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Astrobiologist Sara Imari Walker is developing a deeper description of what life is. She tells Thomas Lewton how it has already led to discoveries about the minimum complexity something must have to be alive
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 Sara Imari Walker, 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 why certain complex objects have become more abundant 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.
If Assembly Theory proves correct, she tells New Scientist, 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?S
A lot of people focus on the RNA world scenario, for example, which posits that selfrepli
