Emory University paleontologist, geologist and ichnologist Anthony J. Martin talks about his new book, The Evolution Underground: Burrows, Bunkers and the Marvelous Subterranean World beneath Our Feet.
Emory University paleontologist, geologist and ichnologist Anthony J. Martin talks about his new book, The Evolution Underground: Burrows, Bunkers and the Marvelous Subterranean World beneath Our Feet.
Steve Mirsky: Welcome to Scientific American’s Science Talk, posted on May 3, 2017. I’m Steve Mirsky. On this episode -
Anthony Martin: Burrows have been around for a minimum of about 550 million years, maybe going back a little bit more. That’s only about maybe halfway through the history of animals over the course of geologic time.
Mirsky: That’s Anthony J. Martin. He’s a paleontologist, geologist, and one of the world’s premiere ichnologists. So what’s an ichnologist? Well here’s a hint. He’s the author of the new book The Evolution Underground: Burrows, Bunkers, and the Marvelous Subterranean World Beneath Our Feet. He’s based at Emory University, but he was in Manhattan recently where I met him at the office of his publisher, Pegasus Books. So as often happens when I do an interview at the Manhattan office, you can also enjoy the sirens of the police cars, fire trucks, and ambulances of New York City.
So you have a picture in the book of the worm stone it’s called at Down House, Darwin’s home. And when I was in England, I made sure to go to Darwin’s house, and I stood next to the worm stone. And talk about the worm stone and what it represents and how kind of cool it is.
Martin: The way I’ve described it to people was a thrilling experience because first of all, just going to Down House and understanding that this is all part of the history of science, the theory of evolution, and Darwin’s connection to that theory. But I really wanted to see the worm stone. And the worm stone in the backyard there of Down House represents his research on earthworms. And a lot of people don’t know that Darwin’s last book and one of his bestselling books was about earthworms.
So this was something that I wanted to write about eventually is this connection between Darwin as a biologist but also a geologist, thinking about the long-term effects for what he described as the leveling effects of earthworms on the surface of the earth and connecting this to this teaming underground, this fauna that is underneath our feet that is actually shaping the earth and changing the earth with the evolution of all these lineages.
So for me, it connects a lot of themes in ecology, geology, and evolutionary biology, and Darwin was right there in 1880 with it.
Mirsky: And he used a device that his son invented to actually make his measurements possible.
Martin: That’s correct. In typical Darwin fashion, he had a long-term experiment in mind, and his son Horace invented a measuring device that the stone – it would be set above the stone, and then it would measure the amount of sinking the stone was doing over time. That sinking represented how the earthworms were burrowing and bringing up sediment from below through fecal piles and depositing that on the surface, which was causing the stone to go down relative to the surface. So he was able to measure this year after year over the course of – oh, I can’t remember how long. Twenty years or so.
But he was able to then calculate a rate of burrowing and a rate of deposition. This was beautiful, quantitatively based reasoning back in the 1870s, 1880s. Darwin was doing it.
Mirsky: And the stone was connected or braced to the bedrock. So he knew that was not moving.
Martin: That’s correct.
Mirsky: The difference in the relative positions was due only to the soil being pulled up.
Martin: He was a very careful experimenter.
Mirsky: And even if that was all he had ever done was this earthworm research, he would have been a recognized scientist.
Martin: He would have been famous just for that. I’m very confident of that. And even today, we still study Darwin, and we look to him in my field of ichnology, the study of traces, as someone who is the first quantitative ichnologist. That’s not something Darwin would have called himself. But we can justify in calling him that, and he was very influential then for our science of being able to measure the amount of burrowing that animals do.
Mirsky: What are traces?
Martin: Traces are tracks, trails, burrows, nests, or other indirect sign left by an animal either in modern environments or in the fossil record. So if it fossilizes, we call it a trace fossil, but if it’s around today, we just call it a trace. Implicit in that is there’s some sort of behavior behind it. So I often times use what I call the holy trinity of ichnology. Substrate, anatomy, and behavior. That the substrate could be say sediment, mud, or sand, or it could be rock or wood. That’s going to be preserving the trace.
Then we have the anatomy of the animal affecting that substrate and leaving its mark, so to speak. Then we have the behavior that’s behind that and the behavior then reflects that animal in real time. What was it doing right then and there?
Mirsky: So in the most basic example would be a footprint is a trace.
Martin: That’s right.
