#22 Sara Walker - Origin of Life, Assembly Theory, Biosignatures

Episode Overview

• Theoretical physicist and astrobiologist Sara Imara Walker introduces Assembly Theory, a framework for detecting life based on the quantifiable complexity of objects, applicable to both chemical biosignatures and technological artifacts.
• The conversation reframes the origin of life not as a single historical event, but as a continuous, planetary-scale process where life is a singular, interconnected global phenomenon.
• Walker argues that selection is not just a biological principle but a fundamental law of physics that drives the universe to explore its vast combinatorial space and create novelty.
• The discussion provides a new, practical strategy for searching for life on exoplanets: analyzing the entire atmospheric network for a high diversity of molecules built from common bonds, rather than searching for a single complex biomarker.

Key Concepts

• Assembly Theory: A method to quantify the complexity of an object by determining the minimum number of steps required to construct it from its basic parts. A high "assembly number" indicates an object is unlikely to have formed by chance and is therefore a likely signature of life or technology.
• Chemical Space: The immense, hyper-astronomical set of all possible molecules. A planet can only explore a tiny fraction of this space, acting as a "combinatorial search engine" guided by selection and historical contingency.
• Planetary-Scale Life: The idea that life is not a localized phenomenon but a global process that co-opts a planet's entire chemical network. This makes multiple independent origins of life on the same planet highly improbable.
• Continuous Origin of Life: A reframing of abiogenesis as an ongoing process. Any time an evolving system opens a new combinatorial space (e.g., multicellularity, technology), it constitutes a new "origin-of-life" event.
• Selection as a Physical Law: The proposal that natural selection is a universal physical principle, not just a biological one, that drives the emergence of complexity and novelty in any system with memory and vast combinatorial possibility.
• Non-Ergodic Systems: Complex, history-dependent systems like life or the universe cannot explore all possible states. Therefore, traditional statistical concepts like entropy are insufficient to describe their evolution toward higher complexity.
• Atmospheric "Joint Assembly": The true biosignature in an exoplanet's atmosphere is not a single complex molecule, but the collective evidence of a biosphere at work: a high diversity of different molecules that are all constructed from a common, smaller set of chemical bonds.

Quotes

• At 0:34 - "She's been working on this very radical idea. It's been controversial, but radical I would say in the field of academia, of assembly theory applied to this question of astrobiology." - Kipping introduces the main topic of discussion for the podcast.
• At 0:47 - "The basic notion being that life is by definition the emergence of complexity... this emergence of complexity is itself kind of a signature that we could look for." - Kipping offers a simplified explanation of assembly theory's core concept.
• At 18:57 - "If you wanted to exhaust all possible molecules of that complexity with one copy per molecule on a planet our size, the planet would collapse to a black hole." - Sara Walker explains the staggering vastness of chemical space using a powerful analogy.
• At 21:36 - "The origin of life is a continual process on a planet anytime you open up a new evolutionary space, whether it's a technological space or some other substrate that life has discovered, it's a new origin of life event." - Sara Walker elaborates on her concept of continuous "origins" tied to major evolutionary and technological transitions.
• At 22:11 - "I think about it as like there's one example of life per planet... because life is a planetary scale process." - Sara Walker clearly states her position that life is a singular, global phenomenon on any given world.
• At 22:29 - "This process of life is this process of information structuring matter on a planet over billions of years." - Sara Walker offers a concise, information-centric definition of what life does at a planetary scale.
• At 42:31 - "But if you look at an engineered material, like a silicon chip, which is a technologically manufactured mineral, you get high complexity at large size scales because you can precisely put defects in the material." - Sara Walker explains the key difference between the low assembly of natural minerals and the high assembly of man-made technology.
• At 46:21 - "I think selection and what life constructs on planets is a universal feature of our universe that emerges in highly complex combinatorial spaces as a fundamental physics." - Sara Walker proposing that selection isn't just a biological principle but a universal law of physics that drives the creation of complexity.
• At 53:58 - "Entropy is a concept that works really well in systems that don't have memory and causal constraints because you can explore ergodically the entire space. But the universe is not an ergodic system." - Sara Walker explains why the Second Law of Thermodynamics doesn't fully capture the behavior of complex, evolving systems like life.
• At 1:02:36 - "So actually no, that's not how I think we'll do it in an atmosphere because it's very unlikely that a really complex molecule would be volatile." - Sara Walker on why searching for a single, large, complex biosignature molecule in an exoplanet's atmosphere is the wrong approach.
• At 1:04:01 - "[On a plot of planetary atmospheres], Earth really stands out... because it has a high diversity of molecules in high abundance that have the same bonds, but many different molecules." - Sara Walker explains the specific atmospheric signature Assembly Theory predicts for a living world, focusing on the collective properties of all molecules rather than a single one.

Takeaways

• Shift your thinking from life as a "thing" to life as a "process" of information structuring matter over geological time.
• Understand that life is not the result of random chance but of selection and historical contingency navigating a planet through an infinitesimally small fraction of what is chemically possible.
• View major evolutionary transitions, like the development of technology, as new "origin of life" events that open up entirely new domains for evolution to explore.
• Apply assembly as a quantitative tool to distinguish between objects formed by random chance (low assembly) and those formed by directed, informational processes like biology or technology (high assembly).
• Consider that the universe's evolution towards complexity may be driven by selection as a fundamental physical law, challenging the conventional view that entropy and disorder are the only dominant forces.
• When searching for life on exoplanets, focus on detecting the collective signature of a biosphere—a diverse atmospheric network of molecules built from common parts—rather than hunting for a single, elusive biomarker.
• Recognize that a biosphere operates by maximizing the exploration of possibilities and generating novelty within a constrained physical space, a process fundamentally different from the statistical mechanics of non-living systems.