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22| Sean McMahon — Astrobiology: what is life & how to know it when we see it?
163 Plays11 months ago
Life. What is it? How did it start? Is it unique to Earth, rare or abundantly distributed throughout the universe?
While biology has made great strides in the last two hundred years, these foundational questions remain almost as mysterious as ever. However, in the last three decades, astrobiology has emerged as an academic discipline focused on their resolution. Already we have seen progress, if not aliens. The success of the space telescope Kepler in discovering exoplanets may come to mind. Equally important is the work to understand how we can demarcate biological from abiotic patterns — when we can be sure something is a genuine biosignature (evidence of life) and not a biomorph (looks like life, but is the product of other processes).
Our guest this week is Sean McMahon, a co-director of the UK Centre for Astrobiology. Sean takes us through the field in general and gives particularly thoughtful insights into these epistemological problems. He also cautions that we may need a certain psychological resilience in this quest: it may require generations of painstaking work to arrive at firm answers.
Corrections
In the intro, I say Enceladus is a moon of Jupiter. Nope, it's one of Saturn's moons.
Milestones
(00:00) Intro
(3:22) Start of discussion: astrobiology as where biology meets the physical science
(6:00) What is life?
(9:30) Life is a self-sustaining chemical system capable of Darwinian evolution — NASA 94
(10:44) Life is emergent, therefore hard to define
(12:00) Assembly theory — beer, the pinnacle of life?
(14:22) Schrodinger & DNA
(15:45) Von Neumann machine behavior as defining life
(17:00) All life on Earth we know comes from one source
(22:55) How did life emerge on Earth
(26:40) The most important meal in history — emergence of eukaryotes
(28:20) The difficulty of delineating life from non-life
(33:30) How spray paint looks like life
(35:30) ALH84001
(39:00) How false positives invigorated exobiology
(44:05) The abiotic baseline
(46:30) Chemical gardens
(49:30) Is natural selection the only way to high complexity?
(54:55) Sci-fi & life as we don’t know it
(58:45) Kepler & exoplanets
(1:00:00) It may take generations
(1:03:40) Sagan’s dictum: Extraordinary claims require extraordinary evidence
(1:08:50) Technosignatures: Gomböc, Obelisk, not Pulsar
(1:12:00) Can we prove the null hypothesis (no life)
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Transcript
The Quest for Extraterrestrial Life
00:00:00
Speaker
It's easy to find life on Earth, it surrounds us, and even in the most extreme environments that our planet has to offer, it often frides. So far, our quest to find life on other planets has been unsuccessful, and the question of whether life exists outside of Earth is one for which we have no definitive answer yet, and it's not even clear whether it's going to be easy or hard to give an answer.
00:00:23
Speaker
In principle, it could be easy. An alien could turn up in a flying saucer and announce itself, or we could receive a message from a distant civilization, or we might have to do a little bit more work, do a flyby of one of the moons of Jupiter and celadus, pick up some water vapor and then find that it's teeming with microbes, but it could be much, much harder still.
00:00:45
Speaker
I guess this week is Sean McMaken. He's one of the co-directors of the UK Centre for Astrobiology based here in Edinburgh. And we have a fairly wide-ranging discussion which is appropriate because this is a field which touches on many things, on chemistry, on physics, astronomy, of course on biology, but also on questions of philosophy. We begin discussing what is life, for example.
Philosophical Explorations of Life
00:01:09
Speaker
But where we end up is with Sean cautioning us that this quest to find life on other planets could be a very painstaking undertaking. It may not be obvious. Life may not come up to us and sort of proudly say, here I am. It may be that we need to accumulate evidence slowly and over the course of many years, even generations, before we can be sure that we've identified life on another planet.
00:01:33
Speaker
As an example of some of the problems in store, we can think of one of Sean's particular areas of research into biomorphs. These are things that look life-like but are produced abiotically, i.e. without biology. So they could be things that look like cells or mycelium but are just the products of much simpler chemical reactions.
00:01:57
Speaker
Another example is, we can think back to the episode with Gabor Dimokosh, where we talked about Uma Mua Mua, this difficult to pronounce asteroid, which looks rather like a
Challenges in Defining Life
00:02:09
Speaker
pencil. And for that reason, many people, or some people at least, thought that it was an alien spacecraft. However, Gabor's research shows that actually this kind of pencil, thin pencil-like shape is exactly what you'd expect.
00:02:22
Speaker
for a rock which is being abraded by smaller rocks and dust as it moves through space.
00:02:29
Speaker
So we need to be very careful here, but I'm really excited about this field of astrobiology. It's a young field, and it's seeking to answer one of the most important questions that is answerable. Are we alone in the universe? So we may have to buckle up and wait many years, but I'm really looking forward to following along with this research. Final note, the UK Centre for Astrobiology has its own podcast. So if you want to follow along in more detail, and I encourage you to do so,
00:02:57
Speaker
You can check out the Tartan Tardigrade. It's on iTunes and also on the web, not on Spotify. And with that, this is Multiverses.
00:03:23
Speaker
Sean McMahan, welcome to Multiverses. Thank you. Although it feels a bit odd saying that, given that you've invited me to your lovely office filled with rocks and books here in the University of Edinburgh. So Edinburgh has the largest astrobiology department in the UK. What is it that you study here? I guess every aspect of what's called astrobiology, so the origin, evolution, distribution of life in the universe,
00:03:47
Speaker
part of which, of course, is search for extraterrestrial organisms. But that's not all of it. It's also thinking about how life interacts with its planetary and astronomical environment over long timescales, including on Earth. One of the things that strikes me about the term astrobiology is
00:04:03
Speaker
we think of zoology as a subfield of biology or ichthyology. I was looking up all these last night as a subfield of zoology, so that's fish. And then there's elasma branchology, a subfield of that, which is sharks and rats. But astrobiology is sort of prior to all those things in the chain. It's sort of above, I guess, biology as we study it generally.
00:04:26
Speaker
knows what it's concerned with and it's concerned with things that we have here on the Earth. The aims of astrobiology are somehow prior to that or broader. The way I think of it is astrobiology is about trying to make connections between biology and the rest of the physical sciences and to really understand how this thing called life, whatever it is, actually fits into our broader understanding of how the universe works.
00:04:52
Speaker
Because at the moment, we don't really know. We don't know how life starts. We don't know how many different possible forms it could take on different worlds and different conditions. It's just this sort of island that's divorced from the rest of our understanding. And astrobiology is about trying to fix that. Yeah.
00:05:07
Speaker
But I don't think we should get too hung up on the term or the definition of what astrobiology is or, you know, whether it's a discrete discipline or just a kind of meeting place for other disciplines. Ultimately, it's just a word. It's convenient at the moment. Maybe it won't be convenient as our knowledge evolves in some other way in 20 years.
00:05:26
Speaker
But the work that astrobiologists are actually doing I think makes contributions to traditional disciplines as well, from microbiology to astrophysics to geology.
00:05:39
Speaker
And the important thing is that the science is good, whether we call it astrobiology or call it something else. Yeah. You mentioned something that maybe we don't want to get too hung up on as well, but you said whatever life is, that's kind of what astrobiology is looking at, where that comes from, where it's going and where it could be sustained, I guess, in the broadest terms. But how important is it that we understand what we're looking for? Do we need a sharp definition of life or
00:06:10
Speaker
is a woody one or no definition at all, can we just say, I know it when I see it? It's kind of a controversial question. I'm not sure we always do know it when we see it. But one thing we do know is
00:06:23
Speaker
Or one thing we know a lot about is the behavior of life, what life actually does. We don't necessarily have a general theory of biology. Obviously, we understand Darwinian natural selection to be, in a sense, the unifying theory of biology. But it doesn't really tell you the difference between life and non-life.
00:06:45
Speaker
I mean, evolution by natural selection is something that populations do. It's not necessarily something an individual organism can do. So it doesn't really work as a criterion for testing whether a given object is alive. It might be a product of natural selection. But perhaps the very first organisms actually weren't, because at some stage evolution had to take over materials that had been produced some other way.
