Speaker 1: Welcome to Stuff to Blow Your Mind, a production of iHeartRadio.
Speaker 2: Hey, welcome to Stuff to Blow your Mind.
Speaker 3: My name is Robert Lamb and I am Joe McCormick.
Speaker 3: And hey, here we are together again, Rob. After you
Speaker 3: you were out for a little bit.
Speaker 2: Oh yeah, yeah, we've recorded a few things here or there,
Speaker 2: so I keep forgetting that, you know, when the actual
Speaker 2: new Core episodes are that I'm around for. So yeah,
Speaker 2: I am back. I was just in a little vacation
Speaker 2: with my family.
Speaker 3: Wait a minute, now, you refreshed my brain because I
Speaker 3: forgot that we did a listener mail episode before this,
Speaker 3: So listeners, you are reacclimated to Rob.
Speaker 2: Now, yeah, I mean listeners I think always have a
Speaker 2: skewed understanding of when we're out when we're present, because
Speaker 2: there's you know, even if one of us is out,
Speaker 2: you may hear us on a vault episode on the weekend.
Speaker 2: We might have recorded through on something recorded ahead of
Speaker 2: time on one piece of content, but not on other
Speaker 2: pieces of content, So you know, you'll just have to
Speaker 2: take our word for it. Sometimes we are away, sometimes
Speaker 2: we were present.
Speaker 3: We're like a duo of mysterious time traveling cats that
Speaker 3: appear in your ears.
Speaker 2: Yeah all right, Well, you know, I have to say,
Speaker 2: on this most recent journey with my family, I did
Speaker 2: not venture into a cave. I don't think I ventured
Speaker 2: into a cave. Let me refresh. No, no cave venturing.
Speaker 3: No, I would think you'd remember.
Speaker 2: Come on, I mean, sometimes, you know, these trips, you
Speaker 2: see a lot of things, and yeah, you know, and
Speaker 2: you know, cave environments are are always fascinating, but they're
Speaker 2: not necessarily always going to be like the top of
Speaker 2: a particular trip. It just varies. But you know, we
Speaker 2: we frequently discussed biological wonders on the show that leads
Speaker 2: us to consider extreme environments, and the cave environment is
Speaker 2: one such environment. I tend to find caves endlessly fascinating,
Speaker 2: even if it's a lower tier cave, you know, And
Speaker 2: I've been to some of those where it's like, okay,
Speaker 2: this is this one was opened by humans a while back.
Speaker 2: It doesn't have much in the way of a robust
Speaker 2: ecosystem and so forth, but it's still fascinating. There's still
Speaker 2: something about the subterranean world.
Speaker 3: Have you ever done the cave exploring experience where you
Speaker 3: go down into a cave and all artificial light sources
Speaker 3: are turned off and you get to see like the
Speaker 3: blackest black night you've ever seen. It's a kind of
Speaker 3: darkness that you cannot imagine otherwise.
Speaker 2: Yeah, absolutely done this in a couple of Tennessee caves,
Speaker 2: I think growing up. One that was like one of
Speaker 2: these big official caves. I'm blinking on which one it
Speaker 2: would have been, but went there was like a scout group,
Speaker 2: and then there was another like local cave that was
Speaker 2: just on somebody's land and got to go in there
Speaker 2: and see the main cavern and then have the lights
Speaker 2: go out. Sometimes they'll do the lights out with a
Speaker 2: little ghost story or something right, just sort of drive
Speaker 2: home the creepiness of it. But yeah, you find yourself
Speaker 2: in just the appsol darkness.
Speaker 3: We did this with our tour guide years ago at
Speaker 3: Oregon Caves National Monument in southwestern Oregon, which is a
Speaker 3: very cool cave system. And it's hard to explain because
Speaker 3: again I had not seen anything like this anywhere before,
Speaker 3: where your eyes are totally wide and you see not
Speaker 3: even a pinprick of light, just nothing.
Speaker 2: Yeah, it's haunting and and it is key to understanding
Speaker 2: cave environments, as we'll be discussing in this episode. Now,
Speaker 2: not only are caves home to rather unique organisms in
Speaker 2: certain caves, as we'll discuss more as we progress, but
Speaker 2: they've of course always fascinated humans, and our global myths
Speaker 2: and traditions are full of caverns that house monsters, passages
Speaker 2: to the underworld, magical treasures, and much more. Also, as
Speaker 2: we've discussed, these were some of the first enclosed spaces
Speaker 2: that humans entered and were occupied to some degree in
Speaker 2: a way, laying the ground for the humble and elaborate
Speaker 2: shelters that we would construct thereafter. You know, because what
Speaker 2: is what are some of the rooms in our houses
Speaker 2: but caves that we have built for ourselves.
Speaker 3: Some houses are more cave like than others.
Speaker 2: Yeah. Yeah, So we may not think of ourselves as
Speaker 2: cave people, but we are, in many respects still people
Speaker 2: of the cave at home, with enclosed spaces, sometimes with windows,
Speaker 2: sometimes without places in general like this, be it constructed,
Speaker 2: be it like a large, you know, enclosed concert hall,
Speaker 2: or an underground facility, or a naturally occurring cave or
Speaker 2: some sort of a tunnel. I mean, you can't help
Speaker 2: but dream about those spaces, to find dread and fascination
Speaker 2: in those spaces. And you know now that I mention it.
