Without a good definition of life, how do we look for it on alien planets? Steven Strogatz speaks with Robert Hazen, a mineralogist and astrobiologist, and Sheref Mansy, a chemist, to learn more.
Scientists don’t really agree on a definition for life. We may recognize life instinctively most of the time, but any time we try to nail it down with set criteria, some stubborn counterexample spoils the effort. Still, can we really search for life on other worlds, or understand the earliest stages of life on this planet, if we don’t know what to look for? On this episode, Steven Strogatz speaks with Robert Hazen, a mineralogist, astrobiologist and senior staff scientist at the Carnegie Institution’s Earth and Planets Laboratory, along with Sheref Mansy, professor of chemistry at the University of Alberta, to learn more about how new taxonomies and a “cellular Turing test” might help us answer this essential question.
Transcript
Steven Strogatz (00:02): I’m Steve Strogatz, and this is The Joy of Why, a podcast from Quanta Magazine that takes you into some of the biggest unanswered questions in math and science today.
In this episode, we’re going to be talking about what it means to be alive. What is life? Can you define it?
The question of what life is also matters, because if we’re going to be looking for life on other planets, don’t we need to at least have some idea of what we’re looking for?
Strogatz (01:40): Great. Well, let’s jump right into this. Why is it so hard for scientists to agree on something that, common-sensically, most people would say they already understand? Like, we know that a plant is alive and a rock is not. Why is it so hard to come to some agreement about the definition of life?
Hazen (01:57): Yeah, that seems strange, doesn’t it? Because we all know things that are alive. And we all know things that aren’t alive. And yet, it’s that gray area in between. So when we start saying, this is alive and this is dead, that’s fine. But when you say everything either has to be alive or dead, you’re setting up a false dichotomy. Because the taxonomy of what it means to be alive, I think is much, much richer than just dead or alive.
Hazen (02:29): Well, think about it, you have an origin of life. So, that’s a really good metric. There was a point in our Earth’s history when there wasn’t a single living thing. It was a blasted surface, it was covered with volcanoes and magma, and it was just basically inhospitable. There was no place that life could even get a tiny foothold. But gradually, as the Earth cooled, as oceans formed, as the atmosphere became more palatable for some kind of living thing, we think there was a process. A historical process, the origin of life, in which chemical systems became gradually more complex, became more interesting. And at some point, yes, there was a first cell that probably had proteins and DNA. But there had to be something before that, and where do you draw the line? It’s just difficult to say there’s an absolute point in space and time when there was no life, and then the next point in space and time there was.
(05:21): But life is very, very particular. And one thing I think we can say is, if something is alive, it’s going to put its energy into making a few molecules that work really well. And ignoring the vast number of molecules that don’t do much of anything. So, if you have a system that has the biological overprint, it’s going to show very specific groups of molecules. Maybe molecules that are what are called “chiral,” or left- and right-handed, maybe you’ll have a predominance of just the left-handed or just the right-handed molecule. Maybe you’ll have just strings of carbon that have multiples of 2, 2-4-6-8, rather than all the other odd numbers as well. Maybe you’ll have some other characteristic that wouldn’t form just by a random process, but forms by a selective process. So that’s what NASA was looking for. And I think that’s a smart thing to do.
Strogatz (06:13): That’s very interesting. The idea of chemical selectivity, you say, could be or at least was proposed by NASA to be a possible — well, nowadays, we speak of biosignatures, I don’t know if that would be the language they would have used at that time.
Hazen (06:26): Yeah, exactly right. That you’re looking for biosignatures. So I think if you see those chemical idiosyncrasies, you can say, wow, something really interesting happened here. And it doesn’t look like just the normal natural process, it looks like there was some real selection for function. Molecules that did a job, you know, they metabolized or they, they help build strong cellular structures or something like that. So, I think that’s what they were looking for.
Strogatz (10:31): So then, getting back to NASA for a second, are there are some kinds of things that you think they should be looking for when searching for life on other planets? Or should they just be kind of going for the most glorious, rich, bountiful taxonomy they can come up with?
Hazen (10:46): Aha! Why not both? Because, you think about it. One thing we do have a hunch about is habitability. That is sort of the range of temperature, pressure, composition. A water-rich world, a sunlit world, you have to have energy, you have to have various other criteria that allow chemical systems to do interesting things. If it’s, if it’s, everything’s molten, or a vapor, it’s much too hot. If everything’s frozen, and nothing moves, like on Pluto, then that seems much too cold. So, so we do think there’s some sweet spots. And we do think there are things we can look for, like liquid water, or some other fluid, but water is the only one that really seems to do the job.
(11:28) We need to look for carbon-based molecules, because it seems like carbon’s the only element that forms the kind of richly varied backbones that you need for the structures of what we think of as life. And I really don’t believe in cloud-based life or, you know, electronic life, or life in a plasma or something like that. I mean, that just, you don’t see the kinds of structures that you need, that spell what I think of as the complexity of a living system. So there are parameters, and that’s what NASA is looking for. Let’s look for water-rich worlds, let’s look for worlds that have the right kind of temperature and pressure and atmospheric composition. And rocks and minerals play a really interesting role. And they provide all sorts of chemical elements in addition to carbon, that might be essential for a complex chemical system.
Strogatz (15:39): Wow. It’s a cosmic thought. You know, I’m sort of encouraged by how quickly life started here. Speaking of geology, like let’s put it in a geological perspective. Give me the numbers, roughly, how old the Earth is and how soon it starts to teem with life.
Hazen (15:56): Sure. So, Earth began to form at 4.567 billion years ago. And it was not habitable for the first period of time. It may have had a window of habitability for a few tens of millions of years, and then that huge impact, the Theia impact that formed the moon, and that just smooshed everything — the whole planet was encircled by a magma ocean, glowing, red hot, that had to cool. So that may have been 4.45 billion years ago, I think, something on that order, maybe as recently as 4.4. But that’s the kind of extreme beginning date that we can think about. And we know that by 3.8, life was well established. We have stromatolites, we have other signs of life that were clearly there.
(16:47) So that’s a block of, what, 600 million years, but I think life started much, much more quickly. But that’s a hunch, I think probably we are looking at millions or tens of millions of years for a process to occur. If it’s going to happen, you know, chemistry, you’ve got a vast surface area of Earth, you’ve got millions of years to play with, you’ve got all different kinds of chemical systems and fluxes. And so, Earth is a great experimental laboratory for chemistry. And with hundreds of millions of years to play with over the entire surface of the planet. Wow, that’s, that’s a lot of combinations of chemicals you can try. And life pops out of it.
OK, that’s enough. This lengthy transcript contains much more dialog, but from a scientifically sensible point of view, I think we’ve reached the end of the track… “And life pops out of it.” What kind of science is that?! Surely, it’s not too difficult to understand how utterly improbable it is for even a single protein molecule to “pop out” of a random mix of abiotic ingredients, let alone a living cell, with its exponentially greater complexity.
The full transcript can be read at Quanta Magazine.