Episode 9: Life Bound

(sound FX: crunch of footsteps on rocks, wind)

[0:04] Narrator: About fifteen years ago, geology student Abigail Allwood was hiking around the red rocks of Northwestern Australia, in a rugged region known as the Pilbara.

This part of Australia has some of the most ancient rocks in the world, dating back between two and three-and-a-half billion years.

Finding rocks this old on Earth is exceedingly rare, because our planet’s surface is always in flux, and over our planet’s lifespan, most rocks have been recycled -- a slow but violent process in which buried rocks become compressed, heated and melted down to eventually form new rock. But some rocks have so far managed to escape this fate.

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Narrator: The vast span of time represented by the rocks in a relatively compact area like the Pilbara is even more rare -- it’s practically a museum collection for geology buffs interested in deep time.

[1:01] Abby had come to see a particular section in the ancient rocks called the Trendall Locality. This outcrop is remarkable not only for its extreme age, but also because it has strange shapes – like upside-down ice cream cones and egg cartons. One interpretation of these shapes was that they’re the fossilized remnants of a form of life called stromatolites.

Abigail Allwood: Stromatolites are structures that have fine layers within them that formed as microbial mats on the sea floor or lake floor. So imagine sand, or silt, or mud or whatever, depositing on the sea floor, and the microbial mat would trap and bind and cement those grains of sediment in place, and build it up layer by layer over time until they have a high-standing structure on the sea floor. And that high-standing rocky structure is something that can be readily preserved through geologic time.

Narrator: The microbes build their layered towers because they use sunlight to grow. And so as the microbial mat layer becomes buried by sediment or clogged by waste products, the microbes migrate upwards as they seek the light, and create new mats on top of the old.

[2:11] You can still find living communities of stromatolites in a few places, like in the shallow waters of Shark Bay, Australia. For fossilized stromatolites, the tiny life forms that made them are long gone, and it takes a lot of work to prove that such columns of layered rock were made by microbes rather than through a non-living, geological process, such as regular eruptions of underwater volcanic structures known as hydrothermal vents.

Scientists had been arguing for more than twenty years about whether the Trendall Locality was evidence for ancient life when Abby made her journey to the site. At the time, she was on the side of the skeptics.

Abigail Allwood: It's funny, I went out with a very preconceived notion. Two of my advisors had spoken a lot about hydrothermal processes, and so I kind of went out there with that in my mind and sort of set about trying to prove that. And I remember I'd drawn this really detailed map of an outcrop, and I was so proud of it.

[3:07] And I went back and showed this map and I said, “This is proof of the hydrothermal association.” And Charlie Lineweaver said, “That's really nice, really beautiful and detailed, but how do I know it's right?” “What, are you kidding?” I was so offended.

It wasn't until the end of that first field season, I realized that was totally wrong. I had to do a huge back flip. And so I went back the next year with a completely new fresh look, and that was when I started seeing what was in the rocks, rather than looking for things that I thought should be there. Because that really biased my view.

Narrator: She looked at the Trendall Locality as though for the first time, and after some reflection, walked away from it. By doing so, she blew up the debate over whether the rock structures were evidence for ancient life.

Abigail Allwood: The best decision I ever made was to turn my back on that locality and just walk along the outcrops in the other direction for what ended up being tens or hundreds of kilometers, and seeing how things changed over space and time.

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[4:07] Narrator: Abby realized the rocks told a larger story of an ancient ocean reef – an underwater ridge of rock and sand. The reef was tilted so that one end had been submerged in deep water, while the other end ascended into shallower water. Stromatolites grew in number and complexity along the reef as it rose up, thanks to the greater availability of sunlight.

This simple scenario fit the overall pattern far better than the rocks being the result of an unusual combination of geological and chemical events. And later findings only confirmed Abby’s insight.

Abigail Allwood: Things that became part of the evidence for life you would never have dreamed of putting on some shopping list criteria beforehand, such as the existence of a sea cliff or the existence of rocky shoreline conglomerates. All of that became such a complex, amazingly detailed, consistent picture that you couldn't pull it apart. And out of that was the evidence that these things had to be biological.

[5:09] Narrator: The stromatolites in Abby’s reef had formed when the young Earth was still a barren wasteland, dotted throughout with erupting volcanoes that filled the atmosphere with carbon-rich gases.

Abigail Allwood: 3.5 billion years [ago] on Earth, it was quite literally an alien planet to what we have today. Which is why it's a good analog for searching for life on Mars.

