The Humble Microbe Could Help Us Understand Life Itself

FLORA LICHTMAN: Hey, I’m Flora Lichtman, and you’re listening to Science Friday.

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On the podcast today, a conversation from our live show in Redwood City, California. We’re talking about one of the most profound questions in science– how did life begin?

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I want you to sift through your memories and excavate an image of a fossil. Maybe you’re picturing dinosaur bones, or an imprint of an ammonite, or the fronds of a fern etched into stone. But there’s a whole other category of fossilized remains that can tell us about life way before T-Rexes or even twigs existed on this planet. I present to you fossil microbes– well, technically, the fossilized evidence of them.

My next guest uses these ancient remains to understand how life began on Earth, which may also help us find life elsewhere in the universe. Joining me now is Dr. Paula Welander, Professor of Earth System Science at Stanford University. Paula, welcome to Science Friday.

PAULA WELANDER: Thank you.

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FLORA LICHTMAN: How do microbes show up in the fossil record? What are you looking for?

PAULA WELANDER: Like you said in your introduction, when we think about a fossil, we think about Jurassic Park, the dinosaurs. Those are morphological fossils, and those are usually hard structures. But microbes are soft. They’re single-celled. And so a lot of their tissue is– what we would consider tissue gets dissolved in that process of a rock forming and fossilizing.

And so geochemist had this idea to extract organic molecules from these fossilized rocks. And a whole slew of chemicals came out and turned out to be molecules that microbes make– we also make them, but that can be preserved in the rock record. And these are primarily fats, lipids structure.

FLORA LICHTMAN: So not rock.

PAULA WELANDER: Yeah.

FLORA LICHTMAN: Very clearly not rock.

PAULA WELANDER: Yes, not rock. And over the years have developed different techniques, instrumentation that allows them to pull these molecules out of the rocks and identify their structures, their chemical structures, so they’re chemical fossils. And it’s exciting because then you don’t need a hard morphological fossil, and you can get interpretation of what is actually being deposited at the time.

FLORA LICHTMAN: I’m going to nerd out a little bit here. What makes lipids good at preserving?

PAULA WELANDER: So lipids are really important. So you all know cholesterol, right? We all have cholesterol. It’s bad for you. It kills you. No. We need cholesterol, and that’s a lipid. It’s one of the macromolecules that sustain life.

And these lipids for microbial organisms are their external barrier from the environment. So it makes them resistant to a lot of the stresses they might encounter, say, in a hot spring. And what we have found over time is that microbes that live in these environments that can be very extreme have modified their membrane lipids so that they can withstand high temperature, low pH. And those structures are then preserved in the rock record.

So if you look back deep in time and you see a specific structure that we see microbes making in a hot spring, you can make the connection between that ancient environment and this modern environment because the lipid molecules are the same. Those are preserved over billions of years.

FLORA LICHTMAN: Wow. OK, so tell me about the extremophiles you study that give us this window into the past.

PAULA WELANDER: So the organisms that we work with primarily are usually found– they’re found in hot springs, but they can also be found in hydrothermal vents. They can be found in mud volcanoes that are really high in methane. These types of organisms can require high temperature, low pH, and extremes of different kinds in order to be able to survive. They need that.

So, for example, a stress for this organism, it normally grows at, like, oh, I’m going to say 75 degrees Celsius. I don’t what that is Fahrenheit, like 200 Fahrenheit?

FLORA LICHTMAN: Hot.

PAULA WELANDER: Yeah, very hot. You bring it down to 100 degrees Fahrenheit and it dies because it needs to grow at that temperature, because it has evolved not only its lipids, but its other macromolecules to be able to thrive in that environment. So these environments are really exciting to go find these microbes because they push the edges of what we consider life, of what we consider how we survive in an environment. They really push the chemistry.

FLORA LICHTMAN: Do you physically go out to the field and scoop them up?

PAULA WELANDER: I don’t. [LAUGHS] So I’m a lab rat. I like to work in the lab. We can actually grow these in the lab. And so there are people that go out and do that. I’ve had grad students who want to go do that, and they go into the environment and they pull them out. And they’ll take new organisms, and we’ll grow them in the lab.

FLORA LICHTMAN: So you particularly would rather be in the lab.

PAULA WELANDER: Yes.

FLORA LICHTMAN: Like, the people in your lab go out.

PAULA WELANDER: Yeah. [LAUGHS]

FLORA LICHTMAN: Would these fossil– these fossilized remains of microbes, I mean, how far back in time can you go?

PAULA WELANDER: We can go back as far as 1.8 billion years ago.

FLORA LICHTMAN: Whoa.

PAULA WELANDER: Yeah. Which, honestly– I know seems very, very old to geologists– is not that old because the planet is about 4 billion years old. Life’s been on this planet for about 3.8 billion years old. So geologists wish we could go all the way back there. But the process of preservation doesn’t go back that far. But these are definitely the oldest fossils we can find.

FLORA LICHTMAN: And what are you learning about this time period that we didn’t before?

PAULA WELANDER: So we’re learning when– so one of the key important things about the Earth in the past is very different than the Earth is now. For example, for the first 2 billion years that the Earth was in existence, there was no oxygen, like at all. And now we are 20% oxygen.

So how did that happen? That turns out that microbes have interacted with the Earth. Microbes invented the ability to do what we call oxygenic photosynthesis, what plants do, and generate oxygen and oxygenate the Earth. So what we learn when we study about these microbes deep in the past is, when did that happen? Geologists are very interested in when that happened.

