What is life? It seems like a simple enough question. And yet the truth is that we can’t explain why one lump of matter is alive and another is not, which is a problem if you want to figure out how life on Earth began – never mind whether it exists elsewhere. But , a theoretical physicist and astrobiologist at Arizona State University, has a radical new theory that purports to transform our understanding of what it is to be alive.
Most attempts to describe life use Earth as a blueprint. Instead, by pushing past cells and their chemistry to general principles about how complex objects come into existence, Walker claims to have reached a deeper understanding. The idea, known as Assembly Theory, explains than others by placing fresh emphasis on their histories. Now, Walker and her colleagues are testing the theory on lab-grown microworlds. In experiments, they have already discovered a threshold – namely the number of steps on the way to complexity – that seems like it must be met for something to be considered alive.
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If Assembly Theory proves correct, she tells 91av, it will redefine what we mean by “living” things and show that we have been going about the search for life beyond Earth all wrong. In the process, she says, we could even end up creating alien life in a laboratory.
Thomas Lewton: How do we define life at the moment?
Sara Imari Walker: A popular definition, often used by NASA, is that life is a self-sustaining chemical system capable of Darwinian evolution. Every word in there is problematic. I don’t think life necessarily needs to be chemical. It’s a much more abstract phenomenon. Life is about how information structures material objects and what objects are selected to exist, regardless of whether those things are chemical or not.
As for “self-sustaining”, well, first you must define the boundary of the self. Parasites are interesting because they’re not self-sustaining, but if you include their host then they are. Life depends on its environment. A lot of the issues with defining life always come from us wanting to draw a hard boundary. We want to be able to categorise things and put them in the life bin or the not-life bin. But there are always these challenging boundary cases. We shouldn’t assume that we know what life is from the outset because that whole enterprise has failed over many decades.
Haven’t we made some sense of what life is and how it began?
A lot of people focus on the RNA world scenario, for example, which posits that self-replicating RNA molecules appeared as a first step, before DNA or proteins. But they’re still missing the bigger story about how complex chemical systems actually arise. This is a problem because we’re building the answer we expect into the design of origin-of-life experiments. Everybody has narrowed in on the features of life that they think are important, but we haven’t gotten to that deeper understanding that allows us to connect all these pieces together.
How are you getting to a deeper understanding?
A lot of people want to argue that the universe generates complexity for free. In standard physics, we think everything can happen spontaneously and life is just some very rare fluctuation. But the universe is this vast space of possible things. There are 118 known elements, and molecules are made from many of these elements, but there’s not enough material in the entire universe to make even one copy of every possible molecule. And that’s just counting simple molecules, I’m not including big molecules like DNA or proteins. So the likelihood of creating even a moderately complex object, say a DNA polymer, by randomly attaching atoms is exponentially low. If you try to create that twice it’s almost impossible. There’s never been a physics that has dealt specifically with this problem.
How does Assembly Theory try to deal with this?
The key conjecture of Assembly Theory is that the only way for us to observe complex objects is through a process of evolution and selection: where selection is based on things that have been built in the past, and they are used to build subsequent objects. This series of stages leads us to a “complexity threshold” and only above this do you see things that are products of life. Along with Lee Cronin at the University of Glasgow in the UK, our hypothesis is that life is the only physics that builds these high complexity objects. Sometimes I say that life is the physics that decides what gets to exist.
What do you mean by “objects”?
The fundamental objects in Assembly Theory are the emergent complex structures, not fundamental particles like quarks or electrons or photons. We define objects within an “assembly space”, which contains all of the ways of building up an object from its basic building blocks. So an object like a molecule isn’t defined by its three dimensional configuration that you might hold in your hand, and it’s not defined by its mass or electric charge. The object is actually the ways of building the molecule. These histories, which converge on a particular structure that we see, are the object.
An electron can be made anywhere in the universe and has no history, so it’s not a very interesting object. You are also a fundamental object, but with a lot of historical dependency. You might want to cite your age counting back to when you were born, but parts of you are billions of years older. The ribosomes that play a key role in translation of information from DNA to protein, for example, are believed to have been around on Earth for nearly 4 billion years. The specific molecules in your body aren’t that old, but the lineage of these objects being reconstructed goes back that far.
From this perspective, we should think of ourselves as lineages of propagating information that temporarily finds itself aggregated in an individual. We are our history. So, we’re reframing life by thinking about it as a temporally extended structure. It’s a lineage, not an individual.