Mirsky: But you could tell based on the footprint whether the animal was walking or running, and then you have behavior.
Martin: There’s a tremendous amount of information contained just in a single footprint, and it reminds me of how Sir Arthur Conan Doyle, he expresses that through his character of Shirlock Holmes, also as an expert tracker. So this is something I also like to point out is a part of being human. That we likely evolved this search image capability, and then linking that to a visual imagination of looking at a series of imprints on the ground and translating that into behavior.
So I’d like to point to ichnology as being one of the original sciences, that this is a science we use to survive, and then evolve as a species.
Mirsky: Because if you know there’s something edible down some hole 10,000 years ago, you’re an ichnologist. You’re a hungry ichnologist.
Martin: That’s right. It’s also good to know if there’s an animal in the area that considers you edible.
Mirsky: Right. And some of the burrows are just absurdly large, especially the fossil burrows of the sloths.
Martin: Oh, these are incredible. I haven’t visited these burrows yet, but I have colleagues who studied them in Brazil and Argentina. These are massive four to five meter wide tunnels that go on for tens of meters or hundreds of meters below ground. Scratch marks on the walls can be linked to the animals, and the paleontologists who have studied those have linked them to giant ground sloths, or as I call them, giant underground sloths, and then giant aardvarks that lived during the Pleistocene epoch – in that area. And yeah, they’re just mind blowing when you look at the photos of them and the maps of them, to imagine these animals carving out these huge subterranean systems.
Mirsky: And some of the – not the very biggest underground sloths, but very large sloths.
Martin: Large enough.
Mirsky: Were living at the same time as people.
Martin: Yeah, that’s right, so that’s the interesting thing to think about. I don’t know if humans had arrived in that area yet at the time that these tunnels were made. I don’t think we’ve determined that. But knowing that people were alive when these gigantic burrowers were also alive is a pretty cool thing to think about.
Mirsky: So somewhere deep in the book, you talk about a series of theoretical catastrophes. Volcanic eruptions, meteor strikes or whatever, and the answer to how to survive them for each one is the same, which is basically go underground.
Martin: That I think is a large part of the answer. Of course, how do you survive a mass extinction? It’s really survival of the luckiest in many of the instances.
Mirsky: This is the David Raup that you talk about in the book.
Martin: That’s right, good genes or bad luck, I can’t remember what it was, what the title of David Rowp’s book a paleontologist who also looked at this.
Mirsky: I think it’s Bad Genes or Bad Luck.
Martin: I think you’re right, yeah. And with that, what he was pointing to is that animals don’t necessarily have a pre-adaptation for surviving say a meteorite impact or a volcanic eruption or a massive storm, for instance. So the answer then is burrows. You have burrows that protect you from these ravages that might be happening on the surface, and these could be slow motion catastrophes, such as climate change that causes extremes in weather or droughts or other posities of resources that might be in an ecosystem. Again, if you’re in a burrow, you are more likely to survive.
Mirsky: The book opens with you and some students on this island off of Georgia, and you’re looking at what you assume to be empty holes in the ground.
Martin: Yeah, the line I use to open up my book is the alligator then had a big surprise for us. It was I would estimate about an eight to nine foot alligator that was inside this hole in the ground that was in the middle of a pine forest on this bare island, St. Catherine’s Island on the Georgia coast. I had assumed it was empty because there was no water nearby. This was the seed for an idea then. Part of my doing research on these alligator dens was to look at how do the alligators use these to adapt to changes in their environments. And when there’s a drought, they go below, and they intersect with the water table, and they’re perfectly fine there. Also, animals are attracted to the water that’s in their den.
So basically, their groceries come to them, and they turn into ambush predators, and you don’t think of alligators as terrestrial on land predators, but this is again the adaptability they have through burrows. So with this chapter, this opening chapter of the book and having my students witness my ineptitude in that respect -
Mirsky: One of the students was kind enough to say, “Hey, I think I see teeth in there.”
Martin: Yeah, I had no idea they would be living teeth either. But – and we got a good verbal warning from its occupant, and I’m sure that’s a lesson that they would not have missed that on the exam.
Mirsky: Well when you say verbal warning, we should say, “Hey, hey, buddy. Watch out.” The alligators make a really memorable hissing sound.
Martin: Yeah, it’s a deep throated hiss, open mouth, and with the – from within the den, let’s just say it adds some resonance that caused everybody to jump back a little bit and go, “Whoa.”