And what are the markers that we can see in the past that can let us know, like, oh, this is when it happened. And you can see that the chemistry of the Earth changed. And this idea that the evolution of life, so the ability for microbes and life to impact the Earth, is also impacted by the evolution of the Earth. Because the Earth is also changing from processes that are not involved in biotic mechanisms.

So it tells us a lot about how this push and pull happens between the evolution of life and the evolution of Earth, but it also tells us about a lot of perturbations in the past. For example, we can see times when the Earth was completely covered in ice and was really cold, and we can see how the microbes changed over time at that stage.

FLORA LICHTMAN: And you can see that in the lipids?

PAULA WELANDER: Mhm. Well, the lipids actually help determine that state. Because we can see that the lipids are a certain– it’s interesting because the way geologists do this is they have stratigraphy. And each layer of a rock represents a certain age in time. And you’ll see blips of, like, the lipids are low, and then they go up, and then they come back down. And they can see where those changes are happening. But that’s like 100 million-year time scale.

FLORA LICHTMAN: What do these earthling extremophiles tell us about possible life on other planets?

PAULA WELANDER: So we basically need to understand how life might evolve; so how might life have evolved and what are the signatures, what are the markers that we can find in that. So a lot of study of the origins of life is focused on understanding how it occurred on this planet, because this is the only planet we know where life has actually originated, where it’s evolved.

And then, what are the signatures that are left? We know that the first forms of life were microbial. They were definitely single-celled organisms. And if we look deep in the past at these rocks, what are some of the molecules that we can find?

Because if we go, say, to Mars or the moons of Jupiter and we are able to pull a sample, what do we look for, in order to be able to find there? So if you find molecules that are preserved that are similar to what we see in the past here on this planet, you can start to make that connection.

FLORA LICHTMAN: Is it possible that we don’t what to look for?

PAULA WELANDER: Exactly. That’s the challenge, right? We’re kind of biased. We’re trying to– there’s this idea of “life as we don’t know it.” And so that is a little bit of having to be open-minded in thinking about what are some other signatures we might be able to find. But we are scientists. We’re grounded in what we do know. And I think fully understanding the possibility of life, both in the rock record and in the lab, will allow us to think creatively about what we might not be able to see right now.

FLORA LICHTMAN: OK, while I have you, there was some exciting news just a couple weeks ago about a possible biosignature found on Mars. I feel like these stories happen kind of regularly. Was this one– how hyped were you for this one?

PAULA WELANDER: Well, I love the Mars Rover stories because they really exemplify what we can do when we come together as scientists, as a community. The number of people it takes to do those kinds of projects is insane in the years of planning, years and years. And so to see it come to fruition is really, really cool.

But it is incremental. And so the signature–

FLORA LICHTMAN: Here we go.

PAULA WELANDER: –they called it a biosignature possibility. Because what we saw was these iron phosphate, these iron, I think sulfur compounds that were being made that can be signatures for microbial life. It can be the waste products of microbes that get preserved in rocks. But there can also be nonbiogenic sources of this.

So, for example, when you think of when we breathe, we breathe out CO2 as our waste. Right? So CO2 can be a biomarker for life. But you also get CO2 from volcanoes, so it can be a marker for volcanoes. The same thing with these iron minerals.

But I think what we really need is to be able to bring those samples back. And I really hope that return mission is not cut from NASA’s funding, because that’s the most exciting thing, to be able to bring these rocks back that the Rover and put them under our machines here would really be exciting to study.

FLORA LICHTMAN: Yes, that would be amazing. Let’s go to the audience. Let’s go here.

AUDIENCE: When studying these ancient microbes, what information isn’t in the fossil record, you know, in the same vein as dinosaurs and feathers? What are your blind spots? What are you not seeing?

PAULA WELANDER: So I think the hard things to see is what exactly the microbe was– the metabolism that it did. So when we talk about metabolism, microbes are really cool because they can eat almost anything, and they can secrete almost anything. We eat sugar, and we breathe in oxygen, and we spit out CO2. That’s all we do. We’re pretty boring metabolically.

But microbes will eat arsenic, and they’ll eat iron. And so I think a lot of times what we’re missing is what are the structures, the proteins that are involved in that process. We see the beginning and the end, and we miss what’s in the middle. And that, for me, being a protein biochemist, is the most exciting part. So those are some of the blind spots that we have.

FLORA LICHTMAN: I mean, so you’re a basic scientist. Have you had to justify your work in recent days?

PAULA WELANDER: I mean, I always have to justify basic science, [LAUGHS] but not in the current environment. I mean, I think for me, the ability to discover has been always the draw to science. The ability to bring in new knowledge is irrelevant, as it may be to everyone in the world, me discovering a new protein that makes a new lipid is always super, super exciting.

But, I mean, like many of my fellow academics, I am funded by the government. I have federal grants, and I fully believe I do need to justify that to the taxpayer, what am I doing with your money. And I’m funding students and scientists in my lab to carry out this research because we don’t know what we’re missing. We don’t know what we don’t know.

And so just wanting to study the world and understand the physical, biological world that we live in is so important to give us all the wonderful discoveries and advancements that we’ve made over the years.

FLORA LICHTMAN: Paula, thank you so much for being here. This is fascinating.

PAULA WELANDER: Thank you. Thank you.

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FLORA LICHTMAN: Paula Welander is a microbiologist at Stanford University. Thanks for listening. Don’t forget to rate and review us, and you can always leave us a comment on this segment on Spotify. We’d love to hear from you. I’m Flora Lichtman. Thanks for listening.

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