What is life? Despite all scientific advances, we still don’t really know.
Lee Cronin, professor of chemistry at the University of Glasgow’s Regis School of Chemistry, says there’s a huge disconnect between the physics of the universe and the biological processes we observe, making it difficult to understand how inanimate matter can breathe, think and evolve into life.
The solution? Cronin proposes assembly theory, which harnesses large-scale complexity to create adaptive life by piecing together all the components that work together. Assembly theory suggests that life emerges through two primary processes: replication and existence. These two simple words, Cronin explains, are the essence of life as we know it.
Thanks to Cronin, this theory is now being used by NASA in their search for life on other planets. Together, we are getting closer to understanding the mysteries of life: how it began, what it looks like, and how it evolves.
Lee Cronin: We don’t know what life is. That’s crazy right now. We still don’t know what life is. Not even life on Earth. Our current definition of physics, or what we think of physics today, is missing something like a timeless universe. The universe unfolds like a clockwork music box. You play music and it just happens. And you can go back in time, or go back in time to the future. But that doesn’t fully explain how life began, or the novelty and the limitlessness of biology.
In contrast, biology is doing all kinds of crazy things. New species are invented all the time, and humanity is developing new technologies, new cultures, or extra features, new memes. There’s a huge disconnect between the physics of the universe, the physics of the universe as we understand it, and what’s going on in evolution. The fact that evolution exists in the physical universe and is defined by the same reality or rules means that there might be room to understand how to bridge the gap between physics and biology.
My name is Lee Cronin and I am the Regis Professor of Chemistry at the University of Glasgow where I research all things in the universe related to chemistry, the origin of life and the creation of life. There are two strange things in the universe. One is the physical universe, from the Big Bang to the formation of stars, we understand the Standard Model, gravity, time and quantum mechanics. The other is the evolution of biology, which began on Earth around 4 billion years ago, and has seen innovations ever since, all the way up to the technologies we’ve created.
I realized there was an interesting problem: the physics of the universe didn’t predict the emergence of biology, and it doesn’t really explain why biology started to evolve. There was the theory of evolution proposed by Darwin, and Darwin did a great job of understanding the changes that happen slowly, but there was no moment that bridged the gap from physics to biology in one fell swoop. “Assembly theory” actually allows us to explain how inanimate matter evolves, and how we can literally turn sand into cells by the process of selection.
What does life actually do? Simply put, life creates complexity at scale. Let’s take a simple example. Take something as bizarre as the iPhone. If you went to Mars and found one iPhone, you might be a little surprised. Maybe it was just a coincidence. But if you found two or three or ten or a hundred iPhones, all working properly, you could be pretty sure that those iPhones were made by a technological process that was coupled with a living system. So assembly encompasses the ability to create complexity at scale for different things.
As a chemist, I think about molecules all the time. A molecule is a series of atoms connected by what we call bonds. When I was coming up with the idea of the “assembly index” and applying it to molecules, I imagined that you could cut different atoms out of a molecule, and keep cutting, and keep cutting, until only the atoms were left. For example, say you have a molecule made of carbon, nitrogen, and oxygen. If you cut all of those out, you’re left with a base at the bottom of the tree that has carbon, nitrogen, and oxygen atoms. To reconstruct the molecule, you combine them in the right order to form a molecule. The assembly index is about chopping up a molecule. How many times at the very least do you need to chop that molecule up to get it back down to its original components? The assembly index is literally a measure of the minimum amount of information needed to create that molecule.
One of the motivations for initially developing Assembly Theory was to help NASA find out whether they could find life elsewhere in the solar system. What NASA wanted was to understand Assembly Theory and the Assembly Index as a universal marker for biology. I wanted to challenge the very geocentric approach that NASA currently takes. NASA was looking for molecules that you find on Earth in biology, which I think is fundamentally wrong. Molecules on Earth are not necessarily a universal signature of life, but complexity is. When I started to convince NASA, they started to get involved and said, “Okay, we’ll give you meteorite samples from different places in the universe, and we can use this to actually map not just Earth, but parts of the universe as well.” Currently, the only place in the universe where we know there is life is Earth. We want to use this technology to benchmark life on Earth and then go even further out to see if we can find life on Mars or in the outer solar system.
Can I explain how matter became life? Yes, rocks became dinosaurs. Assembly theory challenges the idea that life is not possible because it explains how rocks are ground down in stages and subjected to selection, creating complexity in stages. This is something that can be very carefully quantified. The environment on Earth, the origin of life, created the chemistry and the environment, and cooking began. The origin of life, and the key to life in the universe, is one word: existence. For an object to exist, it must live longer than its natural lifespan. This process of copying existence against the law of extinction is how life begins to emerge. It’s a fight to survive out of the vortex of randomness, and it’s very simple. It’s like the simplest observation ever: copy and exist. That’s what life is all about. Life is extremely fragile chemistry, and it found a way to replicate itself to continue to exist. We, living organisms, are the oldest relics on Earth, older than some rocks, because we can replicate ourselves and continue to live. This is very interesting. So existence and reproduction are what give rise to biology.