Narrator: Abby is currently the principal investigator for an instrument on the Perseverance Rover, which is now heading for Mars.  The instrument, called PIXL, or the Planetary Instrument for X-Ray Lithochemistry, will help search for evidence of life on the Red Planet. Abby has tested PIXL on proposed ancient stromatolites here on Earth, to figure out if they were in fact fossils, or just geological patterns unrelated to life.

[5:56] Abigail Allwood: When we go to another planet like Mars, we want to look in the rocks there and try and find the same thing. I think the problem is probably going to be not finding anything that's a candidate, but interpreting it. And we'll be scratching our heads, just like we have been on Earth with these very old rocks. We scratch our heads and try to figure out how to tell the biological apart from the non-biological. And you have to be a lot more rigorous on Mars than here on Earth, because it really is a super extraordinary claim to try and defend.

Narrator: The search for life elsewhere in the universe makes us question what we think we know about life. All life on Earth is built on the spiral-staircase structure of DNA – the genetic material that codes for inherited traits. But will that be true for life everywhere?

Abigail Allwood: I think if we try to say that it has to be DNA-based as being the test of whether life exists or not, then I think we're going to be too narrow and too specific. We’re looking for something that evolves, and life as a system that has to be able to work together to achieve some sort of outcomes -- you can think of them in terms of processes that would be recordable in the rock record as evidence of those outcomes. So microbes being able to sense the intent or some sort of signal from another part of the microbial mat. “Hey guys, we've got to move upwards to escape being buried, or to get closer to the light.” And so they do that, as opposed to just behaving as individuals.

[7:19] I’m always very suspicious of a biosignature of which there was only one example. People ask me, “Then what would you do if you saw a stromatolite on Mars?” And my response is, “I'd walk away from it, look in the other direction.” (laughs) “What, are you kidding me? You just found a pot of gold, the Holy Grail!” But if there's only one, there should be more. If it's just one, then it's not biology. It's just stuff.

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[8:17] Narrator: Welcome to “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory. I’m Leslie Mullen, and in this third season we’re traveling to the ends of the Earth with scientists who explore every aspect of our living world. This is episode 9: Life Bound.

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Narrator: On Earth, life is everywhere. Scientists have found life in the driest deserts, in the deepest, darkest caves, and flying on dust particles high in the atmosphere. Different forms of life can survive the high pressures at the bottom of the ocean, the total absence of pressure in space, live happily in highly acidic conditions, not break a sweat in volcanic temperatures, and hibernate for years on end, frozen within ice.

[9:07] Even in sterile clean rooms, we can only reduce the number of microbes; we can’t eliminate them entirely. Our planet is completely saturated with life, and over the eons, no matter what cataclysm hit Earth, life has proven to be amazingly resilient and persistent.

Planetary scientist David Grinspoon held the first NASA chair for astrobiology at the Library of Congress, and has written extensively about the relationship between life and our planet. In his view, life doesn’t just exist on Earth; it has shaped the world we have today.

David Grinspoon: The idea is to think of life as a property of a planet, not just as something that exists in a particular place in a particular micro-environment. It's a global phenomenon. And the perpetuation of life for very long timescales, and the robustness and survival of life on geologic timescales, seems tied to the global nature of life.

[10:13] So that got me thinking, when we think about life on other planets, maybe this global nature of life on Earth is not a fluke or an incidental property. Maybe it's an integral property. Maybe it's part of what allows life to be on Earth. In which case, when we think about life on other planets, maybe we should look for this global-scale kind of phenomenon that we see on Earth.

“Living worlds” is sort of my term for the idea that life may be something that doesn't just happen on an otherwise dead planet, that maybe life is something that happens to a planet. That life and the planet co-evolve over billions of years to be a very different planetary state than you would have on a planet without life.

[11:00] Narrator: There is no question that our world has shaped us. We Earthlings are bound to this planet – our bodies are made of the same elements, like carbon, phosphorus, nitrogen, and hydrogen, that make up the mountains and valleys and ocean.

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Narrator: We breathe in our planet – humans and other animals fill our lungs every few seconds with the oxygen exhalations of photosynthesizing plants, which in turn breathe in our exhalations of carbon dioxide. We are, truly, the children of Mother Earth.

