How do scientists measure—and define—life?
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Mar 29, 2025
How do scientists measure and define life in the natural world? Dr. Lee Cronin gives us a definition, in 4 minutes:
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0:00
The description for life is very confused
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It doesn't need a gene, doesn't need this, doesn't need that. And if you ask 10 different biologists what life is, you get a thousand different answers
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My definition of life is any system that is able to produce complexity at scale
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That's it. It's really simple. So basically you want to have this object, this machine, if you like
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and it can spit out complex objects, identical numbers of them, and that thing that can do that is living
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The way to determine if a system is alive or not is to calculate its assembly
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Assembly, Big A, is really how much selection has gone on into this stuff
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And the way to do it is to basically look at the stuff, count the number of objects that are identical
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and then for each object, class, cut them up and get the assembly index
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And so the assembly index times the number of objects gives you a number
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and you sum them together, and you get this amount of assembly. So really, this then compresses the definition of life
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is life does non-random things at scale. If you can measure if something is alive or not
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then you can then start to understand the process are giving rise to life
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You can find artifacts associated with life and that tells you how evolution
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is occurring in the universe as a universal phenomenon. So how much selection is going on
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And I think that's really important if we are to kind of understand the phase transitions that occur
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between kind of sand, inorganic material and biological material that we going to find on Earth and in the solar system and in the universe How close am I to creating life
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One of the most exciting things about assembly theory is not only does it allow us to understand
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potentially the transition from non-life to life, number one. Number two, it allows us to quantify life
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but even more importantly in the way, it allows us to think about how we can make life in the lap
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And because we now understand the three parameters, and those three parameters are all about time
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the time to make the objects, the time which the objects falls apart naturally
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if there's no living system, time for which the object can persist in a living lineage
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Those three times have allowed me and my colleagues to come up with a design to make a design
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to make an origin of life machine. Similar to the kind of predictions
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that were made by physicists when they were looking for the Higgs boson
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what we've said is if we build a selection engine that basically looks for objects
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that have these three timescale characteristics, then we will have made life
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And so what we've got to randomly do is keep basically doing random chemistry to look for these things
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And we now know what they are and we are building engines in my lab
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How long is it going to take to do? I don't know. How much money is in the cost? I don't know. How many people need to help me? I don't know
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But I don't think it is going to be many decades. I think we are going to be able to find
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how selection produces life-like systems from sand very quickly and is really exciting
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