00:07:13
Speaker
So, I don't know, every definition of life that's been proposed has some problems. Of course, in philosophy, coming up with definitions for things isn't always a productive research program anyway, because definitions in general tend to have flaws and gaps, and you can think of things that kind of break them.
Debating Definitions and Complexity
00:07:35
Speaker
I suppose some people have this idea that
00:07:38
Speaker
life might be such a kind of discrete, natural kind that it would be comparable with something like water, where, okay, water is H2O. There's no question that we have a perfect definition for something like water. Yeah, that's what we call, I guess, an analytic definition, right? It's just there is...
00:07:56
Speaker
No doubt. It's either H2O or it's not water, so there's never any question. Whereas if we didn't know that water was H2O and we tried to define it based on its observed properties, then it would be much more difficult because its properties are different in different conditions. Somebody said to me the other day, it would be as if we were trying to define water by watching, getting as much data as we can about the way the waves come in on the beach.
00:08:21
Speaker
It doesn't matter how much data you get on the way the waves come in on the beach. You're never going to arrive at a perfect definition of water until you have a proper scientific understanding of what water actually is, which is H2O. It just might not be possible to get that kind of precise definition when it comes to something like life, which is such a messy phenomenon where we can imagine forms of life that aren't based on the same chemistry.
00:08:44
Speaker
about which intuitively we would want to call life if they have certain behaviours. But again, there isn't a simple list of behaviours that constitute the necessary and sufficient conditions for something being alive. Nobody's really been able to come up with one. So yeah, the whole issue is kind of thorny. There's disagreement even about whether we need a definition. Clearly we can do research on the core issues of astrobiology without having a definition.
00:09:10
Speaker
um and we do and maybe it will eventually come out of that research that we we arrive at one but i'm not too worried about it no fair enough yeah i mean i i'd agree that um yeah all of the definitions of of life that i've seen seem to and i fail to capture the spirit of what's interesting about life for us like you say there's this famous uh definition from from nasa 94 where they say oh it's um
00:09:38
Speaker
chemical system, self-sustaining chemical system capable of Darwinian evolution. Do I really care that I'm capable of Darwinian evolution or that you're capable of Darwinian evolution? Well, you by yourself are not, I'm sorry to tell you, because you're not a population. What does self-sustaining actually mean? Nothing alive is self-sustaining. Everything requires energy and nutrients from its environment.
00:10:05
Speaker
And many things are obviously not self-sustaining. I mean, if you've ever tried to keep sea monkeys alive, you have to do a lot of work, but you're not gonna say they don't qualify as being alive. I guess when they came up with that definition, they were maybe trying to capture the distinction between organisms and things like viruses, which can only be produced by hijacking the machinery of the host cell. And so in that very technical, quite specific sense on not self-sustaining.
00:10:31
Speaker
Yeah. But I'm not convinced, you know, any parasite requires something from its host organism. Yeah. We still want to acknowledge that parasites are alive. Yeah. So I don't know. And viruses are doing something really interesting that they can only do in the presence of things that we would accept as life proper. So yeah, it sort of like, well, you know, viruses on the fence, you can put them on either side and say it seems kind of arbitrary. But I guess there is always going to be some
00:11:01
Speaker
issue drawing a line with looking at life because it is something that emerges from non-life. And presumably it's not like an on the switch, right? It's not like there's nothing there and then it's there. I mean, similar to consciousness, right? I think it's kind of, the features that we're interested in are going to come out in degrees. And of course, when people think about alien life, they're always excited about the top of the chain, like some little green men, but actually what we're more likely to find, at least in our own backyard, is something very, very small and
00:11:32
Speaker
I think it's important to say that it's not some magical, mysterious thing about life that it's so hard to find. Most things are hard to find. If you're trying to find a chair or a table, you quite quickly run into cases where you're not really sure if it belongs in the category or not. Definitions are just not the sort of thing that science readily produces for the various natural objects that it studies, and generally doesn't bother even trying.
00:11:57
Speaker
Yeah, I guess it's an emergent property. I mean, it's interesting, you know, one of the approaches, we shouldn't spend too long on this because we want to, but I do find it fascinating to me, but one of the approaches is this kind of constructor theory or assembly theory, where
00:12:16
Speaker
you look at a number of stages that you'd need to go through some, you know, combining atoms into molecules and molecules into molecules and then polymers and you say, okay, well, it's almost, it's very similar to, in my mind, to Kolmogorov complexity in kind of algorithms where you say, okay, well, this is the minimum number of, this is the minimum amount
00:12:43
Speaker
way that you could express this function. The shortest program you can write. The shortest program you could write, yeah. So it's sort of like the shortest way you can assemble something. And of course, well, there's problems with that definition in my mind, because you can probably come up with something extremely complex that just isn't life-like.
Earth's Life Origins and Evolutionary Path
00:13:00
Speaker
But it does capture something, which is that, well, there is going to be a certain arbitrary, arbitrariness, in where you say, oh, now it's sufficiently complex. That's absolutely true. There are minerals, quite rare minerals, whose lattice structure is so complicated that if you actually calculate the number of steps it takes to put atoms into that lattice, it starts to overlap with certain proteins and certain things that we would think of as biological.
00:13:28
Speaker
My favorite little factoid, by the way, about the assembly theory stuff is that when they, so this is work by Lee Cronin's group in Glasgow, they published a paper in which they used mass spectrometry to analyze the number of components that basically, so they're sort of doing it in reverse, they're breaking material down and looking at the number of fragments that you can make as an indicator of how many steps it takes to put the thing together.
00:13:55
Speaker
and they analyzed a range of different substances and the most complicated thing that came out in their data set and they had various organisms rocks minerals different organic compounds and so on the most complicated thing was beer okay it's like far far and away more complicated than all the other things including including even whiskey that's beer yeah well yeah maybe beer is what we'd call i guess a techno signature or something could be yeah yeah something that or it could just be more alive than we are who knows yeah
00:14:21
Speaker
I've got one last thing to say on this, which is that Schrödinger had this book, What is Life, where frustratingly he doesn't answer the question. He just asks more questions. He hints at some things, you know, entropy is important. Yeah, like entropy as he calls it. Yeah, which annoys a lot of people. But yeah, there's this aspect of sort of stealing energy or free energy. But then a fire does that, of course. And fire, fire has that. But the one thing, back onto this point of kind of lattices and so forth, he
00:14:50
Speaker
he kind of second guessed the structure of DNA before it was revealed. So that is quite impressive. And he said, we need some kind of aperiodic crystal structure that's going to encode information. And it's got to be a pretty solid structure so they can be passed on.
00:15:09
Speaker
but not so solid that it can't evolve and change and also have interesting variation within it. And I think that's quite key. Life lives somewhere between complete randomness and complete homogeneity, but it's very, very hard to pin down. The fact is there's lots of other things that live on that interface, not live. There's lots of other things that exist on that interface. The kinds of self-organised, dissipative structures that form in far from equilibrium conditions
00:15:33
Speaker
everything from convection cells to the sorts of mineral precipitation processes that we study here generate really interesting structures that look very biological on that interface but are not themselves biological. For my money the person who came closest to coming up with a properly abstract universal image of what life is might be von Neumann. I really like the von Neumann machine where you have
00:16:00
Speaker
these sort of entities that are capable of assimilating material from the environment, using it to build a kind of instruction tape together with the machinery necessary for decoding the instruction tape, and then sort of executing the instructions to produce a copy of themselves, including the machinery and the tape. And this is something that you can write a computer program to do this.
00:16:24
Speaker
And for me, that's the most satisfying general, I think that's the level of abstraction you can get to where it's still useful and interesting. Because for me, if we found a von Neumann machine that was sort of, let's say, implemented in a different chemistry,
00:16:40
Speaker
I would be very tempted to regard that as alive. So that kind of works for me. I don't know if it qualifies as a definition, but I think it's as close as anybody's really come that is still useful, in my opinion. Yeah. Yeah. So we don't know precisely what life is, at least in the most general sense. But we've got lots of good examples of life on the Earth. It's all around us. In one sense, of course, we only have one example. Well, that's true, actually.