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Narrator: This is an ancient concept. In the Biblical story of Genesis, the first human was breathed into life from a pile of dust. His name, Adam, comes from the Hebrew word for “ground” or “land.”  The word “human” is likewise related to the word “humus,” which means soil. To be human is to be “of the earth.”

[12:03] (sound FX: voices)
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Narrator: In death, we crumble back into the earth – whether buried or cremated, we are eventually dispersed and recycled. In this way, no matter who you are or when you lived, you continue to live on. Earth’s memory is long, and nothing is really forgotten. You carry the history of life within you now, just as you will be carried by all the generations to come.

David says the life cycles of all these generations have been just as fundamental to the state of the planet as the cycles of water, rock, and chemistry.

[12:55] David Grinspoon: There are these cyclic phenomena in Earth's biosphere that encompass what we think of as the living world, but also the non-living world -- you know, the carbon cycle, and the nitrogen cycle, and the hydrologic cycle, and so forth. And these go very deep, and are part of the living world, but also part of the sort of overall physical planet.

You know, if you look at the composition of seawater and the composition of Earth's atmosphere and the composition of the soil, what you see are all of these complex feedbacks between life and the planet. And if you just took away the life on Earth, and then let the planet evolve physically without life for billions of years, it would be unrecognizable from the way it is today.

And so if you look at all these cyclic feedbacks, we can think of a planet as entering a state where it is “alive.” That's the notion of a living world.

[13:55] It's very similar and very much indebted to the idea known as the Gaia hypothesis, which Jim Lovelock and Lynn Margolis first advanced in the 1970s, thinking about life on Earth as a planetary-scale phenomenon. And the Gaia hypothesis is often represented as the idea that the Earth is a living organism, which isn't quite right and has led to a lot of confusion about Gaia. Because it's not so much saying that the Earth is exactly the same as a living organism. Obviously it's not. It's a planet (laughs).

But it has some qualities in common with organisms: the idea that it sort of self-regulates some environmental properties through negative feedbacks in a way that helps to keep a healthy environment for life. Which is very much the way the internal physiology of an organism results from the series of feedbacks where, basically if it starts to deviate from that healthy range, then all these mechanisms sort of kick in and push it back into that range. You know, it's not just an accident that your internal temperature when you're healthy stays at 98.6 Fahrenheit or whatever, and the pH of your blood is within a certain range.

[15:23] The idea of Gaia is the observation that the life on Earth acts in concert with the environment, that things stay within a certain healthy range of climate, and all these other properties of the environment result from the collective action of all of these different organisms kind of feeding back on the environment, acting in a way that's similar to the internal regulation of a living organism.

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Narrator: The chemist Jim Lovelock first thought up the Gaia hypothesis in the 1960s, when he was working at JPL. His challenge was to come up with ways to search for life on Mars, and that got him thinking about how the constant presence of oxygen in Earth’s atmosphere is a big red flag that life is at play here. Without photosynthetic plants continually producing oxygen, our air probably would be dominant in carbon dioxide, like it is on Venus and Mars.

[16:23] By the early 1970s, his collaboration with the biologist Lynn Margulis led them to publish an article outlining their concept of biology as an active aspect of a planet’s overall balance. Critics of their Gaia hypothesis argue that life and the planet don’t work in harmony to keep Earth in a habitable state. But an enduring aspect of their reasoning was the notion that a planet can be seen as a system of interacting processes – and, on Earth at least, one of those planetary processes is life.

[16:59] The strategy of looking at a planet’s atmosphere for evidence of life is now a hallmark of the search for habitable planets beyond our solar system. Exoplanets – planets that orbit other stars – are so far away that we can never hope to send a spacecraft there in our lifetime. But we can see them in a telescope, and read in their reflected starlight the gases that make up their atmospheres.

So far, we haven’t found evidence for life beyond Earth, but considering how vast the universe is and how many diverse planetary systems are out there, we’ve only just begun to look.

Apart from life, another aspect of Earth that we haven’t found elsewhere is plate tectonics -- that slow roll of Earth’s rock cycle, driven by our planet’s hot interior, where the gigantic plates that make up the surface burrow underneath or grind past each other, triggering earthquakes and volcanic eruptions.

[17:59] David Grinspoon: Plate tectonics, the motions of these rigid plates on Earth's surface, which creates all the geology we know and love, and sometimes fear, is something that's unique to Earth, as far as what we’ve found so far in our early attempts to understand the planets elsewhere in the universe. And since life is unique to Earth, at least so far in our explorations, you wonder how closely tied together those two facts are, or is that just sort of a coincidence?