Mirsky: Maybe we’ll look at this one from a little bit of a distance.
Martin: That’s right.
Mirsky: Yeah, I’ve been the recipient of alligator hisses in the Everglades, and it really gets your attention.
Martin: Yeah, you don’t forget it.
Mirsky: So the book then goes on to talk about basically the history of life on earth, and the role that being underground has played in that history, and everything burrows to some extent.
Martin: Yeah, that was one idea that I really wanted to get across in the book is how many different animals burrow, or they owe their lineage to burrows as either a means of survival or as a way of adapting to changing ecosystems through time. And yeah, the menagerie of burrowing animals, it staggered me once I started doing a little bit of research on it that most people, sure, they know about worms, for instance, but then crustaceans. There are crayfish, crabs, lobsters, all of those kinds of animals that burrow, and insects.
All of these different species of insects that burrows. Mammals, so many species of those that burrow. In between that, you even have lung fish that burrow. So lung fish, amphibians, reptiles, you can just go through all the different animal groups, and you can pick out burrowers from those.
Mirsky: In fact, the jacket leaf of the book has a picture of you holding a cast of a burrow system, a network, and it doesn’t show you on the leaf itself what organisms made that, but somewhere in the book, it explains it was a crustacean network, which surprised me. Because it looked so huge that I assumed it was naked mole rats or something – shrews. Something fairly big, a small mammal that was living underground. But no, this was a crustacean network.
Martin: That’s right, yeah, it was probably from a burrowing crab in some of the salt marshes on the Georgia coast, and that’s where the cast was made in the late 1970s from a PhD student at the university of Georgia who is working with the person who would eventually be my PhD advisor, Bob Frye at the University of Georgia. He was very interested in casting these burrow systems to use those as models for what you might see in the fossil record, and sure enough, some of these crustacean burrow systems that I’ve seen in the fossil record are massive and complex systems that you just stand back in awe of how much energy went into making these systems.
And crustaceans evolve that ability very early on in their evolutionary history.
Mirsky: They have to be good engineers without – I mean they’re not consciously engineering, but some engineering skills have been selected for, or else these things would collapse on themselves.
Martin: That’s right, or as I like to say to my students, it’s evolution baby.
Mirsky: That you can pretty much say that truthfully all the time.
Martin: Yeah, that’s right.
Mirsky: So take us through the origins of burrowing in the fossil record. It’s incredibly ancient.
Martin: Burrows have been around for a minimum of about 550 million years, maybe going back a little bit more, maybe 550 and 560 million years ago. Now actually in the history of animals, that’s only about maybe halfway through the history of animals over the course of geologic time. So it’s a relatively recent innovation by animals, but obviously it has taken off ever since. Early on, all animals were superficial. They either just stayed on the ocean floor or just below the ocean floor, and they burrowed horizontally. At some point in their evolution, it behooved them to start going down and start plumbing some of those depths.
Some of that may have been selected for through accessing resources. There’s more food down there. Or it may have been avoiding predators. I don’t want to be food. Or other adaptations, just say changing climate conditions that were making the surface of the ocean floor more inhospitable. So going below, again, made sense.
Evolutionarily speaking, or it may have been a combination of all those. But at some point, about 545 million years ago, it really started to take off, and then animals started going down.
Mirsky: The ability to burrow required some physical attributes.
Martin: That’s right. So there are several ways these animals had to evolve. Either the ability to move the sediment from around their bodies, so that would be say expanding and contracting their bodies. So think about an earthworm on the sidewalk, how it moves along, doing exactly that. A peristalsis. Or you could get a hard skeleton that if you have a shell, for instance, and then you have some soft part, like a clam with its foot anchoring, and then pulling itself through the sediment. Then you’re basically like a little ship.
Where you’re just plowing through that sand or mud, and you’re able to better get yourself below the surface.
Mirsky: And even the shape of the head becomes a factor.
Martin: That’s right. So a lot of animals, when you look at their heads, and if they’re head first burrowers, their heads are very well adapted for doing that. When we look at early trilobites from the Cambrian period. And you look at their head shape. Their head shape also looks a bit like a plow, and you can start thinking about how some of those early trilobites were indeed burrowers, and we sometimes find them directly associated with their burrows.
We certainly see a lot of their burrows from the Cambrian period on that reflect their anatomy where they were going below the surface.
