Walker, Sara
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Walker opens by arguing that every standard biology-textbook definition of life ("self-sustaining chemical system capable of Darwinian evolution," self-reproduction, metabolism, etc.) fails under scrutiny — citing Carl Sagan's observation that aliens might mistake cars for Earth's dominant life form, the zombie-ant fungus (where the "living" ant is arguably already dead, piloted by a fungus, making the individual boundary meaningless), and the fact that humans aren't literally self-sustaining (we depend on societies, which raises the paradoxical possibility that societies are "more alive" than individuals). Her resolution is to abandon the individual as the unit of analysis entirely: life is "the process of how information structures matter over time and space," and any individual organism is just a brief, low-dimensional cross-section of a much larger causal lineage stretching back roughly 4 billion years. She argues we're perceptually blind to this because we evolved to track space, not time — living things are "some of the biggest structures in the universe," but big in time (causal history), not space, and if we could perceive that full causal structure the way we perceive spatial extent, individuals wouldn't look like isolated, ephemeral things at all. This reframing lets her include as "alive" (or life-like) things far outside chemistry: language, mathematics, and human societies, since what unifies them isn't a chemical substrate but an open-ended combinatorial space too large to ever be exhausted, within which historically contingent, self-reinforcing structures persist.
She lays out assembly theory (developed with chemist Lee Cronin) as the formal machinery behind this view: any object's "assembly index" is the minimum number of recursive construction steps needed to build it from elementary parts, and objects only appear in high copy number if they cross a complexity threshold requiring a self-reinforcing causal history to exist at all (experimentally, around an assembly index of ~15 for organic molecules) — below that threshold, molecules are essentially generic and abundant regardless of complexity; above it, only historically-selected, self-perpetuating structures appear in abundance, because the space of possible molecules explodes combinatorially (a molecule as modest as Taxol, molecular weight ~853, would fill 1.5 universes if every 3D isomer were enumerated). This reframes evolution/selection not as competition among things that already exist, but as an ongoing exclusion of the vastly larger space of things that will never get the chance to exist — "the most significant struggle... is not among the objects that do exist, but between the ones that do and those that never have the chance to." She describes this as making the origin of life abrupt rather than gradual: once a set of molecules stumbles into mutual, self-reinforcing production (illustrated by Cronin's molybdenum-ring autocatalytic set — a large ring template that catalyzes the very smaller rings that build it), the possibility space "snaps" onto that structure very quickly. A related research thread in her lab concerns chirality (molecular handedness): below a certain molecular complexity almost no molecules have a mirror-image form, but above it almost all do, and she argues this transition in chemical space coincides with the origin-of-life transition, since each "choice" of handedness effectively locks in a molecule's entire future set of possible reactions — chirality, she says, breaks symmetry in time, not space.
On aliens and the "great filter," Walker reframes SETI-style questions around what she calls a great perceptual filter: since we didn't even recognize microbial life until inventing microscopes despite it being everywhere around and inside us, she worries advanced life elsewhere (or even our own biosphere's future) may become perceptually inaccessible to us not because it's rare, but because sufficiently evolved, "virtualized" life (deep in time, packing enormous causal/informational structure into little physical space — she draws an analogy to VR headsets, Freeman Dyson's proposal that life could survive an expanding universe by running exponentially slower, and Paul Davies's speculative "quinteligence" retreating into vacuum-based quantum computation) might become effectively undetectable, even resembling a black hole from the outside. She connects the search for alien life directly to solving Earth's origin of life, arguing they are literally the same problem: understanding how likely abiogenesis is requires the same theory needed to recognize truly alien biochemistry, and she frames humanity's current technological moment — the "technosphere," the global integration of life and technology — as effectively a second origin-of-life event: just as geochemistry once transitioned into biology, human civilization plus technology may itself be crossing into a new form of planetary-scale life, one that (if it eventually spreads to other planets) would represent life reproducing itself across worlds, closing the causal loop back to Earth's original abiogenesis. She explicitly rejects framing this in Kardashev-scale, energy-conquest terms, preferring to see life's expansion as the universe "creatively expressing itself" rather than as resource-consuming conquest.
On consciousness, computation, and AI, Walker resists treating consciousness or intelligence as more fundamental than life, or vice versa — she wants to build the same physics-of-time framework under all three rather than assume any of them as a starting axiom. Her working hypothesis is that conscious experience follows from being a temporally "deep" (causally large) structure with a correspondingly enormous, mostly private inner space, and that two beings can share understanding to the extent their causal histories haven't diverged too far in time (which is why humans can communicate with each other but not, say, with something separated much earlier along life's tree). She is skeptical of treating computation as ontologically fundamental (criticizing Stephen Wolfram's "ruliad" project directly, arguing computation is itself a product of billions of years of biospheric evolution, not a base layer of reality, and that string-lights programmed to run a cellular automaton don't thereby possess "computation" as an intrinsic physical property — that requires an external agent/designer), and instead sees computation as a powerful but partial language, occupying "the top of reality" rather than the bottom. She likens large language models to a new "genetic system for language" — a way of archiving humanity's distributed, dynamic linguistic/cultural structure into a compact, read-writable physical artifact, analogous to the evolution from primitive genes to the eukaryotic genome — while rejecting AGI-doomsday framing built on notions of "smarter" as a single scalar; she argues (via David Deutsch's "universal explainer" concept) that humans already have unbounded explanatory reach, so AI is better understood as an extension/enhancement of a human-technology ecosystem (a "technosphere") than as a separate, rival species that could simply out-general us. She still takes seriously near-term harms (deepfakes, weapons, fraud) and views public AI-doomerism partly as a useful societal "immune response," while arguing real philosophical resolution of AI consciousness or existential risk likely won't happen for a century or more.
On time, free will, and randomness, Walker describes reality as random "at the bottom" (in raw, unconstrained chemistry and quantum mechanics) with deterministic structure — including us — emerging only where self-reinforcing causal histories manage to persist against that randomness; she rejects the many-worlds interpretation's branching-universes picture in favor of imagining all those branches "folded up" into the single deterministic, causally dense object that a person actually is. Free will, in her account, is never instantaneous — it operates over the combinatorial space of one's own accumulated causal history, and one steers toward futures that still contain a coherent "you." The conversation closes on the hard problem of existence itself ("why does anything exist at all"), which she considers likely unanswerable even by a complete theory of life, and on her firmly physicalist rejection of anything existing "outside" the universe (including mathematical Platonism and the multiverse as literal separate realities, both of which she treats instead as useful conceptual tools generated within, and constrained by, our own biosphere) — ending on what she says gives her the greatest joy in the work: the shock of genuinely new understanding, and the hope of contributing an idea that changes how humanity as a whole thinks.
Excerpts
On why standard definitions of life all fail, using the zombie-ant fungus as an example:
“Anytime that you want to draw a boundary around something and you say, 'This feature is the thing that makes this alive, or this thing is alive on its own,' there's not ever really a clear boundary... it's like the thing that you would've thought is the living organism is now dead, except that it has another living organism that's piloting it. So the two of them together are alive in some sense.”
On the core claim of assembly theory — that existence itself is the real site of struggle, not competition among existing things:
“The most significant struggle for existence in the evolutionary process is not among the objects that do exist, but between the ones that do and those that never have the chance to. This is where selection does most of its causal work.”
On why she rejects treating computation, or AGI-as-single-superior-mind, as fundamental:
“I think it defines the boundary of the space of human languages... computation was built by our biosphere. It's something that happened after many billion years of evolution. It doesn't happen in every physical object... I see it at the top of reality, not at the base.”