00:17:11
Speaker
Yeah, I suppose that's right. I mean, do we, I guess all the indications are that life has evolved singly on the earth and everything comes from the same, despite the incredible variation that we see in forms. We're all cousins. We're all cousins. And actually under the hood, there's a lot of similarities in the way that, you know, everything's got ATP and you look at the branching structures in trees and the, you know, the computer reason ourselves and so forth.
00:17:38
Speaker
Well, just more fundamentally. I mean, we were all genetically related to each other. You could read that straight from the DNA. Yeah. One of the things that kind of blew my mind out of this here is that, you know, even chemosynthetic life, so life which is not part of the food chain which we're part of, doesn't ultimately get its energy from the sun, but from chemicals typically in hydrothermal vents or under the ground.
00:18:01
Speaker
We're all part of the same family. Yeah, that's right. Which isn't to say that there weren't other forms of life at some stage in the history of the Earth, or even that there aren't still somewhere hidden today. We don't really know. There's this shadow biosphere hypothesis that
00:18:19
Speaker
that kind of postulates that there might be radically different forms of life, even on the Earth, and we just don't really know how to find them, which is maybe more useful as a thought experiment than it is plausible as a scientific hypothesis. I imagine that when life began, there might well have been various sort of different versions of it, which may have arisen more or less independently.
00:18:45
Speaker
When you start to build an understanding of how life originated, very quickly you start to think, well, this had to happen or that had to happen. And if this had to happen and that had to happen, then it probably happened more than once and in slightly different ways. And the more we understand about the origin of life, the more tempting it is to predict that it would have arisen.
00:19:07
Speaker
potentially multiple times with slightly different features and it's just there's no there's not going to be any surviving trace of that now and the fossil record isn't good enough to reveal forms of life with slightly different chemistry that appeared maybe transiently around some hydrothermal vent four billion years ago and then disappeared again got swallowed up by the rest of the biosphere so we may never really know for sure so this is probably a dumb question but
00:19:34
Speaker
can we even be sure that we are all related? I mean, we know that there's a lot of evolutionary features which have, you know, convergently evolved. So the same thing has popped up independently because it's a useful adaptive feature. So yeah, how certain can we be that we all come from the same random event or
00:19:57
Speaker
I don't know how best to describe it, but the same. Point of origin. Yeah. Well, I think we could be pretty sure because of the universality of the genetic code. It is theoretically possible to build genetic codes in other ways. You can have a different number of base pairs in the alphabet. And it still works. People have sort of tinkered in the lab to show that you can build organisms along different lines by sort of slightly modifying the genetic code that they use.
00:20:27
Speaker
And yet, all life that we know of on Earth uses the same code. And if you compare the codes of different organisms, you can fit them into this tree of relatedness, which shows you exactly how things are related evolutionarily. And then you can calibrate that against the fossil record to start putting dates on when different branches of the tree diverged.
00:20:53
Speaker
And it's all wonderfully coherent and satisfying and it fits together beautifully. Of course there are always little areas of the tree where there's some confusion about
00:21:06
Speaker
why, let's say, a divergence seems to have happened if you believe the molecular clocks either much earlier or much later than you would think from the fossil record. And it's a technically challenging task to do this calibration and make it coherent. But so far, it's been very successful. So we can be pretty sure that everything is just one tree of life on Earth. The other thing that popped into my head was this kirouti
00:21:36
Speaker
as well, which is just, I guess, everything is either left-handed or right-handed, depending on whether you're thinking of sugars or... Or amino acids. Yeah, I mean, we can actually reconstruct some of the properties of the last universal common ancestor of all living things because there's this core set of conserved genes and proteins that all organisms produce
00:22:02
Speaker
Or if they don't, we can infer how and when it was lost so that we can reconstruct what the ancestral set of genes and proteins probably was. It's not easy to do that. It's complicated by the extent of horizontal gene transfer between different branches of the tree of life, which can make things appear more universal than they really are.
00:22:22
Speaker
You can have characters that are lost and then secondarily acquired again, and there's this sort of stochastic element, but nevertheless you can say certain things about what that organism was able to do and which kind of metabolic cycles it was able to carry out. And it obviously wouldn't be possible to do that if we weren't starting with organisms that were related to each other. Let's catch that out a little bit.
00:22:49
Speaker
as best we understand it, what do we think that earliest life form was like and how do we think it came about in our kind of tech stack of life, I guess. Yeah, of course, that's a different question from what the last universal common ancestor was like, because the first organism was ancestral to that, possibly by hundreds of millions of years. Take us back as far as you... But the first organisms, I mean,
00:23:17
Speaker
It's quite likely, I think, that they were chemosynthetic microorganisms. They would have had a cell membrane that was recognizably similar to the cell membranes of microorganisms that we find on the Earth today. They would have had
00:23:45
Speaker
some genetic code that was, depending on which model you go along with, either based on RNA or DNA, they would have needed to assimilate nutrients from their environment, and they would have needed some source of energy that was quite likely to have been the reaction between carbon dioxide and hydrogen, although we don't know that absolutely for a fact.
00:24:14
Speaker
There are models of how, particularly in alkaline hydrothermal vents, metabolism of this kind might have begun. The cell membrane is probably in some sense the easiest part of the puzzle because fatty acids more or less spontaneously organize themselves into little vesicles.
00:24:40
Speaker
That's easy to make, but you need to get the right stuff inside it. Exactly. What's really difficult is to figure out how these different systems became kind of coupled to each other so that you have cells that are reproducing and are correctly passing on and inheriting the machinery that they need.
00:25:01
Speaker
so that they can carry on metabolising and assimilating nutrients and at some point repairing damage. You need to couple this reproductive system, the metabolic system, the homeostatic machinery and get it all working together. That's where it's technically quite challenging to figure out exactly how this could have happened.
00:25:23
Speaker
I mean, is it just a case of rolling the die enough? You're creating a lot of cell membrane structures. You also maybe have some RNA or DNA type things in this kind of soup. I'm sure. Yeah, I think there's obviously a huge random element here where you have
00:25:44
Speaker
We don't take this too literally, but in a sense, millions of failed experiments per second. It's not like nature is trying to produce life, but this sort of chemistry is happening. And at some point, just by chance, because of the law of large numbers, it's going to generate something that has the right balance of structures and properties as it's able to start reproducing itself in this synchronized way.
00:26:13
Speaker
And it might be sort of vanishingly improbable in which case, I mean, that's totally consistent with the evidence that we have since we've never found life anywhere else. And since we wouldn't be here if it hadn't happened. So the probability can be as low as you like, and we're still in the realm of what's consistent with the evidence. There are certain other events in the history of life on Earth that may also have been sort of staggeringly unlikely. And the obvious one would be the origin of the eukaryote cell, which involved
00:26:44
Speaker
One microorganism being consumed by another one and rather than being digested and destroyed, it was able to carry on living inside its sort of new host organism and reproduced in sync with its host organism and became a kind of symbiont that ultimately was just assimilated into the host so that you can no longer say where the host ends and where the symbiont begins.
00:27:08
Speaker
So this is mitochondria, the power station of... Yeah, chloroplasts. Somehow the nucleus may have arisen out of some of these symbiotic events, that's not very well understood at the moment. But if that hadn't happened, and it seems to have only happened at least the acquisition of mitochondria and the origin of the eukaryote cell,
00:27:27
Speaker
seems to have happened only once. Eukaryotes have their own single tree that's nested within the larger tree of life. There aren't multiple different origins of eukaryotes. Even though it could presumably happen any time, it only seems to have happened once.
00:27:44
Speaker
So if that hadn't happened, we'd still have archaea, which are, in many ways, way more varied than eukaryotes, and really good at living in lots of places and surviving, but they're not going out building spaceships.
Astrobiology's Role in Mars Exploration
00:28:00
Speaker
Yeah, we'd still have microorganisms, but we would never have achieved large organisms capable of building complicated, differentiated tissues and all the rest of it, multicellular life. OK, so life
00:28:14
Speaker
we know it, it's pretty diverse, but still just a single point of origin. Let's talk about the kind of tech stack that we know for life at the moment. So we're kind of carbon-based, water's important, there's a few, a handful of other elements that are really important. In fact, I think it's like six elements or something account for 99% of the of the biomass. So we have a kind of good idea of
00:28:39
Speaker
of the ingredients that are working here on Earth. But as I understand it, even trying to look for those and use that kind of fingerprint elsewhere, that's pretty hard. But that's probably our first, is that sort of the most obvious place to look, I guess, if we want to find either another instance of life originating here on Earth or if we're looking elsewhere? So it's not even clear. So there's a related issue, which comes back to something you said earlier, that there's this fuzzy boundary between life and non-life.
