What is life? Assembly theory, information and the limits of definition
Sara Walker and Lee Cronin argue life is information structuring matter across time, while Adam Frank and Sean Carroll frame it through planetary processes and the emergence of complexity from entropy. How we define life determines how we'd recognize alien life and even mind.
Why does Sara Walker reject the definition of life as a self-sustaining chemical system capable of Darwinian evolution? Show answer
Nothing is self-sustaining, life is not tied to chemistry as such, and Darwinian evolution happens to populations, not individuals.
How does Walker define life? Show answer
As the process of how information structures matter over time and space, with any individual being one instance of lineages structured across time.
What is the assembly index? Show answer
The minimum number of steps needed to build an object from basic building blocks, where anything already built can be reused. For molecules the blocks are atoms and the steps are bonds.
Why does copy number matter in assembly theory? Show answer
One complex object could arise by chance, but many identical copies of a complex object could not, so abundance plus complexity signals selection.
What did Cronin's blind test with NASA samples show? Show answer
Biological samples produced molecules with an assembly index above 15 and abiotic ones stayed below, and NASA confirmed the lab had sorted them correctly.
What is Adam Frank's unit of life? Show answer
The biosphere, not DNA or a cell. He says you have to think about the whole planet and its history to judge whether it is a good place for life.
Why does Frank say the hard steps argument fails? Show answer
The fate of Earth and the fate of life are tied together, and a step that happened only once may not be hard, since old innovations get lost and filled niches block repeats.
How does Sean Carroll relate life to entropy? Show answer
Life is a stage in the growth of complexity as the universe moves from low to high entropy. It maintains itself by burning low-entropy fuel and using information.
Why does complexity rise and then fall in Carroll's picture? Show answer
Because both very low and very high entropy look simple, so complexity peaks in between.
Where do Walker and Carroll disagree most sharply? Show answer
On entropy. Walker says it is the wrong approach because the question is how life makes order from randomness; Carroll makes the entropy gradient the engine.
What is Walker's great perceptual filter? Show answer
Her worry that a biosphere that becomes more virtualized and abstract may be unable to perceive other such structures. She stresses she isn't sure she agrees with it.
What signs of life does Frank's planetary approach look for? Show answer
Things like oxygen together with methane, chlorofluorocarbons that only industry makes, and chemical disequilibrium in an atmosphere.