Walker, Sara

astrobiologist and theoretical physicist, Arizona State University and Santa Fe Institute

Sara Walker: The Origin of Life on Earth and Alien Worlds

Lex Fridman Podcast · Jul 9, 2021

• Sara Walker
• origin of life
• RNA world hypothesis
• assembly theory
• physics of existence
• chemical correlates of life
• shadow biosphere
• panspermia
• consciousness and neural correlates
• free will and imagination
• planetary-scale life
• AGI as planetary phenomenon
• meaning and creativity

Core ideas

Walker surveys the two leading historical origin-of-life hypotheses — the "RNA world" (RNA as the first molecule able to both store genetic information and catalyze reactions) and metabolism-first scenarios (self-sustaining catalytic chemical cycles, likely powered by hydrothermal vents, preceding genetics) — but is openly critical of the strongest version of the RNA-world claim (that an RNA molecule spontaneously self-assembled and began Darwinian evolution), while accepting a softer version (RNA was simply the earliest genetic material in life's actual lineage). She argues the deeper problem is that origin-of-life research has been fighting the wrong question: rather than trying to retrodict the exact historical sequence of events on early Earth (which requires filling in unknowable details), she wants a general, testable theory of what physical conditions allow life-like phenomena to arise anywhere — a question she says is tractable in the lab, unlike the historical one.

She proposes reframing "what is life?" entirely: rather than seeking a definition (e.g., "a self-reproducing system capable of Darwinian evolution"), she wants an explanatory physics of existence — missing fundamental principles, akin to how gravity unified terrestrial and celestial motion, that would explain why certain complex objects (a cup, a protein, a human) get to exist in the universe at all, out of a vast space of things that could exist but don't. She resists calling this "information" or "computation" (both feel like ill-fitting borrowed words) and instead leans toward describing it as "the physics of causation": complex objects require a specific, non-trivial causal history to come into being, meaning they can't be dismissed as random fluctuations (rejecting Boltzmann-brain-style spontaneous appearance) — a view she credits partly to collaboration with chemist Lee Cronin. She argues the laws of physics as classically conceived (a fixed rule plus an initial condition) are themselves value-laden human inventions describing regularities, not eternal external truths, and that biology exhibits something physics doesn't model well: rules that co-evolve with the system's state, rather than staying static — and speculates, following collaborators like Lee Smolin, that a future theory of quantum gravity might turn out to be the same theory that explains life, since both areas grapple with problems of emergent space, self-reference, and causal history (drawing loose parallels to Stephen Wolfram's physics project and David Deutsch/Chiara Marletto's constructor theory).

Walker and her collaborators (primarily Lee Cronin's lab) propose "assembly theory" as a concrete, experimentally measurable alternative to searching for "chemical correlates of life" (looking for specific molecules like oxygen or amino acids on other planets, which she considers a weak, non-generalizable heuristic). The idea: break a molecule down into its component parts and calculate the shortest possible assembly pathway (the "assembly number") needed to construct it from elementary building blocks; complex objects with high assembly numbers are argued to be so combinatorially improbable that their existence in abundance requires a system with "memory" of how to build them (i.e., an evolutionary/biological process), and this can be measured directly via mass spectrometry, independent of the specific chemistry involved — making it a chemistry-agnostic signature potentially useful for detecting alien life that might not share Earth's biochemistry. She stresses that a scientific validation of a general theory of life would look less like "a bacterium crawling out of a test tube" and more like demonstrating that information generated spontaneously within an experimental system, without being put there by the experimenter, comes to govern that system's own future dynamics — while cautioning that lab origin-of-life experiments often unknowingly "inject life" into their setups through the researcher's own careful, information-laden experimental design (choice of reagents, pH, containers, etc.).

On extraterrestrial life, Walker is skeptical that panspermia (life traveling between planets via asteroids) would be scientifically interesting even if true, since finding identical biochemistry elsewhere wouldn't reveal a second, independent origin event — the real question she cares about. She frames life as potentially a "planetary-scale phenomenon" deeply coupled to a planet's geochemistry, which has significant implications for space colonization: sending humans to Mars, in her view, effectively requires "reproducing" Earth's planetary conditions there, much like a cell must replicate its internal environment to propagate. She discusses the "shadow biosphere" hypothesis (the idea that a second, independent origin-of-life event could exist on Earth right now, undetected, alongside the one all known life descends from) as a serious, testable hypothesis in principle and cheaper to investigate than interplanetary missions, but expresses skepticism it would be found, given evidence (from her own group's research) that Earth's biochemical networks exhibit consistent scaling properties across individual, ecosystem, and planetary levels that a truly independent chemistry would likely disrupt; she is more open to isolated pockets (e.g., deep-crust or volcanic environments) potentially hosting chemically distinct life forms that never interact with surface biochemistry.

On consciousness, Walker treats it as deeply related to (but distinct from) the physics-of-life problem, and proposes sidestepping the "hard problem" (whether something has subjective experience, which is unobservable from outside) by instead asking whether subjective experience causes physical effects that wouldn't otherwise occur — making consciousness scientifically tractable as a question about causal consequences rather than inner feeling. She draws an explicit parallel between neuroscience's search for "neural correlates of consciousness" and astrobiology's implicit search for "chemical correlates of life," arguing both fields mistake proxies (a molecule, a brain signal) for the underlying explanatory phenomenon itself. She connects imagination specifically to free will: humans (and potentially other physical systems) can causally bring into existence things that were never part of their own historical lineage (rockets existed as ideas long before physically), and she considers this — the capacity to draw from an unconstrained space of possibilities rather than only local history — a meaningful, non-mystical definition of choice, compatible with a universe she suspects may be fundamentally deterministic even though its description (and our felt experience of free will) is inherently probabilistic.

On existential risk and future intelligence, Walker rejects survival/replication-centric Darwinian narratives (fittest-replicator-wins) in favor of framing life's fundamental "drive" as creativity/existence-maximization, with replication as a secondary mechanism for carrying accumulated creativity forward — a reframing she considers less deterministic and more optimistic (she explicitly favors building "optimistic" physical theories, arguing that since theories have real causal power over what gets built next, choosing hopeful framings is itself scientifically consequential, not just a psychological preference). Extending this to AI, she argues AGI is unlikely to emerge as a discrete, boxed system that "replaces" humans, but rather as an integrated, planetary-scale phenomenon growing out of humanity's existing technological and informational infrastructure — with humans persisting as a kind of foundational "genetic architecture" within that larger system, analogous to how earlier evolutionary information hasn't been erased but layered upon.

On mortality, Walker says she isn't rationally afraid of her own death (since she won't be around to experience its absence) but fears it in terms of its impact on her children, framing death's significance as fundamentally collective/relational rather than individual — while acknowledging she still "likes existing" and doesn't find genuine comfort in physics-based arguments (e.g., that past moments persist "in" spacetime) against the loss that death represents. She closes by describing her own meaning-making framework: that humans (and life generally) constitute "the creative process of the universe," with beauty and the emotional pull of intellectual/creative activity possibly serving as intrinsic signals or correlates of that deeper creative drive, and offers career advice centered on pursuing one's own genuinely compelling "driving question" rather than a discipline label, crediting her own path (from community college physics to astrobiology) to sustained fascination with the relationship between minds and physical reality.

Excerpts