00:29:10
Speaker
It must have been the case that on the early Earth, or wherever the origin of life actually happened, there was some kind of a transition from fairly complex but not biological organic carbon chemistry to biology.
00:29:24
Speaker
And we can argue about where exactly we want to draw that line. But wherever you draw the line, you have the same problem that there's stuff on one side of the line that looks very much like the stuff on the other side of the line. So wherever you draw the line, you have this difficulty of drawing any real distinction between prebiotic chemistry, as we say, and biological chemistry.
00:29:45
Speaker
And so when we're looking out into the universe and finding quite complex organic chemistry it's going to be hard for us to say is this life or is this just maybe the sort of chemistry that could have given rise to life but perhaps didn't in the environment where we're looking. And this is going to be particularly difficult in the context of Mars
00:30:05
Speaker
where we are going to be extracting from rocks four billion years old a suite of organic molecules which may be quite complicated and quite reminiscent of biology and it's going to be difficult to know whether it actually is the remains of biology or is it the remains of the sorts of chemistry which on Earth led to biology but maybe on Mars didn't. So yeah, it's difficult. I guess in some ways, like we said,
00:30:33
Speaker
the absence of a perfect definition of life it almost needs to be the last explanation once we've exhausted oh it's it's not uh let's say
00:30:46
Speaker
geophysics, right? Or just organic chemistry, but without these additional things that we're interested in. So I guess you kind of need to exhaust the other possibilities, to be really sure. But are there, I mean, what sort of things, like if we went to Mars, what stuff are they looking for exactly? So it's, you know, complicated organic molecules. Are there particular, within that, is there, can we narrow down a bit?
00:31:12
Speaker
I mean, it's not just the molecules, first of all. Basically, they're looking for fossils broadly construed, which might include structural remains of cells or colonies of microorganisms, which we know have a fossil record on the Earth. So they might find things that are just recognizably fossils, which maybe they wouldn't be able to see until they get these samples back and put them under the microscope. Or maybe in some very optimistic scenarios, they will actually see things with the cameras on the instrument
00:31:42
Speaker
that correspond to the kind of macroscopic expression of communities of microorganisms. So I have on my desk here, for example, an ancient stromatolite. OK, so I've read a lot about these. I've not seen one in the flash. You can see it's got a polished slab of rock that's got these sort of branching tree-like structures in the polished face, which are kind of capped by these dome-like layered
00:32:10
Speaker
features. And this is a product of microbes that lived on the seafloor several hundred million years ago that were trapping and binding sediment grains and precipitating calcium carbonate around the cells to form a basically kind of living rock that grew upwards as the microbes had to keep moving towards the light. And these sort of large mushroom-like rocky structures that they formed, stromatolites, are often large enough that if they exist on Mars, the rover cameras would be able to see them.
00:32:40
Speaker
It's interesting because to the untrained eye, which one is, that looks like a rock. Well it is a rock. It is a rock. But it looks like it's never been living is what I want to say. But I suppose we can be really confident that Stramanolites are a good biasing manager to use the kind of term of art.
00:33:01
Speaker
because we have living ones now. We have living ones and sometimes the fossil ones have the fossil microorganisms trapped inside them. Although I wouldn't say that these dramatolite sort of shape or structure is by itself a completely compelling biosignature because there are ways that non-biological processes can make structures very much like that.
00:33:24
Speaker
Some of which have been studied experimentally, and here's another fun sort of factoid. A real impetus for this line of research was the observation paint in car factories building up around where the cars were being sprayed.
00:33:40
Speaker
doesn't form just flat layers of paint on the floor like you might expect. Instead it builds up into these remarkable convex columns, domes, branching structures. And this comes out of the kind of physics of particle deposition in a way that you can actually quite easily simulate with some computer code.
00:34:01
Speaker
and it's just a general property of systems where you have the deposition of lots of small particles that are sticky in the right way that they form these rather complex branching growth structures and you can do the same thing for yourself if you have a can of spray paint and just spray it onto a table maybe use a couple of different colors so that you can build up layers that you can see you'll find that it doesn't just make a flat layer of paint but it makes this rather interesting
00:34:29
Speaker
sort of knobbly structure which if you cut into it will look a lot like stromatolite. So the shape by itself is maybe not diagnostic but there are other features you can look at. Real stromatolites tend to have a characteristic spacing between them that comes from the fact that these are communities of organisms that depend on light and can't grow if they overshadow each other. So they tend to organize with this sort of spacing like eggs sitting in an egg carton. You can look for microfossils inside, you can look for certain organic molecules inside and ultimately
00:35:00
Speaker
The hope is that if we do find evidence of life on Mars, it might consist of several different lines of evidence that all agree with each other, any one of which by itself you might query. But if you have enough lines of evidence that are mutually independent but mutually supportive, it can kind of push the balance of probabilities high enough that you can be reasonably confident you found life.
00:35:21
Speaker
that I think for most sort of feasible scenarios, it's always going to be, there's always going to be a bit of room for doubt. And the job is to try and make that room as small as possible. Yeah, this maybe seems like a good time to mention LH-8401, because, I mean, that was a case of this asteroid that was picked up in Antarctica in 1984. And then a decade or a bit more later,
00:35:49
Speaker
They found lots of evidence that there was life inside. That's right. But then it turned out to be not as compelling as was thought. That's right. I remember being nine years old and seeing on the television news Bill Clinton on the lawn of the White House announcing to the world America had found fossils from Mars.
00:36:07
Speaker
And it was very exciting. The journalists, incidentally at this press conference, were not excited at all. You can find a clip on YouTube. The first question they ask him is not anything to do with life on Mars. It's about some measure that was going through Congress at the time to do with abortion law. Kind of sad that this was, they just were so blithe about the whole thing.
00:36:29
Speaker
But anyway, so there were structures inside that meteorite that looked kind of like bacterial cells, although rather on the small side, composed of minerals. There were globules of carbonate minerals that looked similar to what happens where you have microorganisms producing carbon dioxide that mineralizes around them.
00:36:49
Speaker
There were magnetite crystals, so an iron oxide, that had a prismatic shape quite similar to the magnetite particles that certain bacteria form inside their cells. There were hydrocarbon compounds.
00:37:07
Speaker
which although they weren't very complex organics, could have been the kind of altered remains of what was originally biomass. That was the hypothesis. So any one of these lines of evidence by itself, you might query, but to have so many lines of evidence, all seeming to point to life, was thought to be pretty compelling. I mean, there was always some doubt. Nobody ever said this is 100% watertight, even when the first announcement was made. And even in Bill Clinton's speech, there's a line about, you know, more tests will need to be done.
00:37:37
Speaker
And of course what actually happened is that the tests were done and none of these lines of evidence actually held water in the end. The things that looked like fossil cells turned out to be the edges of kind of crystal lattice planes which where they kind of appear on the surface
00:37:53
Speaker
can look like little worms combined with the effects of coating the sample for electron microscopy that can kind of make things look more round because they're being coated. The organic compounds or hydrocarbon compounds turned out to be a combination of probable contamination and some organic compounds that actually did come from Mars and were formed by non-biological chemical reactions.
00:38:18
Speaker
And you can tell this from the way that they're associated with certain minerals that are also produced by those same reactions. So if you look on a really fine scale, you can see that where these carbon compounds are is also where these minerals are that tell us about these reactions.
00:38:31
Speaker
So unfortunately, not biology. The magnetite crystals seem to be the product of the shock decomposition of iron carbonate minerals in the kind of heat and pressure of the, or particularly the shock pressure of the impact event that sent this thing on its way to Earth. And the carbonate globules turn out to be the product of
00:38:48
Speaker
of hydrothermal mineral precipitation processes. So every one of those lines of evidence ended up falling down. So there are probably still some holdouts in the community who would still claim that this on balance looks like evidence of life. But I think if there's a consensus now, it's that actually, no, this was a very misleading, very unfortunate kind of false positive. Well, in some ways it was. I mean, so there's a cautionary tale there for sure.
