Celia Blanco I read in an article that Luke Kemp, from Cambridge University, had identified a series of civilizations over the history of Earth and calculated that the average duration of a civilization is 336 years. That is not surprising. We know that civilizations have risen and fallen throughout Earth's history. But it made me wonder how the pattern would transfer to a technological civilization, and in particular to futures we haven't lived yet. Obviously we can't go and look, because we can't time travel (unfortunately!), but we can simulate. And the beauty of simulation is that you actually get to peek at all the futures we can plausibly envision.

Celia Blanco To me, the most uncomfortable scenario is the one with recurrent collapse, Ouroboros, which collapses and recovers continually. On paper, it looks like a successful scenario because it stays up and running except during the collapse windows, so the duty cycle looks respectable. But for the people in it, it's a world where every few generations experience severe hardship. Every few generations, humanity loses the infrastructure the previous generations built. And that, at least to me, is uncomfortable in a way that the dramatic scenarios (like a meteorite falling) aren't, because it doesn't require a catastrophe. Nothing really ends it, but it also never gets far enough ahead of its own consumption to build anything that lasts.

Celia Blanco Yes, definitely. The main take-home message for us is that silence doesn't mean absence. We might just be looking at the "wrong" time.

I think it is actually more severe than it sounds, because the effect multiplies. If the emitting civilization is only detectable part of the time, and the listening one is only listening part of the time, the chance of overlap is the product of those two fractions, not the average. So, for example, two civilizations each active 40% of their history have roughly a one in six chance of ever being visible to each other. In other words, a galaxy full of intermittent civilizations and a galaxy that's genuinely empty can look identical from where we're sitting. I think that's a humbling conclusion for life detection, but a hopeful one for the possibility of life existing elsewhere.

Celia Blanco It is indeed a vertiginous range of scales. But if we leave the scale aside, these two topics actually have a few things in common! First, there is only one type of life we know about, and there is only one technological civilization we know about. I find it very interesting to think of all the ways in which life could have emerged and all the ways in which civilizations could go.

Second, both systems exhibit persistence over time, each on its own terms. I think one of the hardest questions in prebiotic chemistry and early evolution is not so much how something starts, but how it keeps going. A chemical system that organizes itself has to keep running and keep consuming to stay organized. A technosphere does the same, at a very different scale. Its structure stays ordered only as long as energy and resources keep moving through it.

There is also something poetic about these two topics. One is about the beginning of us, and one is about the end. I feel tremendously lucky to be able to work on both ends of it.

Celia Blanco I think how amplification can happen is not a big issue. We've known since the 1950s that autocatalysis plus mutual inhibition will drive a system of monomers to one hand. Since then, we've found several more routes that work: polymerization, crystallization, attrition, and others. So it's not that we don't have explanations. We might actually have too many! The field's uncertainty comes from the fact that we have no way to tell which one actually took place in the history of life on Earth. That's a different kind of uncertainty from the one caused by a missing mechanism or an unexplained step, and in some ways it's a more frustrating one.

In my opinion (and I don't think this is said enough), there is a difference between breaking symmetry and breaking it everywhere. The models usually describe well-mixed systems: one vessel, one outcome. But the early Earth consisted of a huge number of partly isolated environments, and if each one breaks symmetry on its own, you'd expect roughly half to land on each hand. So you need the local outcomes to communicate somehow, through transport, or through one environment simply winning and exporting its chemistry. In other words, the mechanism that makes a pond homochiral is not the mechanism that makes a planet homochiral, and we don't usually separate those questions as much as we should.

Celia Blanco As a physicist, I'd say both. Many systems change smoothly in some underlying parameters but abruptly in others, like a change of phase. Water can gradually get colder, but it doesn't gradually get more solid. I think the transition from chemistry to biology might have looked something like that. While it makes sense to think that the diversity of molecules and reactions increased gradually, I imagine closure happened suddenly. As the number of possible molecules and reactions grew, the system must have reached a critical density of interactions, at which a subset of the network could start producing everything it needed to keep itself going. I think that's the point at which the system stopped being a collection of unrelated reactions and started being a thing. All this to say, there might be nothing special or dramatic happening close to the transition, no spark or anything like that. It's just that the loop closed.

Celia Blanco I think the term "frozen accident" is actually two claims in one. "Frozen" is well supported. Once the code is used by everything in the cell, any reassignment changes the meaning of every existing gene at once, so the cost of a change is enormous. Even so, it isn't completely frozen. We know of dozens of variant codes and two amino acids that were added after the fact, and some labs have expanded the code deliberately. So it changes; it just changes rarely. "Accident" is a much weaker claim, because the fact that the code can't easily change now tells you nothing about how it got where it is. In fitness landscape terms, "frozen accident" is a statement about the depth of the valleys around the peak. It says nothing about whether we climbed to that peak or landed on it.

My view is that it's probably neither pure accident nor pure selection. Some years ago, we mapped the fitness landscape of RNA molecules that attach amino acids to themselves (a model system, not the actual ancestor). We found that evolution could easily climb the nearest peak, but the valleys between peaks were too deep to be crossed without losing function. So which peak you end up on depends a lot on where you start. That can be seen as the accident part. But in a follow-up study, we found that related RNA molecules prefer chemically similar amino acids. So if the code grew by reusing existing molecules for new amino acids, it would end up tolerant to errors as a by-product, and that tolerance is usually seen as the strongest evidence for selection. More recently, we found that the order in which amino acids entered the code tracks their molecular complexity, with the simple ones in basal positions. That's availability, not accident. It also isn't selection for translation, which is what most of this debate assumes. So it seems that translation inherited an order that was already set.

Celia Blanco I think theory offers explanations and predictions, and I would argue both are as important as the measurements themselves. This is especially true in astrobiology, because we are working with one example, and you cannot do statistics on a sample of one. Theory doesn't fix that, but it lets us explore the space of what could have happened here, and of what could still happen elsewhere.

Where theory falls short is the same place the chirality field is stuck. It is much easier to produce models than to test them experimentally. I can give you three mechanisms that explain the same phenomenon, but I can't tell you which one actually took place on Earth. In some sense, theory generates candidates without ranking them; for that you need measurements. But that is also the beauty of theory. It can tell you which experiments are worth doing, because it shows you where the candidates disagree. So I don't think of it as a hierarchy. It's more like two halves of the same job.

Celia Blanco We both have changed. Everything has changed. I left CAB at the very start of my career and came back with a very different set of questions. My decade in the U.S. is where my work stretched from molecules to civilizations, and I owe a lot of that to the people I worked with there. So it feels less like going back and more like coming full circle: the same place, seen with different eyes.

Celia Blanco The honest answer is that I think it's the same answer at every scale, which is part of why I work on all three. Something begins when a loop closes, and it lasts as long as it can stay ahead of what it consumes. That's true of a chemical network, and I think it's true of a civilization. If that's right, then understanding how the first molecules kept going might teach us something about how we can keep going.