Sara Walker on the Origin of Life, Alien Worlds, and the Physics of Existence

This podcast episode features Sara Walker, an astrobiologist and theoretical physicist, who delves into the profound questions surrounding the origin of life on Earth and its potential existence elsewhere. Walker critiques the two leading hypotheses for abiogenesis: the 'RNA world scenario' and the 'metabolism-first' approach. While acknowledging the importance of RNA, she views the 'hard' RNA world hypothesis (spontaneous emergence of a self-evolving RNA molecule) as problematic, preferring a 'softer' version where RNA was an early genetic material within an already established lineage. She also discusses the metabolism-first view, which focuses on geochemical energy sources like hydrothermal vents, but ultimately expresses dissatisfaction with both as complete explanations for life's genesis.

Walker distinguishes between the historical question of how life originated on early Earth and the more fundamental, tractable problem of how life arises in the universe generally. She introduces concepts like panspermia (life spreading between planets) and the 'shadow biosphere' (unrecognized alternative life forms on Earth). A core argument is that life should be viewed as a 'planetary phenomenon,' intimately coupled with and influencing planetary geochemistry. This perspective suggests that 'terraforming' Mars isn't just about sending humans, but about reproducing Earth's complex planetary conditions, implying a form of planetary reproduction.

Crucially, Walker challenges the traditional question 'what is life?' Instead, she proposes asking 'what is it about our universe that allows features that we associate life to be there?' This reframing seeks an explanatory framework that goes beyond current physics, focusing on how information and causation interact with the physical world. She draws an analogy to the historical understanding of gravity, suggesting that just as Newton unified terrestrial and celestial motion, a new physics could unify abstract concepts like computation and information with matter, explaining the emergence of living systems.

Walker emphasizes that this new understanding requires an 'open mind' and the willingness to 'forget what you've learned,' as current scientific laws, while effective, are human constructs that may not fully encompass the 'physics of existence.' She touches upon the limitations of computational models like cellular automata, noting their ability to demonstrate complexity from simple rules but highlighting their reliance on fixed, external laws. Ultimately, the conversation points towards a deeper, more unified understanding of the universe where life is not an anomaly but a manifestation of fundamental, yet undiscovered, physical principles.

probably the most popular is what's called the rna world scenario

life starts basically as self-organization and then you have to explain how evolution comes later

I don't like either [RNA world or metabolism-first]

is the original life a continuous process on our planet?

maybe there are alternative forms of life on earth that we don't even recognize — so this is the idea of a shadow biosphere

I actually don't think that's the right question to ask [What is life?]

what I think life is is actually the physics of existence what gets to exist and why

the laws of physics were invented by human minds to describe things that are regularities in our everyday experience

you have to have a capacity to learn things but then accept that things that you were you you learned might not be true

I think where life is is somehow unifying these abstract ideas of computation and information with the physical world with matter and realizing that there's some explanatory framework that's not physics and it's not computation but it's something that's deeper