00:39:16
Speaker
It surprises me that despite those journalists on the White House lawn not seeing impressed, the lasting effect of this seems to have been like a reinvigoration of the astrobiology program. At least that's what I've heard it as. And so the kind of first impression we had life is kind of what mattered. Whereas if one thinks back to Viking, where they sent to Mars some experiments which weren't
00:39:44
Speaker
particularly well thought out. And I think the people who built those experiments were like, actually, yeah, we wanted to do something slightly different and NASA just wanted to go kind of gung-ho looking for life. Whereas in hindsight, what would have happened with Viking is that they would have looked at more basic questions like what is the chemistry here? But anyway, the Viking results came back and were kind of disappointing because people had really high expectations. They didn't see anything. In reflection, they weren't so bad. They weren't so damning as that at all.
00:40:13
Speaker
But there was a little bit of a kind of astrobiology winter, I suppose, after that, in which everyone was slunk off. And yeah, it seemed that the research was somewhat depressed after that. So yeah, I don't know. There's also a first impressions count in a LTR. There's a lot to unpack here. I mean, astrobiology has sort of been born twice out of NASA initiatives. The first time it was called exobiology. And this would have been around 1960.
00:40:39
Speaker
And at that time, a big stimulus was a then-recent result that there seemed to be vegetation on the surface of Mars.
NASA's Early Exobiology Efforts
00:40:50
Speaker
And a Harvard astronomer called William Sinton had observed features in the spectrum of Mars that seemed to appear when Mars was showing us its darker regions and disappear when it was facing the other way.
00:41:02
Speaker
And it had long been thought that the darker features that you could see on the surface of Mars, which kind of come and go with the seasons, must be vegetation. And the features that were observed by William Sinton in the spectrum corresponded to organic molecules. And this was cited as an important result by people like Josh Lederberg, who were really influential in sort of getting NASA to do exobiology.
00:41:26
Speaker
Later it turned out that actually these features were not on Mars. They were water vapor in Earth's atmosphere along the line of sight in the telescope observations. And actually it turns out that heavy water, so water with a bit of deuterium in it, has absorption features around three microns in the spectrum, the same place as organic molecules. So this was all a mistake. And yet it probably played an important role in the origin of exobiology at NASA.
00:41:56
Speaker
And then you could argue that this meteorite, ALH84001, was also kind of interpreted in a mistaken way, but also played a really important role in the kind of second origin of astrobiology. Because not long after that, NASA got a bit more funding to do astrobiology, and the NASA Astrobiology Institute started, and astrobiology started to take off as a field.
00:42:19
Speaker
But a more positive framing of the whole narrative would just be to say, these observations were not understood because the relevant science hadn't been done. And the consequence of these observations being made was that people started to put the resources in to get the relevant science done. And so I guess the thing about cautionary tales is that, almost by definition, you learn from them. And we have learned a lot from the study of that meteorite. And we now have the scientific understanding
00:42:47
Speaker
to explain some of those features and to think more in a more subtle and sophisticated way about how to find evidence of life. And a lot of that understanding comes from the fact that NASA has injected a lot of cash into astrobiology. So, yeah, it's an interesting story. Yeah, I think it's, I mean, one of the lessons or one of the things it highlights is just how hard
00:43:12
Speaker
It is. I don't think anyone was doing bad science here. Sintam, his hypothesis is a pretty good one, but he just didn't know that you could have deuterium in the atmosphere. Who would have thought that just by coincidence, heavy water has these features in just the same place as organic molecules? Just a really annoying coincidence. It shows that level of
00:43:32
Speaker
Yeah, both caution and, I don't know, breadth of knowledge that one needs in the astrobiology community to really check out these things. There are lots of these cautionary tales.
00:43:44
Speaker
But I don't think that's really a problem. In a way, that's just sort of how we learn. You've got a history of any science. And you'll see mistakes being made. The things that in hindsight we now understand to be mistakes, but were logical interpretations based on the understanding people had at the time. And it's just, you know, that's how science makes progress.
00:44:04
Speaker
And this is, we've scratched a lot of my kind of general issues on the field, but I really want to come on to some of your work because it completely touches on this issue of make contact with it in that.
00:44:15
Speaker
You're trying to look for things which look like life, but aren't life. I guess they sort of make us streetwise, as it were. And so we don't kind of go out there and be like, I've got it. And like, no, this is like paint on the wall of a garage. Yeah. I think, so here's a way of framing it that was put to me by an astrophysicist, Kevin Heng at Munich. He said, searching for life, what we're really doing is looking for a signal above a baseline.
00:44:44
Speaker
We're looking for an anomaly above some baseline, whether we're talking about the spectrum of light captured from an exoplanet or the complexity of shapes that we can measure in the lab. In order to recognize the signal, you need to know what the baseline is. So in order to recognize life, you need to know what are the non-biological processes doing in that environment that might generate interesting things that are not life.
00:45:06
Speaker
And it struck me and a few other people as well that most work that's been done in astrobiology has been focused on what the signal might look like. Very little has been focused on what the baseline looks like in these environments where we're looking for life.
00:45:22
Speaker
And all of these cautionary tales, and I've got a list somewhere of like 20 of them. We've only touched on two or three. In every case, there was something about the baseline that had been missed. And so I think it's reasonable enough in a field that has thousands of people working in it for somebody to put the effort in to thinking about what the baseline is going to look like in these contexts where we're looking for life.
00:45:44
Speaker
So if we're looking for organic molecules on Mars, what are the non-biological organic molecules going to look like? If we're looking for the shapes of microorganisms that have been replaced by minerals and fossilized, what are the shapes of those minerals going to be that are not fossils? If we're looking at spectra from exoplanets, what features of those spectra are going to be produced by gases in the atmosphere that have nothing to do with life, or minerals on the surface that have nothing to do with life?
00:46:10
Speaker
And it seems to me that these questions are logically prior to any detection of life. We need to know the baseline to recognize the signal. And so that's what I work on.
00:46:18
Speaker
Yeah, you're using surprisingly old techniques, I guess, to look at this quite new problem. So yeah, take us through some of the things that you do to produce beautiful things that look life-like. Well, yeah, so one example is the chemical garden. This is such an old area of science that in a way predates modern science, comes out of the alchemy that was being done in the 17th century.
00:46:48
Speaker
when sort of European scholars realized that certain reactions would generate tree-like structures that you could see growing before your eyes. And to them it was kind of proof that there's this sort of rudimentary vitality in all matter. And this was the time when people believed in spontaneous generation and stuff like that.
00:47:10
Speaker
We now understand that these processes are the consequence of just physics and chemistry. But if I can describe to you what the experiment looks like, you take some crystal grains, about the sizes of grains of salt that you might have in your salt shaker, you drop them into a liquid, which is water glass, an alkaline
00:47:35
Speaker
sodium silicate solution and then that's it you just watch and what happens is that these crystal grains as they start to dissolve into the liquid
00:47:45
Speaker
produce a little kind of bag like membrane around themselves which is sort of flexible and porous and pressure builds up on the inside of this membrane until it eventually ruptures and you get these finger like jets of fluid coming out which spontaneously are sort of wrapped around by new membrane that grows along the walls of these fingers. So you end up with tubes radiating from a central blob which corresponds to where the crystals originally were
00:48:15
Speaker
and the tubes continue to grow outwards from that central blob, and they can branch in a tree-like way, and the branches can converge and join back together again, like a fungal mycelium. And the whole thing just takes place in minutes. And then the last thing that happens is that this sort of gelatinous membrane becomes encrusted by crystals of metal oxides, so like iron oxides,
00:48:43
Speaker
so that you end up with, instead of this flexible bag-like thing, a solid object that's kind of brittle, which is sort of these little hollow tubes and branching tubes made of solid minerals, which, if you do the experiment on the right scale, are almost indistinguishable from the metal oxide tubes that certain bacteria will make, for example.