Mirsky: And throughout the history of life on earth, we’ve had these major extinction events, mass catastrophe, and being underground gives you an edge when something like that happens.
Martin: Yeah, it’s one of those ways that it’s almost fool proof. That if there’s something terrible happening on the surface and you’re already below ground, you’re more likely to survive, and you can just do a little check list of the different type of disasters that could be happening. Let’s say there’s a tropical storm, and you’re an animal living on the sea floor. You might get buried, but you can always unbury yourself. You’re not going to get tossed away, for instance, or put up on land. If you’re in a river and there’s a saltwater wedge going up the river, for instance, and you’re an animal that loves saltwater, burrowing is the way for you to survive freshwater coming into your ecosystem.
Then you can think about even volcanic eruptions, some sort of massive change in your environment that’s totally unexpected, in no way you could adapt for that ahead of time. If you’re in a burrow, again, you’re more likely to survive.
Mirsky: If an asteroid smashes into the Yucatan, it really helps to be underground. It would also help to be thousands of miles away, but being underground does give you a big advantage.
Martin: That’s right, if you’re in the impact zone, your burrow is probably not going to save you, but if you’re farther away from the impact zone, you were more likely to survive. So the end cretaceous extinction, which happened about 66 million years ago is linked with a meteorite impact. What happened in terms of the animals who did live through that is that they had in common being small, maybe being burrowers, and maybe being in freshwater environments where they were able to escape the ravages of fire or extreme temperature fluctuations that were happening on the earth’s surface. I think the key factor to look at in this instance is looking at burrowing and what animals were burrowing at the end of the cretaceous that could have given them an advantage going into this new world of the Cenozoic era.
Mirsky: You talk about that – the ability to burrow and to live in burrows has evolved many many times.
Martin: Right. It’s not necessarily something passed down through a lineage is that it can re-evolve in a lineage. So throughout geologic time when you think about different animal lineages like let’s take worms, for example, and annelids, for example, which include earthworms, leeches, and among my favorite worms, polychaetes, like bob head worms that live on the seafloor. Wonderful little predators. A lot of those animals evolved burrowing multiple times in their languages.
Hence when we look at those kind of animals and how they’ve used that to adapt, whether they use it for predation, whether they use it for gaining other food resources or not becoming food, burrowing definitely has some advantages that you could see it getting selected for again and again in a given evolutionary lineage.
Mirsky: We don’t think of humans as burrowers, but you have a whole chapter on underground living among humans, and you talk about these cities in ancient Turkey, and it’s just crazy. It’s absolutely amazing what they built down there and how they lived down there. You’ve got a photograph in the book of one of these places. There were two cities that were five miles apart that had a tunnel connecting them. I mean people had plenty of free time back then I guess. They didn’t have the internet. They didn’t have television. So they could do things like this, but it’s just unbelievable.
Martin: Well and that’s one of the reasons I wrote the book also was to help people understand that we come from a lineage of burrowers, that mammals survive mass extinctions at the end of the Triassic period, which was about 200 million years ago. The end of the cretaceous period, 66 million years ago, and then we really took off – we being mammals, really took off in our diversity through those last 66 million years. So this burrowing behavior might have actually been a kind of bio-mimicry, where humans were seeing badgers in central Asia for instance.
Badgers and going, “Hey, that looks like that works.” And, “Huh, here comes some conquerors,” and we happen to have some soft sandstone that’s very malleable in our – in this area of Cappadocia, Central Turkey. So you could see people being able to use this as a strategy for escaping invading armies, and it did work. So for generations then in that area, people lived underground if they needed to. Not necessarily all the time, but this was an option for them, and they carved out these vast underground cities where 5,000 to 10,000 people could live underground.
Mirsky: Amazing, and you talk about how they had these sort of tight little – I was thinking of Thermopylae, the battle of Thermopylae and how there was that little channel that they were forcing the Persians to come through. They kind of did that in these underground caverns that they built where if any enemies came in, they’d be blocked up so that it was only one soldier at a time who could get through, and you dispatch that soldier and you block their whole entrance. Must have been a pretty frightening scenario for any invader to go down there.
Martin: I write a little fictional scenario in the book about that. Let’s say a century shows up with a centurion, 100 Roman soldiers, and you decide you’re going to invade this little underground city. And just all of the defenses there and knowing that anyone who has watched a horror movie where you’re going into a dark place, and the people down there know the terrain and you don’t. That’s what those Roman soldiers would have faced. Then later, that would have been the Ottomans would have faced the same sort of scenario.
