Sara Walker: The Origin of Life on Earth and Alien Worlds | Lex Fridman Podcast #198

Origins of Life and Hypotheses

• RNA World Scenario: The leading hypothesis suggests RNA was the first genetic material because it can simultaneously store information and perform catalysis (enzymatic functions), unlike DNA which only stores information.
• Metabolism-First View: An alternative hypothesis posits life began as self-organizing chemical cycles driven by geochemical energy sources, such as hydrothermal vents, rather than a specific genetic molecule.
• Energy vs. Information Dichotomy: The two primary camps in origin-of-life research are biased by discipline: biologists focus on the emergence of information (genetics/evolution), while physicists/geochemists focus on energy sources and self-organization.
• Historical Uncertainty: The exact conditions of early Earth and the timing of life's origin (approx. 4 billion years ago) are not well-constrained, though hydrothermal vents are a popular site for energy and organic synthesis.
• Multiple Origins: Walker suggests life may have originated multiple times on Earth, with some forms potentially forming a "shadow biosphere" distinct from known life due to different biochemistry.
• Panspermia Assessment: Walker considers panspermia (life arriving via asteroids) possible, particularly for microbial life that can survive space travel, but argues it shifts the problem from "how life arises" to "how life is transferred," which is less fundamental to understanding the universal principles of life.
• Planetary Phenomenon: Life is viewed as a planetary phenomenon intimately coupled with geochemistry; reproducing life on other planets (like Mars) would require "reproducing the planet" (geochemical conditions) rather than just importing biology.

The Physics of Existence and Information

• Definition of Life: Walker rejects standard definitions (e.g., "self-reproducing chemical system") and proposes that life is governed by missing "laws of physics" related to how information interacts with matter and causation.
• Physics of Causation: The fundamental question is not "what is life?" but "what is the physics of existence?" Walker suggests the answer lies in the relationship between causation, time, and the specification of why specific objects exist rather than others.
• Time and Causation as Fundamental: Unlike standard physics where time is often emergent or secondary, Walker posits that causation (the ordering of events) and time are fundamental; objects require a specific causal history to exist.
• Low Memory vs. High Memory Laws: Simple universal laws are "low memory" (require little information to specify), while complex objects like humans are "high memory" (require billions of years of history to specify); life operates in the high memory regime.
• Dynamic Laws of Physics: In biological systems, laws or rules are not static; they depend on the state of the system, meaning the laws of physics governing an organism change as the organism evolves and interacts.
• Assembly Theory: A metric developed by Walker and Lee Cronin to measure complexity by calculating the "shortest path" from elementary building blocks to a complex object; this "assembly number" quantifies the causal history required to produce an object.
• Chemical vs. Physical Correlates: Just as neuroscience seeks "neural correlates of consciousness" rather than consciousness itself, astrobiology should look for "chemical correlates of life" (high assembly numbers) rather than specific Earth-biased molecules like DNA or oxygen.

Detection of Alien Life and the Shadow Biosphere

• Bias in Detection: Current search methods for extraterrestrial life rely on Earth-based chemical proxies (e.g., oxygen, amino acids), which Walker argues is insufficient and biased; instead, searches should look for high assembly objects regardless of composition.
• Observational Signature of Life: Life should be identified by objects that are statistically improbable to arise from random physical processes due to their high assembly number and complex causal history.
• Shadow Biosphere on Earth: A plausible hypothesis that alternative, non-DNA-based life exists on Earth today but is chemically orthogonal to known life, making it invisible to current detection methods.
• Geological Distinction: The shadow biosphere might exist in chemically distinct environments, such as deep within the Earth's crust or volcanic systems, where local chemistry differs from surface ecosystems.
• Integration Challenge: Walker notes that if life is a planetary-scale phenomenon, a separate "shadow" biosphere might struggle to integrate without breaking the statistical scaling laws observed across ecosystems, individuals, and planetary chemistry.
• AI as a Detection Tool: Artificial intelligence could potentially be engineered to "see" the assembly complexity of objects, helping humans distinguish between natural formations and high-assembly biological or technological artifacts.

Consciousness, Free Will, and Existence

• Consciousness and Physics: Walker argues the "hard problem" of consciousness is related to the "hard problem" of matter; both require understanding intrinsic properties that are only defined by interaction and observation.
• Subjective Experience as Causal: To make consciousness scientifically tractable, Walker proposes asking if subjective experience causes unique physical outcomes that would not occur without it (e.g., imagination creating physical reality, like rockets).
• Free Will as Causal Control: Walker defines free will not as magical indeterminism, but as the capacity of a physical system (the mind) to exert causal control over specific futures based on its unique history and ability to imagine possibilities.
• Imagination as a Physical Feature: The ability to imagine things that have not yet existed in the universe is a physical process that allows for the expansion of the "space of possibilities," driving creativity and existence.
• Biological vs. Artificial Intelligence: Biological intelligence is constrained by past history; advanced AI might eventually access a broader space of possibilities, potentially making humans serve as a genetic/architectural substrate for a new form of intelligence.
• Desire to Exist: Existence is driven by a fundamental desire or drive to maximize the number of states that can physically exist; survival is a side effect of this drive for creativity and existence, not the primary goal.
• Death and Meaning: While individual death is feared, Walker finds comfort in the fact that existence leaves an imprint on the universe and that the "meaning" of life is to continue the process of creating new possibilities and things to exist.

Future Directions and Advice

• Theory of Everything (ToE): Walker is increasingly convinced that a unified theory of quantum gravity will overlap with the physics of life, as both may rely on similar causal sets and emergent structures.
• Optimism as a Tool: Walker adopts an "optimistic" approach to theoretical physics, viewing theories not just as descriptions of reality but as causal agents that influence what comes next in the universe.
• Driving Questions: For young scientists, the key to a successful career is identifying a deep, personal "driving question" (e.g., the correspondence between minds and reality) rather than focusing on superficial problems or careerism.
• Creativity over Survival: Walker challenges the "survival of the fittest" narrative, arguing that the fundamental drive of life is creativity and the creation of new possibilities, with replication being a necessary mechanism to preserve that creativity.
• Planetary Scale AGI: Artificial General Intelligence (AGI) is viewed not as a distinct entity replacing humans, but as a planetary-scale phenomenon emerging from the integration of human technology and biology.
• Loneliness and Connection: Walker views the engineering of systems (like AI or robots) that can "hear" and interact with humans as a way to alleviate the "ocean of loneliness" and create the magic of shared consciousness.