00:49:06
Speaker
So one area of my work has been to investigate whether this type of chemistry could take place on the early Earth or early Mars and how we can differentiate between these microscopic but completely inorganic mineral growth structures on the one hand and real fossil microbes on the other hand. I mean, these seem like really interesting experiments in their own right, like whether or not we're interested in just demarcating that
00:49:36
Speaker
that or delineating, I guess, that baseline. You were discovering really interesting things about how quite simple processes can produce wonderful structures. Yeah, the chemistry is quite simple. The physics is actually quite complicated, fluid dynamics and so on. But yeah, it's one thing you realize when you start attending to the baseline is how
00:50:00
Speaker
interesting that non-biological universe is and how many complicated surprising Completely unintuitive things form out there in nature. They have nothing to do with life. Yeah One thing I do I do wonder is yeah, could we discover something as interesting as life from an anthropocentric perspective? Yeah, that that's non-life the thing that sort of coming back to this Darwinian evolution point and
00:50:25
Speaker
We're saying, well, what's so important about going into an evolution? And we should say that it's the thing which seems to not just add noise over time, but although it's a random process in a sense, it does produce something directed. And actually a previous guest of mine was Simon Kirby talking about language evolution and modeling language.
00:50:49
Speaker
as an organism and remarkably you can start off with some completely random unstructured mapping between sounds or you know some kind of symbols and the world and then just as a process of evolving that mapping through subsequent generations of learners who have kind of constraints it will develop structure. Maybe that's why
00:51:13
Speaker
Darwinian point is so interesting, and why perhaps there's kind of a limit to how interesting you can get with the agarlic structures. Absolutely. Evolutionary algorithms, as you probably know, are quite an important engineering tool. But more generally, natural selection is the only way that we know nature can make structures of the order of complexity that organisms are.
00:51:38
Speaker
However, a fossil can be much simpler than an organism. And if we're looking for evidence of life on Mars, for example, most of the biological information is lost, is gone. What's left behind is a far simpler kind of residue. We have relatively simple molecules, relatively simple structures, maybe some isotopic anomalies associated with that. But you've lost that fingerprint of Darwinian evolution, which is that massive degree of complexity that we see in living cells.
00:52:08
Speaker
Now it took us until the 1850s to get natural selection figured out, let's say. So that's relatively late in the history of modern science.
00:52:20
Speaker
Even if you understand that natural selection, it doesn't really give you the ability to predict that life would have the diversity and complexity that it has, because basically we're not that smart. If you imagine, this is quite a complicated thought experiment, but imagine that you were born without any sense of the fact that you yourself are an organism.
00:52:42
Speaker
And without any access to the living world and you would I don't know kept in a concrete box But given a thorough understanding of physics mathematics chemistry and the logic of natural selection There's no way that you would guess that something like even a bacterium the simplest form of life we can think of Would come into existence as the end of that's one end of that process. Yeah Because you know, we're just not not that imaginative. Yeah, there's a really nice poem by
00:53:12
Speaker
by a Polish Nobel Prize winning poet that's kind of along these lines about how you would never have anticipated a blade of grass or a strawberry or an insect from the best possible understanding of the logical evolution until you'd found them. And I think we might be in that same situation with respect to all kinds of things in the universe. Where it doesn't matter how smart we are, how good our astrophysics is, how good our geophysics is, how good our chemistry is,
00:53:39
Speaker
If we're talking about the endpoints of very, very long evolutionary processes, we're not smart enough to guess what's going to be thrown up. So we have to explore. And the same might be true of processes other than evolution, that they can produce things that we just never would have guessed. And I think that's what we see from exploring the baseline. You find things that are so unbelievably similar to life in their sort of superficial qualities that you just wouldn't, unless you'd seen it with your own eyes, you wouldn't believe it was possible.
00:54:08
Speaker
Yeah, so maybe we will find something as exciting as life. I think it's, yeah, I don't know. I guess it depends what you're excited by. We'll certainly find stuff that we couldn't possibly have predicted that we would find. I mean, there's basically every space mission ends up doing that one way or another. I mean, who knew that Pluto was going to be so complicated? We were expecting a created ball of rock and instead we found this wonderfully dynamic world with dunes and
00:54:34
Speaker
convection cells exposed on the surface with this wonderful polygonal pattern and all these sort of interesting landscapes shaped by the way that nitrogen and ammonia behave at very, very low temperatures.
Creative Perspectives on Life
00:54:46
Speaker
They're just a complete surprise. And that was only, what, 2015 or something?
00:54:51
Speaker
does make me wonder, given that we are so unimaginative. I mean coming back to this kind of tech stack, even with this kind of simple, even if we assume that carbon is the right basis for life, which I think is a good assumption, although I do want to throw out, you know, this
00:55:07
Speaker
you know, wonderful ideas of science fiction writers. I love the story by Ted Chiang of Exhalation, which is, I don't know if you know this one. I don't know this one. But it's machines with a lot of tubes, but their kind of free energy is a pressure difference. Okay. And so their brain is a set of kind of pumps and stuff, and they realize that they're thinking slower, but it's just because the pressure difference is going.
00:55:30
Speaker
They're losing that sort of Cartesian machine that feels like so it's a really nice way of like highlighting the kind of end the free energy aspect to life but of course then there's you know Fred Hoyle's black cloud and Was it dragons dragons egg? Yeah, we teach our students about some of these things. I mean should we treat these as as as sci-fi or You know should we take them?
00:55:56
Speaker
Seriously, like do you teach them to your students for fun? Or is it like, you know, this is something that? Well, there's a couple of different reasons why we might want to think about these things. One reason might be kind of precisely what we were just talking about, trying to make sure that we're open minded, that we're as imaginative as we can be, that we don't shut down minds of inquiry because we have too conservative a view about what life might be. Nobody wants to be too conservative.
00:56:26
Speaker
about what life might be. However, the fact is we have limited resources. We have limited observing time. We have limited time in general. And there are kind of much lower hanging fruit that we can pluck in our observations of the universe, given that organic molecules and water are so common. And as it turns out, rocky planets in the habitable zones of their parent stars are so common.
00:56:53
Speaker
To me it makes complete sense to at least focus the majority of our astrobiological research on these well understood scenarios where we have Admittedly only this one biosphere to go on but at least we know that that form of life is possible. Yeah
00:57:07
Speaker
Does that mean we should be close-minded about other scenarios? No. But just as a matter of pragmatic utility, I think we need to look for the thing that we understand something about how to look for, maybe first. We'll give the weight of our effort to that.
00:57:23
Speaker
while at the same time continuing to explore. Because the solution to the problem of not being able to imagine stuff is to go out and explore the universe and see what you actually find. And this I think is a somewhat different
00:57:40
Speaker
activity from science strictly speaking. I think it's very worthwhile just to go and look and see what you find without worrying about whether you're testing hypotheses or whether you have well-defined scientific research objectives. And sometimes these things can be slightly in tension, which I think is a bit unfortunate. Sometimes the reason for going to look at an environment should just be we haven't really looked at it before.
00:58:08
Speaker
rather than that we have a really nicely defined, well considered kind of model of how a certain phenomenon may or may not take place there that we want to test. We need to do both things.
00:58:19
Speaker
I forget the quote, but I think that Shelley, who said something, you know, science starts in poetry. That idea traces back to the Greeks as well. But I think you're absolutely right that on both points, firstly, that there needs to be a spirit of inquiry, which is kind of prescientific and gives that kind of inspiration to science. But on your first point as well, let's look in the obvious places first.
00:58:42
Speaker
astrology is still really young in many ways. We only confirmed the first exoplanet in 1995.
Exoplanets and Extraterrestrial Life Search
00:58:50
Speaker
But since then, like Kepler, just looking at a quarter of 1% of the sky, and just focused on our galaxy, has found coming up to 3,000 confirmed exoplanets with a bunch of them in what we call the habitable zone.
00:59:10
Speaker
within that kind of carbon-based way of life in the place where we think that could work, right? So there's already
00:59:20
Speaker
It's just been an explosion in the number of places where we could look. And I suppose we've just not had the tools. We're only now building the telescopes that will be able to do the spectosh. Look at the spectral bands from those planets and figure out, does it look like there's some vegetation there that's photosynthesizing and absorbing relevant wavelengths?
