Drawing the Line on 'Life'
Drawing the Line on 'Life'
Key Takeaways from 'Identifying Molecules as Biosignatures with Assembly Theory & Mass Spectrometry'
A paper recently published by a group of cutting edge scientists at Cronin Group puts forth a model that, ‘Proposes a theory of molecular complexity that is experimentally viable.’
The significance of this is that it may be used to determine if an unknown molecule required a biological system to create it or if it occurred randomly. A reliable model for drawing this distinction would be key to determining the existence of life outside our planet.
When I read this paper, I was shocked by how it took something as complex as the formation of life from core building blocks and broke it down into a simple framework that could be used to recognize this process in other parts of the universe. Let’s get into how they did it.
Intro to the Ideas
This paper’s methodology leans heavily on something called Assembly Theory. Assembly theory is a framework for the formation of substances from base materials through Assembly Pathways. An assembly pathway for the word ‘Abracadabra’ from it’s base materials might look something like this fun example from Cronin Labs’ paper —
The further down an assembly pathway nature proceeds, the less likely any one outcome.
Why is this the case? Take a quick look at the below tree of ‘molecules’ —
Let’s call the 4th row down our ‘complex molecules’. At this layer in the tree, the probability of any one molecule being formed is equal in randomness to it’s neighbors. Lets say that in nature we’ve observed an abundant amount of #11 being formed, Cronin Labs’ assumption is that complex molecules that exist in high enough numbers must be signatures of life because it would be near impossible for them to form in great numbers by chance.
Any one molecule may have many different possible assembly pathways. A molecule’s Molecular Assembly Number(MA) is used to determine the specificity of it’s formation and is most simply- the length of the shortest pathway.
A Model to Discover New Life
The Cronin Group used information from a database called Reaxys to determine the MA of 2.5 million unique substances. In doing so, they noticed that in all living samples, the MA was above 15.
This discovery is the basis for their proposal to adopt the system of categorizing a substance as living or not based on it’s molecular assembly number.
Our system avoids the potential of false negatives and allows us to search the universe agnostically for evidence of what life does rather than attempting to define what life is.
Right now, we use Mass Spectrometry among many other things to examine the characteristics of far off molecules, but Cronin Labs’ proposed model might just be the key to drawing the is/isn’t conclusion for whether a molecule we discover is life using an empirical measure rather than a comparison to what life looks like for us here on Earth.
I came across this paper having followed Professor Lee Cronin since he joined us on the podcast a few months ago. (Here’s an 8 minute clip from that interview where Lee defines ‘life’ in his own words, strongly referencing ideas from this paper.) I didn’t fully take in the significance of what he was saying then until reading this publication on the deep dive behind what he only teased in our full interview, and I’ll be eagerly looking forward to what they put out next.
Next week, I’m going run another page crimps post on Carl Sagan’s The Demon Haunted World.
Cheers.
-Benjamin Anderson