There's still so much we don't know about how other planets organize their activity, and which qualities of Earth are really required to find life elsewhere. I mean, certainly on Earth, life is very much hooked into plate tectonics in all kinds of interesting ways. And the more we understand the deep influence of life on Earth, even the interior properties of the rocks in the mantle of Earth are altered, it seems, by life.

[19:00] Narrator: Mineral crystals in rocks, like zircon and quartz, form when certain chemistries react to temperature and pressure and are exposed to water or acidic conditions. Earth has around five thousand mineral species, but earlier in our planet’s history there were far fewer. About two-thirds of the minerals on Earth today are an outcome of the input from life. For instance, because photosynthetic plants have filled the air with oxygen, that has allowed thousands of new kinds of minerals to form.

Life needs minerals to survive, and the constant cycling of the land through plate tectonics replenishes minerals on the surface that would have otherwise stayed buried within rocks. Life itself may have gotten started thanks to the flow of energy and minerals of Earth’s rock cycle, at volcanic vents in the ocean floor.

[19:53] David Grinspoon: Life may have started off where these mineral rich hot waters are spewing out of the bottom of the ocean. That's a popular candidate right now. But some location where there was a lot of chemical energy on Earth seems to be the best candidate, perhaps 3.8 billion years ago, or closer to 4 billion years ago. We're still trying to pin that down. In other words, the origin of life arose out of some kind of geochemical activity, and became sort of an extension of that geochemical activity.

Narrator: The specific combination of minerals, heat, and chemical energy at hydrothermal vents is considered a possible origin site for life because that could have led to the first metabolism, the chemical reactions that all cells use today to convert energy into food.

[20:41] (sound FX: lava bubbles)

Narrator: A team at JPL has set up lab experiments that replicate what hydrothermal vents would have been like when Earth was young, to try to get a better grasp on how life could have gotten started there. They haven’t produced life itself, but over time, their experiments have produced amino acids and other building blocks of life.

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[21:07] Narrator: The first lab experiment to try to capture the spark of life was in 1952, when the chemists Stanley Miller and Harold Urey simulated the conditions of our planet’s early atmosphere. Their experiment used water, methane, ammonia, and hydrogen, and added lightning bolts, in the form of electric sparks. Sure enough, after about a week the experiment yielded some of the same amino acids used by life today.

Since then, we’ve learned the mixture of gases used in the Miller-Urey experiment probably don’t represent the conditions on the young Earth. But more recent experiments to mimic the primal atmosphere also have found that molecules important for life can form under a variety of conditions. 

David Grinspoon: There's still some uncertainty about the composition of the early atmosphere of Earth, but likely it had a lot more carbon dioxide, it did not have free oxygen, and also, there's some debate about this, but there may well have been a lot of organic haze in the upper atmosphere.

[22:12] You know, we talk about the pale blue dot as the poetic image of what Earth looks like from afar. There's the idea that Earth might have been a pale orange dot at that time when it was a young planet, because there would have been all this carbon in the atmosphere: carbon dioxide, and maybe some hydrogen-rich compounds, because you have methane and other stuff coming out of volcanoes. And then in the atmosphere, that carbon and that hydrogen and other elements are getting broken up by sunlight, by ultraviolet light, and forming these organic rich hazes in the atmosphere.

Narrator: So much about the young Earth and early life remains a mystery, due to the scarcity of ancient rocks that provide clues to the past, and the fact that the first forms of life had no hard body parts, like shells or skeletons, that could fossilize.

[23:02] We also don’t know how long it took for life to become widespread on Earth. But the flow of sunlight streaming down from above set the stage for a big shift in the nature of our world around 2 billion years ago, due to photosynthetic bacteria called cyanobacteria.

David Grinspoon: Photosynthesis was probably not first invented by the cyanobacteria, but sort of perfected, patented by the cyanobacteria and mass produced, to the point where they started really multiplying and transforming the environment. But it took a while, and the reason is because oxygen is a very reactive element. It's very sort of chemically promiscuous, and it reacts with everything. And so all the iron in the environment would react with oxygen first and make iron oxides. So it took another billion years after the cyanobacteria started putting oxygen into the atmosphere, before all the available iron and other easily reactive stuff was oxygenated before the oxygen actually started building up in the atmosphere.