00:59:47
Speaker
it look like there's something which is putting unusual concentrations of complicated hydrocarbons into the atmosphere? I think it's tempting to just sort of want to jump to the end of what will actually be a long scientific program that probably takes place over several generations before we get clear answers. I don't buy this line that
01:00:10
Speaker
we're going to know that there's life in the universe within 20 years. And we might get lucky, but I think it's sort of foolish to set up that expectation in people's minds, not least because we still don't know enough about the baseline in these contexts. There's even an argument that before we even start trying to analyze habitable planets, we should start by analyzing uninhabitable planets so that we have an understanding of just how diverse and complex and different from each other
01:00:38
Speaker
Planetary atmospheres can be in environments where we know there's no life, at least no life as we would normally conceive it.
01:00:45
Speaker
So, I don't know. I'm not sure I'd totally by that argument, although I think it's along the right lines. What's actually going to happen is that we're going to find surprises. We're going to find anomalies. We're going to find molecules we didn't expect to find in atmospheres where it may even be hard to imagine how a life could possibly be producing them, but we just can't think of anything else because the science hasn't been done yet. I mean, this situation's already arisen at least three times. First with methane on Mars, then with phosphine on Venus,
01:01:14
Speaker
possible DMS in K2-18B where some of these detections are not very robust, but people have jumped very quickly to the conclusion that it could be life, and maybe it could be, but I just don't think that conclusions are motivated until we have a much better understanding of the physics and chemistry of these environments. So I think it's important that people
01:01:36
Speaker
be cautious and circumspect and don't necessarily believe the first press release that gets put out about these things because usually when the truth comes out it's much less exciting than the journalists wanted it to be.
01:01:52
Speaker
But I'm not being pessimistic. I just think these things take time. And as you say, this is quite a young field, and we're only just beginning to be able to get the kinds of data that we need, let alone understand and interpret those data correctly. So let's be patient, I think is the bottom line. That's an important psychological call to arms in some ways, that this could be, as you say, a multi-generational process, almost like
01:02:18
Speaker
updating your Bayesian updating as you go. Exactly. And it might be a very slow incremental process where even though we have some suggested evidence, it takes a lot more follow-up work over a long time to get that posterior probability above your threshold of credibility. Yeah, I can imagine.
01:02:41
Speaker
we find some things with the next generation telescopes, but then that's it. Oh, actually we need a new telescope or a new experiment. Those things take a long time to build. I think a Bayesian idea sort of approach is the right way to go. And in this respect, I may be actually less negative about things than some colleagues. So you mentioned earlier on that maybe life should only be the hypothesis of last resort. And I don't actually agree with that. I think that's unscientific just to say this hypothesis is like intrinsically unacceptable.
01:03:08
Speaker
I think we just need to follow the evidence wherever it points. And if the evidence isn't very good, then it motivates only a small adjustment to our Bayesian, you know, probabilities. But it still motivates some adjustment. And hopefully, over time, the evidence will get better and accumulate in one direction. You wrote a fun paper called, on Carl Sagan's famous stick term, Extraordinary
01:03:36
Speaker
Extraordinary hypotheses extraordinary claims require extraordinary
Evaluating Evidence in Astrobiology
01:03:40
Speaker
evidence. Yeah. Yeah, this gets wheeled out a lot and I don't agree with it
01:03:44
Speaker
Yeah. Well, it's a nice, pivvy, you know. Yeah. I mean, OK. So it definitely is correct in certain contexts. So if you have a hypothesis which has a low prior probability, then you can see straight away from Bayes' theorem that you're going to need evidence that has certain properties in order to get that posterior probability above the line. You're going to need evidence that you would only get if the hypothesis were true, that you're not going to get otherwise.
01:04:14
Speaker
So there's that formal logical sense in which if your hypothesis has a low prior probability attached to it, then yes, it requires evidence that you might say is extraordinary. Also, if you have a hypothesis, the truth of which would entail the falsehood of many other things that are well known to be true, like maybe your hypothesis is that horoscopes work. Well, if that were true, then a lot of what we know about physics has to be wrong.
01:04:40
Speaker
So it's harder to just assign a probability to that, but it's an extraordinary claim that would require extraordinary evidence because the evidence would somehow have to overbalance all the evidence that we already have for our existing understanding of physics.
01:04:56
Speaker
But the discovery of extraterrestrial life isn't like either of those situations. Extraterrestrial life, the existence of extraterrestrial life, doesn't conflict with any well-understood physics and chemistry. It doesn't require us to rewrite the textbooks on physics. And it's also not something to which we can just straightforwardly assign a very low prior probability. In fact, if you look at the Drake equation, which perhaps you shouldn't, but you can,
01:05:24
Speaker
I haven't actually with this recreation, but we'll come at it. So it's this idea that you can, you know, if you multiply the number of stars by the fraction of those that have habitable planets, by the fraction of those that develop life, by the fraction of those forms of lives that develop something that we can actually observe and so on, you get to the end of this multiplication, you come up with a number that tells you how many
01:05:46
Speaker
observable civilizations or observable forms of life we might predict that we're going to be able to see. And the reason I bring this up is just that the uncertainties attached to each one of the terms in the equation are such that you can end up with any number you want. But that in itself tells you
01:06:02
Speaker
that we can't straightforwardly assign a low probability. Because the evidence we currently have allows us to put basically any number on how much extraterrestrial life there could be that we could observe. So for these reasons I don't think the existence of extraterrestrial life just by itself is an extraordinary claim in the sense that would be required for Sagan's pithy dictum actually to apply to it.
01:06:27
Speaker
So I think the right response when you see a press conference about extraterrestrial life has been discovered isn't to say I'm not going to allow that because that's an extraordinary claim.
01:06:37
Speaker
The right response is to say, how good actually is the evidence? How much do we actually understand about the abiotic processes operating in the environment that this data comes from? And if necessary, make some proportionate adjustment to your beliefs, which can be a very small adjustment if the evidence isn't very good. But that's the rational way to proceed. Yeah. Very quickly on the Drake equation, one thing it seems to be missing. So one of the terms is,
01:07:05
Speaker
proportion of intelligent life, you get to that and then you're like, and the number of, you know, the number of intelligent life or the proportion that then develops the ability to communicate.
01:07:20
Speaker
There's a term which doesn't seem to account for our ability to recognize the communication as such. And so if life is operating on a very different time scale, we just might not see it. And it could be super intelligent. It could be so rapid or so slow.
01:07:38
Speaker
There's two timescales you have to think about. One is the timescale for which that civilization is actually producing stuff. And the other one is the timescale for which the stuff it produces can be observed. And if it makes, let's say, there's this hypothetical idea that extraterrestrial life might produce kind of autonomous robot probes that can reproduce themselves and spread to the galaxy, these could happen.
01:08:01
Speaker
indefinitely extended lifespan long after the civilization itself has come and gone. So that's, I don't know if I buy this, but that's one argument for thinking that we should invest at least a little bit of our time and scientific resources in looking for techno signatures.
01:08:17
Speaker
because they might have this incredible longevity, far more than the biosphere that actually produced them. And then in that case, they might be the form of life that we could most easily detect, even though they might be the thing least likely to be produced in the first place, because we imagine the most biosphere is just going to be microbes.
01:08:35
Speaker
If technosignatures are produced by even one civilisation that decides to propagate their machinery through the universe, maybe to send out probes to do research or whatever, if that's happened only once, we might have a much better chance of detecting that than all these millions of worlds of microbes. Well, one-minded machines are very, very good at replicating.
01:08:56
Speaker
sort of on Neumann machine. That would be a pretty good technician. I want to throw out some other technicians. If we saw an obelisk, like in 2008, that'd be good. Also, a gombots. I don't know what that is. It's this shape. So an obelisk is sort of like at one end of the, just, it's just one bracket to the naturally occurring shapes. Like you never get something so perfectly, you never get a perfect cube in nature. And the way you can sort of get pretty close, so crystal.
01:09:24
Speaker
So the guy who's done work on this is Gawa Dimokosh, and he points out that if you look at the number of balance points, and you use that or the number of vertices as well, you find that things approximate queues, but never quite there. And at the other end of the spectrum, things approximate the gombots, which just weirdly just has one stable and one unstable balance point, whereas most stuff has at least two.