Anyone living down there, they had the advantage, and then you think about this again from the perspective of say a badger. Would you go down a badger hole if you could fit in it? I doubt it seriously. This is a point then I wanted to bring across is also burrows has places of defense that if you are an animal, whether you’re human or not, you can stay defended, you can stay safe in this underground habitat.
Mirsky: And the environmental conditions stay pretty constant, too, so you don’t have to deal with the very cold winter or very hot summer.
Martin: Right. Cavers will tell you that that’s one of the nice things they love about caving is you could be in a tropical environment and it’s going to be nice and cool down there. Or you could be in a more northern environment. It’s going to feel warm down there. Because these cave systems are going to have the average temperature of that region where it’s going to be pretty comfortable living, staying underground, even in inhospitable environments where it’s not so nice up on the surface.
Mirsky: And in that chapter on you’ve been tunneling, you talk about some of these defense networks that have been set up in mountains in the US and other countries.
Martin: It was interesting to dive into some of the literature done on this, on these underground defenses that started with the cold war, and they were looking to the Cappadocians as examples. And other examples through human history where they had used underground defenses. Hence, when humans invented the most destructive weapons known to human kind, nuclear weapons, the first thing that US defenses, Soviet defenses, and the Chinese thought of was well, we need to have underground facilities, and here in the US, we went big time into it where there were underground defenses at Raven Mountain is one I describe.
Site R in Southern Pennsylvania, and several other facilities that were put deep underground, fortified, and set up with abilities for the US government to continue to function in the case of a nuclear attack. That was just a fascinating exploration into that mentality from the 1950s of how do you survive a nuclear war. Again, the answer was go underground.
Mirsky: You and colleagues discovered the first burrowing dinosaur fossil.
Martin: Yeah, my colleagues, Dave Rikio, Yoshi Katsura, and I, we were lucky enough to work on what’s still the only known burrowing dinosaur, the only known species. Oryctodromeus cubicularis. Now that literally means a running – digging runner of the den because this was a two-legged relatively small herbivorous dinosaur that we were lucky enough to find in its burrow. So Yoshi found the bones at the surface in Southwestern Montana in these cretaceous age rocks. Then Dave, my friend Dave Rikio, he noticed that there was this odd structure connected to the bones. He sent me a photograph, and from the photograph then, I was able to say, “Yeah, that looks like a burrow. What’s at the other end?” And then when he told me a dinosaur, I was doing a happy Snoopy dance for that.
Because we knew then that would be really good strong evidence that this was a burrowing dinosaur. In testing that hypothesis, we looked at the anatomy of a dinosaur as well as the characteristics of the burrow, and everything fit. So we were lucky enough – and again, very lucky – I always emphasize that. Very lucky to be able to document that dinosaur, and then we now think about dinosaurs as not just being on the ground surface or up in trees, but actually being below ground, too.
Mirsky: And why not?
Martin: And why not? And because we have so many burrowing birds today, I threw out the idea that maybe that’s how birds as dinosaurs survived the mass extinction 66 million years ago. Maybe those birds had their nests underground, and that would have been a way for their eggs then for the next generation to be able to survive a mass extinction by death from above. Our entire earth has been shaped by burrows. Burrowing animals, once they went underground starting about 555, 560 million years ago, they started changing the chemistry of the ocean floor. That in turn changed the chemistry of the atmosphere.
Everything changed after that. Ecosystems changed in the oceans. Then when animals got on land and started burrowing on land, they changed all of those ecosystems, from freshwater lakes near the shore, all the way up to the highest mountaintops. So this is part of the marvelous world beneath our feet is thinking about how animals have actually changed the world. We live in a world that is shaped by burrows and burrowing animals, and it’s a long legacy that goes back more than 500 million years. It’s awesome to think about.
Mirsky: That’s it for this episode. Get your science news on our website, www.ScientificAmerican.com, or you can also check out the education page and find out about science experiments to do with your kids, as well as citizen science projects in which you can help researchers working on everything from figuring out where plastic ends up in the ocean to finding new exo-planets, and follow us on Twitter where you’ll get a tweet whenever a new item hits the website. Our Twitter name is @SciAm. For Scientific American’s Science Talk, I’m Steve Mirsky. Thanks for clicking on us.
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