Scientists do not really agree on a definition of life. We can recognize life instinctively most of the time, but every time we try to nail it down with established criteria, some stubborn counterexample spoils the effort. Still, can we really look for life in other worlds, or understand the early stages of life on this planet, if we don’t know what to look for? In this episode, Steven Strogatz speaks with Robert Hazen, a mineralogist, astrobiologist, and senior scientist at Carnegie Institution’s Earth and Planetary Laboratory, along with Sheref Mansy, a professor of chemistry at the University of Alberta, to learn more about how new taxonomies and a “cellular Turing test” can help us answer this essential question.
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Transcript
Steven Strogatz (00:02): I’m Steve Strogatz, and this is The Joy of Why, a Quanta Magazine podcast that takes you to some of the most important unanswered questions in today’s math and science.
In this episode, we will talk about what it means to be alive. What is life? Can you define it? Actually, scientists disagree on one definition. Sounds weird, doesn’t it? I mean, most of us would say with some confidence that a bird is alive and a chair is not. But deep down, scientists are asking questions like this: To be considered alive, does something have to be reproducible? Should it be a product of evolution through natural selection? Do you need to have a metabolism and be able to process energy?
(00:51) Any definition of this line is full of exceptions. For example, is a virus alive? Well, viruses do evolve, but they don’t reproduce on their own. They use the host’s cellular machinery to make more copies of themselves. The question of what life is also matters, because if we are going to seek life on other planets, do we not need to have at least an idea of what we are looking for?
(01:15) Later in this episode, we hear Sheref Mansy, a professor of chemistry in the chemistry department at the University of Alberta. But first, Robert Hazen joins me now. He is a senior mineralogist, astrobiologist, and scientist at the Carnegie Institution’s Earth and Planetary Laboratory. Bob, thank you so much for joining us today.
Robert Hazen (1:38): Oh, it’s a pleasure. Thank you so much, Steve.
Strogatz (1:40): Great. Well, let’s get straight to that. Why is it so difficult for scientists to agree on something that, in common sense, most people would say they already understand? Like, we know that a plant is alive and a rock is not. Why is it so difficult to come to terms with the definition of life?
Hazen (1:57): Yeah, that sounds weird, doesn’t it? Because we all know things that are alive. And we all know things that are not alive. And yet it is that gray area in between. So when we start saying, this is alive and this is dead, that’s fine. But when you say that everything must be alive or dead, you are creating a false dichotomy. Because the taxonomy of what it means to be alive, I think, is much, much richer than just dead or alive.
Strogatz: Hmm. How is it?
Hazen (02:29): Well, think about it, you have an origin in life. So this is a very good metric. There was a time in the history of our Earth when there was not a single living thing. It was an explosive surface, it was covered in volcanoes and magma, and it was basically inhospitable. There was no place where life could not even have a small point. But little by little, as the Earth cooled, as the oceans formed, as the atmosphere became more pleasant for some kind of living thing, we believe there was a process. More interesting was a historical process, the origin of life, in which chemical systems became more complex. And at some point yes, there was a first cell that probably had protein and DNA. But there had to be something before, and where do you draw the line? It is difficult to say that there is an absolute point in space and time in which there was no life, and then there was the next point in space and time.
Strogatz (03:26): Interesting, interesting. So, bottom line is that we’re really looking forward to this issue.
Hazen (3:34): It’s a matter of chemical complexity. But it is also a much more basic question of taxonomy. You know, it’s very easy for humans to think of dichotomies. Good bad; black white; day, night. These are the things that make life easy. It means we can sort things out very, very quickly. And at the beginning of human history, this was the defense mechanism, because decisions had to be made very, very quickly. Whether you were going to shake hands with that person or shoot an arrow. So we had to make those decisions.
But we shouldn’t do that when we think about the biggest problems in the natural world. The natural world is incredibly complex and complex. And how these complex chemical systems arise, and at what point a complex chemical system is something we will really call alive, is not at all obvious.
Strogatz (4:24): So I understand your point about the gray area. I mean, black and white is usually too simple, applied to just about anything. That there is always something, a little ambiguity in between. However, let’s say that in the case of the space missions that NASA is running, maybe in the future, we will try, or even with previous missions, when we sent probes to Mars and that kind of thing, there was a search to see if we could detect life. And so you would think that in order to approach this question objectively, you have to have some criteria about what you mean: did you find it or not?
Hazen (4:56): And NASA did. NASA had criteria. And most of all, it has to do with what I would call chemical idiosyncrasies. Therefore, organic molecules, molecules made with a carbon backbone, are found all over the cosmos. Wherever there is carbon, this thing called “organic chemistry” is obtained. Many different types of molecules are just a kind of confusion, a mixture of these things [that] form.
(05:21): But life is very, very particular. And one thing I think we can say is that if something is alive, it will put its energy into making a few molecules that work really well. And ignoring the sheer number of molecules that do little to nothing. Therefore, if you have a system that has biological overprinting, it will show very specific groups of molecules. Maybe the molecules are called “chiral”, or left and right, maybe you will have a predominance only of the left molecule or only the right. You may only have carbon chains that have multiples of 2, 2-4-6-8, instead of all other odd numbers as well. You may have some other characteristic that would be formed not only by a random process, but by a selective process. So that’s what NASA was looking for. And I think that’s a smart thing.
Strogatz (06:13): That’s very interesting. The idea of chemical selectivity, you say, could be or at least was proposed by NASA as possible, well, today we are talking about biosignatures, I do not know if this would be the language they would have used at that time. .
Hazen (06:26): Yes, exactly right. That you are looking for biosignatures. So I think if you look at these chemical idiosyncrasies, you can say, hey, something really interesting happened here. And it doesn’t just look like the normal natural process, it looks like there was a real selection for the function. Molecules that did a job, you know, were metabolized or they, they help build strong cell structures or something. So I think that’s what they were looking for.
(06:56) But the fact is, that doesn’t define life, does it? He just says, we are looking for something that we believe is a feature of the kind of life we are familiar with. How many other types of life could there be? And that’s something we don’t know. We do not have enough information to build a taxonomy. That is to say, these things are alive and these things are dead, and these things have other interesting chemical characteristics that may be real, but they just don’t get us.
Strogatz (07:23): So which ones would be more functional?
Hazen (07:27): There are chemical systems that could make exact copies of themselves, but would not undergo mutations or natural selection. There are chemical systems that can be modeled, so they grow sideways and get bigger and bigger, and they seem to grow, but they don’t really have that characteristic of encapsulating a separate entity that we think is real. But they are all interesting systems and they are all part of a kind of continuum of chemical complexity. And for me, the much more interesting challenge is to develop this taxonomy.
(08:02) Now, think of Linnaeus’ classification system, where you have kingdoms and below kingdoms, you have phyla, and you have orders, and so on. Well, perhaps in our taxonomy of chemical complexity, we would have a realm of non-living things, and the realm of living things, and we would have a realm of ambiguous things. And then, under that, we would have a bunch of other subcategories and subtypes, and we would start to realize that the universe is an amazing and wonderful place, and that chemistry only does extraordinary things, some of which we call life. .
Strogatz (08:36): So far you’ve focused on chemistry, which interests me, since I think of you as a person with a lot of experience in mineralogy and geology. What about these fields? How do they overlap in this vast picture of the question of life and other interesting things …