[24:04] And then once it did, it built up rapidly, and that's what we call the oxygen catastrophe, because it was such a rapid transition, which was deadly to a lot of organisms on Earth. It caused a mass extinction when oxygen appeared, because oxygen's poison. It destroys organics, until you learn how to metabolize it and make use of the great energy of those reactions. And that's called respiration, and that's how we live. But until respiration came along, oxygen was this dangerous stuff which was very environmentally harmful to the Earth at that time.

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David Grinspoon: You know, the oxygen catastrophe is a great example that a life form can come along and really knock the planet out of balance in a way that then the rest of the biosphere has to sort of compensate for. Obviously, Earth has not just been one happy, stable planet throughout its entire multi-billion-year history, or we wouldn't have mass extinctions. Now, some of those are externally forced, like asteroids and things, but some of them are internal, like oxygen catastrophe, when a species just gets out of hand.

[25:14] And if you want to fast forward to now, you could say that's kind of happening again, with Homo sapiens getting out of hand. And it's still an open question, how the Earth is going to handle that perturbation.

People tend to oversimplify and say, “Oh, humans are the first species to ever come along and wreck the world and destroy things for other species.” And actually, no, they're not. It's what life does, is multiply itself. And sometimes organisms are so successful at doing that, that they harm other species. This happens time and time again. And an extreme example is the cyanobacteria who basically wrecked the whole world for other species, because they were so successful at doing what they did.

Not only that, you can carry the parable further because what the cyanobacteria basically did was discover a new energy source and mined that energy source so successfully that they polluted the atmosphere with a gas that wrecked the climate. Hey, that seems sorta familiar, right?

[26:08] And then you say, “Well, that's just what humans are doing.” But the difference is the cyanobacteria bacteria didn't know what they were doing, and they didn't have conversations and make podcasts about it. They just did it. They're bacteria. They're dumb. (laughs)

Now are we any less dumb? I mean, at least we're talking about it, right? And we can conceive of it as a problem and conceive of solutions. Whether or not that will be enough to really change the behavior is still an open question, but the fact that we have some awareness of what we're doing is, in my mind, what makes it not just a repeat of the cyanobacteria or not just a mirror image of it, and it gives us the potential or maybe the challenge of saying, “Okay, so we can see what we're doing. Will that be enough? Can we then act on that knowledge and change our behavior?”

[26:59] It's funny because you could say, “Oh, we're so intelligent. We have all this science, we can send spacecraft to other planets, and we made an internet, and we understand so much, look how clever we are.” And then you can say, “Okay, that's true. But look at our behavior on a planetary scale and compare it to an infant just learning to basically take care of themselves in the world. And one of the first things an infant hopefully learns is to not soil itself. And in a certain sense in our own development, we're still kind of at that stage where we can see that we're soiling ourselves and go like, “Hey, maybe we should find something else to do with our effluence here, rather than just like roll around in it.”

Narration: We humans haven’t had very long to grow up – our species has only existed for the briefest of moments compared to the vast age of the planet. And if you look at the Tree of Life, a chart that documents all the species that are known to have existed on Earth, humans are a tiny, newly-born shoot in an incredibly complex network of lifeforms, with roots that extend far back in time.

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“Life on Earth” by David Attenborough:
David Attenborough: No one can say just how many species of animals there are in these greenhouse-humid, dimly-lit jungles. They contain the richest and the most varied assemblage of animal and plant life to be found anywhere on Earth. Not only are there many major categories of creatures – monkeys, rodents, spiders, hummingbirds, butterflies -- but most of those types exist in many different forms. There are over 40 different species of parrot, over 70 different monkeys, 300 hummingbirds, and tens of thousands of butterflies. If you’re not careful, you can even be bitten by 100 different kinds of mosquito.

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[29:00] Narration: Many books and documentaries, like David Attenborough’s “Life on Earth,” have celebrated the spectacular variety, abundance, and beauty of all the different species on our planet. We have not yet discovered all the species that Earth holds – every year, scientists are finding new and unusual creatures, both those living today and the remains of beings long gone but still preserved in the rocks.

The fossil record also has shown us that species seem to have expiration dates, just like an individual’s life span. Estimates are that 99.9 percent of all species that have existed on Earth have gone extinct. Earth is a planet of constant transformation, and species rise and fall largely based on whether they can adapt to a changing environment.

