006, provided the institutional home for this work, and Walker's appointment there in 2013 marked the beginning of her most productive collaboration with Davies.
Assembly theory. A quantitative measure of complexity—the minimum number of operations required to construct an object—that distinguishes living from non-living systems without requiring knowledge of their chemistry.
Life as physics. The thesis that life is not a biological category but a physical one—a specific class of information-processing systems characterized by high assembly index and top-down causal architecture.
Recursive construction. Living systems build themselves through processes that themselves are products of prior construction—a recursive loop that current AI architectures have not implemented.
Intelligence without biology. The framework that treats intelligence as a physical phenomenon whose biological instantiation is contingent, opening the conceptual space for genuinely alien minds.