It seems intuitive that complex living objects are made from simpler objects. In what way is this an explanation for life?
We’re saying that there’s a different kind of complexity, which is assembled. On a meteorite you can have a chemical mixture of many molecules, but because none of them are produced in enough abundance you get this undifferentiated tar. From the perspective of Assembly Theory, no assembled structures have been selected out of that. It’s a flat complexity that is very different to the kind of complexity we’re talking about. According to Assembly Theory, this is why a meteorite has little to do with life.
Our key argument is that if something is hard to make, and requires many steps, then you’re not going to see an abundance unless there was a selected pathway that makes it. In a meteorite, molecules with high assembly level are produced in such small amounts by random processes that they are undetectable. But if an object is alive, then selected pathways can reuse parts from the object’s history to make an abundance of structures with high assembly. That’s the only way to traverse this exponentially growing space of all possible things and so explain the existence of life. You have to trace out historically contingent paths, and we’re trying to find out what the minimal number of steps needed to get there is.
What makes certain pathways “selected”?
If I was wildly conjecturing, I’d say there’s a sort of force that moves objects through assembly space to generate higher assembly objects, which is why the biosphere evolved complexity over time. But I don’t know if it’s a force like in standard physics, we’re trying to think about that. My intuition is that life is the physics that builds and grows possibility spaces. There’s some sort of driving force that the universe is trying to explore in order to make as many objects as it can. It’s trying to maximise the number of things that exist, and life is the way of doing that.
How are you testing this in the lab?
From our general theory about how objects assemble, we predicted that a threshold for life exists. Cronin and his team took a whole bunch of chemical samples from non-living and living materials. Using a mass spectrometer, which measures molecular fragments, of an object. This is the minimal shortest path required to build it.
The seems to be an assembly index of about 15. We don’t know if the number 15 is universal or specific to chemistry on Earth. But the fact that there is a threshold, above which we only observe molecules produced by life, is really significant.
We haven’t found any non-living materials that have an assembly index above 15. This lack of false positives is unusual in origin-of-life science. When astrobiologists, like me, look for life, we seem to think that false positives are inevitable. We’ll find atmospheric oxygen on an exoplanet, or amino acids on meteorites, even though there’s no life there. This tells us that we’ve been looking at life wrong. If life is a real category of nature, and we understand the physics, there should be no false positives. It’s either life, or it’s not.
So, you could you use this assembly index of life to search for alien life?
Yes. We’re applying the idea of an assembly index of 15 to future flight instrumentation for NASA missions. NASA’s Dragonfly mission, set to launch in 2027, will be the first to visit the surface of Saturn’s moon Titan. It’s a good example of the advantage of taking a more general approach to what life is because Titan is very different to Earth: the surface of Titan has hydrocarbon lakes. We don’t expect anything like Earth life to evolve or live in this environment, so if we want to find out if life is on Titan, we need an agnostic technique. My group is now working on determining how we might be able to detect high assembly molecules. We’re working with NASA to ensure that their existing mass spectrometry instrumentation has high enough resolution to detect high assembly molecules.
Life detection on other planets or moons has so far been done by analogy to life on Earth, but this underestimates just how different alien life could be. Ultimately, I want to use Assembly Theory not just to detect life on other planets, but to predict what kind of life-assembling chemistries we expect to evolve on different planets.
How is Assembly Theory going to help you work out the origin of life, on Earth or elsewhere?
I actually think the origin of life was a planetary scale transition. The opposing, reductionist picture is that cells emerged in one environment, like at an isolated hydrothermal vent deep under the sea, and then they expanded over the entire planet.
My view is that geochemistry started to generate more complexity, which changed features of that geochemistry globally. Through these feedback loops, geochemistry transitioned to biochemistry and that led to cellular structures and eventually to humans. You can almost think of it as a condensation across scales from molecules to cells to ecosystems to the planetary scale, all at once.
Is there any way to test that idea?
We would need to build experiments that can explore diverse geochemistries to determine how different conditions could drive selection and the emergence of evolution. An analogy is in the Large Hadron Collider, which was built to explore the conditions at the start of our universe. We need to build a planetary geochemistry experiment to explore the conditions at the start of life.
We can do this with automated chemistry experiments done at scale, which we are working on with Cronin’s team. The ultimate goal is to experimentally search enough of the chemical space of planets in the lab to observe a new origin-of-life event. That is, we want to discover alien life by making it from scratch in the lab. If we can do that, I think we can say we have solved what life is.
Thomas Lewton is a features editor at 91av
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