01:09:50
Speaker
That's something that never occurs. And it's so precise, like nature can't really manufacture it. So I throw that out. I mean, the other classic example of a techno-signature is a pulsar. And I mention that because it turned out to be really fruitful to look for those things, but not because we found evidence of an alien civilization, but we found pulsars. But coming back to Sagan's dictum, I've wandered a bit off a track here. I think you're right. And I think it's maybe an example of
01:10:20
Speaker
a meme which sounds really good but you know propagates by virtue of its catchiness rather than its... It's got that lovely sort of formal symmetry to it that makes it sound like it must just be true. It also lights upon probably a cognitive bias that we have like I think probably captures something that we do do but we probably shouldn't do and the place that comes to mind is within the interpretations of quantum
01:10:49
Speaker
perennial actually have to scratch. One of the kind of objections to the many worlds interpretation, the Everest interpretation is just like, you cannot be serious, right? And I think that is an instance of, come on, that's just too out of this world. But actually, well, really, you know,
01:11:11
Speaker
In many ways, it's the simplest explanation. And that's what you should be focused on, rather than this kind of gut reaction, gut feel. I think somewhere in the sort of new atheism arguments from the 90s and early 2000s, there was this sort of jokey
01:11:32
Speaker
line about the argument from personal incredulity. Yeah. Just because you happen to find something difficult to believe has no bearing on whether it's lugs to be true or not. Yeah. Maybe that's putting things a bit too strongly because we have certain intuitions for evolved reasons, but in general, yeah. Yeah. So I feel like we've scratched so many of my issues, like this has done a really good self.
01:12:02
Speaker
I don't know if you have some kind
Earth's Unique Position in the Universe
01:12:03
Speaker
of final thoughts. I thought it was really interesting what you mentioned about we sort of need to settle in for the long run on this. But I'm curious about the null hypothesis, as it were, the kind of rare-earth hypothesis that we are unique in the universe, or even if not unique, just so rare that we would not expect to find any other instances.
01:12:32
Speaker
Part of me hopes that we astrobiology proceeds and keeps on incrementally adding evidence that, you know, there is life elsewhere. We see some exoplanet. It's got the right signatures. We find more and more evidence and discard other things that could have produced those signatures. But yeah, there's
01:12:51
Speaker
There's a logical world, at least out there, where that doesn't happen. And in some ways, that in a way is even more fascinating because how long would we have to go? It's much easier to produce, you just can't produce evidence for the null hypothesis here. You can just not find life. You can't prove, I mean, you could prove that it's so rare that
01:13:13
Speaker
There are certain ways the universe might be that we can progressively eliminate. So there's a version where every different planet has its own particular form of life that's evolved to whatever the conditions are there. This was a plausible hypothesis, you know, a century ago. Now it isn't. Because, you know, as Sagan said, we don't see silicon-based giraffes strolling around the surface of Mars. I think it's quite religiously motivated, right? People thought,
01:13:37
Speaker
If God went through the trouble of creating a world, he'd put stuff on it. Yeah, there's a secular version of the same argument, but it's still not a very good argument. It could be that wherever you have liquid water and some sorts of energy and nutrients, life appears. That's another possible world that was maybe getting to the point now of being able to say that's actually not right. Because it seems as though
01:14:03
Speaker
There is liquid water at least in the subsurface of Mars and there certainly was abundant liquid water on the surface four billion years ago and also plentiful carbon and nutrients and energy and so on.
01:14:15
Speaker
And so if there was life on Mars, chances are we're gonna find some trace of it. We have the right approach now to actually answer that question. And one kind of convenient thing about life is that it doesn't just sit in one place, it spreads and evolves and ends up everywhere. And so if we don't find any evidence that there was life on Mars in these four billion year old rocks,
01:14:42
Speaker
For me, the simplest and best explanation of that is that life didn't appear on Mars. You can't absolutely rule out that it maybe didn't appear in some, maybe it appeared in some very local place and disappeared very quickly. But I think if life appeared at all, it probably spread everywhere because that's what life does. I mean, on Earth, every single habitable environment pretty much is inhabited.
Impact of Discovering Extraterrestrial Life
01:15:03
Speaker
We can get to the point where we can say, actually, you had these two planets early in the history of the Solar System, Earth and Mars, they were very similar. They both had all the right conditions for life.
01:15:12
Speaker
and only one of them actually became inhabited. And so now we can eliminate the scenario where just having water and nutrients energy and so on is enough. Which means the whole universe then looks different because it means all these habitable planets that we may find orbiting other stars, we don't have any actual good reason to think just because they're habitable, they're actually inhabited.
01:15:34
Speaker
And so we can make progress in this way, even if we can never absolutely say, you know, we've searched everywhere because we can't search everywhere. And similarly, you know, we only have to find life once on another planet, and then the whole universe looks different. So I don't think these, I think there are several different null hypotheses about how life might be distributed, only one of which ultimately is that there's just no life at all anywhere.
01:15:59
Speaker
That last one, maybe, is impossible to prove, but we can certainly work towards it, even if we never reach it, kind of like an asymptote. I can pretty much prove that there is life somewhere, because I'm looking at you right now. Yeah, OK. Anywhere else. I forgot the word else. I thought you might have been in some kind of simulation argument. But I think even the simulation argument presupposes that there's probably some life to create the simulation. We call your brains in a jar. That's true. We can be a Boltzmann brain, and that probably wouldn't be life.
01:16:26
Speaker
I think I agree also that non-detections are just as interesting as detections. One big motivation for doing astrobiology is to get a clearer sense of how, as I said at the beginning, how life as we know it fits into our bigger picture of our understanding of the universe and how we ourselves fit into our bigger picture understanding. Are we an anomaly or are we the usual thing that crops up in the right conditions? And either answer, as you say, is very interesting.
01:16:56
Speaker
Yeah, absolutely. I think either answer would be. It's weird. It's a situation which I think either answer would completely change our view of the world, which seems wrong. Yeah, I don't know about that, actually, because I think if you ask people, most people already kind of have a hunch that life probably exists elsewhere. Right. And it doesn't seem to change.
01:17:17
Speaker
It doesn't seem to be important to them that they have this hunch. It doesn't seem to entail a radical lifestyle change or... I don't know. It's a bit like we could have said all the same things about God, right? If God exists, then holy shit. If God doesn't exist, then holy shit.
01:17:33
Speaker
But one of those things must be true, and everyone believes one or the other pretty much, and yet we all seem to just carry on our lives much the same way, more or less, at least in our model of civilisation. So I don't know, I think sometimes people exaggerate how important the discovery of extraterrestrial life will be. I'm not even sure it would be in the news for more than a week, to be honest.
01:17:59
Speaker
I'm not as cynical about this as some people. I was at an event once where there was somebody representing the UK Space Agency, a civil servant, and he said this incredibly cynical thing about how actually most people out there care more about what's going to happen in the next episode of his stenders than whether there's life on Mars.
01:18:19
Speaker
And I really didn't like it, but I'm not, I don't know, he might be right. You know, I think there's a sense in which I don't disagree, in which, you know, we have these everyday concerns. Yeah. And yet there's a sense, maybe the same thing happened when Copernicus was like, hey, you know, it's actually the other way around. Yeah. Right. But in the long view, it was massively important. But for people living at the time. Exactly. They're probably like, oh, yeah, where am I going to get my gruel from tonight?
01:18:43
Speaker
But yeah, in the long view, it would be an incredible discovery. I just don't expect it to... I think it will probably be a bit anticlimactic in some ways for people that actually live through it, even though in the long term it might really change the future of humanity. Well, setting us up like that... we're not going to be disappointed.
01:19:09
Speaker
Oh, this has been a real pleasure. I've, um, yeah, I particularly enjoyed looking around your, your labs and saying that this is, this isn't all just, uh, writing the Drake equation on the board over and over and trying to think through. We do, we do, we do actual science. Do actual science. Yeah. I can confirm there are. Brilliant. Um, yeah, thanks so much. Well, thanks for having me along. It's been really fun.
01:19:48
Speaker
So,