David Grinspoon: We live at the surface of the Earth, the interface between these two giant heat engines, the interior heat engine with everything cycling because of that heat in the interior which is causing plate tectonics. And then the exterior heat engine of the restless dynamic atmosphere with weather, and also the solar energy, the sunlight, coming into the atmosphere.

[30:09] It's that dynamic nature of the planet that facilitates life here, but it also means that there's always something happening on Earth, some extreme volcanic eruption or some extreme weather event, that has a certain risk for natural disasters and even mass extinctions.

Narration: One environmental shift that life has historically had a hard time coping with is Earth’s slow cycling between Ice Age and Hothouse Eras. We’ve had essentially the same climate for the last ten thousand years, but based on Earth’s history, that was always bound to change. By burning carbon-based fuels, we’re pushing that change far faster than Earth would have done on its own, and perhaps even reversing a cooling trend that can lead to an Ice Age.

[30:57] David Grinspoon: It is very interesting to take a deep time look at Earth's environment, and then consider the really tiny amount of that, that humans have been here for. I mean, it depends what you mean by humans, of course, because as a species, if you're talking about hominids, we've been here for millions of years, but if you're talking about anatomically modern humans, we’ve been here for something like a hundred thousand years. As civilization builders, when we've actually started doing agriculture and settling down and having recorded history and all that stuff, you can look at that as like a 10,000-year timescale.

And all of that whole 10,000-year history has occurred during a very unusual time on the Earth. It's been a relatively warm and relatively stable climate. So we have this illusion, because of our history, that Earth is this sort of stable, safe place. And then that contributes to this idea that we're messing up the planet now with our industry, and if we just stopped, then Earth would be fine forever, and Earth is this Eden that takes care of life. But actually, that's an illusion.

[32:09] I mean, it is true that we're messing up the climate, but it's also true that on a longer timescale, natural climate changes happen, which are very dangerous, Ice Ages and Hothouses. If we want to last for another 10,000 years, or certainly for another 50,000 years, then we're going to have to deal with the natural cycles of the Earth, where there'll be another Ice Age or another Hothouse Age, which is much more intense than the global warming we're causing now, if we leave Earth to its own devices.

That's a much longer timescale challenge, but I think if we start seeing ourselves as a geological process, which is something I think we need to do, once we accept that we're planet changers, and stop doing a bad job of it, then we'd have to start thinking, “Well, is there a way that we could be planet changers, but do a good job of it? And sort of become stewards of this planet?” I think that will become our responsibility if we are able to survive in the short term and stick around here a bit longer.

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Narrator: We still have so much more to learn about how the cycles of our home planet, including the cycle of life, have interacted over time and led to the conditions we have today. We may have as little control over the evolution of life as we do over the revolutions of Earth around the Sun, but the events of the past at least provide some insight into the challenges of our future.

David Grinspoon: Right now, we're worried about what sometimes people call the sixth mass extinction. There've been five natural mass extinctions, the most recent and dramatic being the events 65 million years ago, the big asteroid collision which wiped out the dinosaurs and most of the other life at that time. And the worry is that we're now causing another mass extinction, you know, loss of biodiversity through climate change and habitat loss through our changes in land use and all that stuff. And so that is a valid concern, and we want to stop instituting a mass extinction, obviously.

[34:09] So, if we see an asteroid heading towards Earth that we think is going to cause a mass extinction, we want to divert that. But then you think about mass extinction in the past, you know, the asteroid 65 million years ago that wiped everything out, or there's even bigger ones, the Permian-Triassic extinction, which is the biggest one of all 250 million years ago, you think those were bad, right? Wouldn't it be great if somebody could go back in time and change them? But then of course we wouldn't be here.

And if you appreciate human beings and art and music and technology, or even if you just appreciate the other species on Earth today that we share this planet with, if you like blue whales and oak trees and whatever, none of that would be here either if those mass extinctions hadn't happened. So, it's a paradox, right? You don't want to stamp out all change because that would be boring, and it sounds dead. But you also want to avoid the extreme changes that lead to mass extinctions.

[35:05] So, I guess the take home here maybe is that long-term management of an environment and even of a planet is tricky, and leads to apparent conflicts. And is something that, if we're going to ultimately become the kind of mature species that can not only avoid short-term calamity by basically soiling our (laughs) our crib -- then we are going to be faced with the challenges of being a mature species. And adulthood has its challenges too.

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Narrator: If you like this podcast, please subscribe, rate us on your podcast platform, and share us on social media. We’re “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory.

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