Speaker 2: On my family vacation, I don't think we went We
Speaker 2: did not go into a cave system, but we did
Speaker 2: go through tunnels at times, and we were stuck in
Speaker 2: traffic at a tunnel at one point. And so that
Speaker 2: alone gives one enough room for pause. It's just as
Speaker 2: good almost well, I wouldn't say it's not as good,
Speaker 2: but it does. It summons possibilities, maybe even the possibility
Speaker 2: of talking about the strange world of cave biology. And
Speaker 2: that's what we're going to be looking at in this series, right,
Speaker 2: that's right, talking about the cave environment, talking about some
Speaker 2: of the organisms and life forms that live there. And
Speaker 2: to get into this, I want to talk a little
Speaker 2: bit about just sort of the idea of there being
Speaker 2: a dual world, something that again is quite reflected in
Speaker 2: some of our more supernatural understandings of the surface world
Speaker 2: and the world beneath the earth, you know, the world
Speaker 2: of the living, the world of the dead, and so forth.
Speaker 2: I was reading a twenty eleven article in The American
Speaker 2: Scientist by Abamaro Romero, and he pointed out that cave
Speaker 2: biologist or biospileiologists divide the world into two. There's the
Speaker 2: epigean world, or the world upon the earth, the world
Speaker 2: of light that we predominantly think of as our home,
Speaker 2: and then there's this other world, this underworld, the Hypogean
Speaker 2: world beneath the soil.
Speaker 3: I've never thought of the surface as epigean before, but
Speaker 3: I love that because I guess with the Greek there Yet,
Speaker 3: like you say, it means upon the earth, on the earth,
Speaker 3: so not just like exposed, but it's like like an
Speaker 3: epiphyte plant that grows on another plant. The epigean world
Speaker 3: of the surface world isn't like the thing that grows
Speaker 3: on the surface of this ball of iron.
Speaker 2: Yeah, And then everything else, the underworld, the Hypogean world
Speaker 2: beneath the soil, and the world beneath the soil. No
Speaker 2: mistake here is vast. So just as the epigean world
Speaker 2: consists of any part of the biosphere that is even
Speaker 2: partially or periodically exposed to light, the Hypogean world consists
Speaker 2: of any underground part of the biosphere. So this includes
Speaker 2: the living soil in your backyard, you know, networks of
Speaker 2: mycilium that rooting through it, as well as life between
Speaker 2: grains of sand buried under the beat underground waters, and
Speaker 2: indeed cave environments, which to be clear, can also be
Speaker 2: part of the Epigean world because they of course are
Speaker 2: gateways and they are going to be transition spaces the
Speaker 2: mouth of a cave, for example.
Speaker 3: Mmmmm yeah.
Speaker 2: Now, caves, as Ramera points out in this article, are
Speaker 2: also This is also a very broad categorization, entailing everything
Speaker 2: from ice caves to lava tubes. And indeed, caves are
Speaker 2: merely the subterranean spaces into which humans can venture. There
Speaker 2: are plenty of subterranean environments and even cavernous spaces in
Speaker 2: the Earth that are just inaccessible to us because there
Speaker 2: are no openings to the surface.
Speaker 3: That's a really good point. And there's so much that
Speaker 3: goes on in those those small or tiny spaces that
Speaker 3: you can't like walk into and observe. You know, you
Speaker 3: mentioned like the idea of the myceelium that underlies that.
Speaker 3: You know, you find a mushroom in the forest. It's
Speaker 3: hard to remember this sometimes, but like the mushroom you
Speaker 3: find is just like one organ of the fungus organism
Speaker 3: as a whole. It's the fruiting body that comes up
Speaker 3: above the surface but underneath there's going to be this
Speaker 3: whole network that's sort of where the organism really is
Speaker 3: is under the soil, and you don't even see it.
Speaker 3: You don't even think to look down there.
Speaker 2: Yeah, there's a vital and rich world in there, and
Speaker 2: it's not a space that human beings can venture into
Speaker 2: in the way that we can venture into a cave environment,
Speaker 2: you know. I mean we can study it, we can
Speaker 2: explore it, we can dig it up, but it's a
Speaker 2: different sort of world. Now. Romero also mentions the freatic
Speaker 2: environments of underground lakes and rivers, but the word cave
Speaker 2: is generally reserved for again, those spaces we can access ourselves,
Speaker 2: where human explorers and scientists or at least their technological minions,
Speaker 2: can physically investigate the world below. At least you can
Speaker 2: get you can get a robot in there, you can
Speaker 2: stick a probe in somehow invoke your presence there.
Speaker 3: A cave is like a space that can be spilunked,
Speaker 3: and maybe our standards for how small a space can
Speaker 3: be spilunked are changing.
Speaker 2: Right When we had Lee Burger on the show, the
Speaker 2: author of Cave of Bones, he was talking about exploring
Speaker 2: various cave environments, a specific cave environment in fact, looking
Speaker 2: at pre human skeletal remains. And one of the really
Speaker 2: telling things about his experience is there was this part
Speaker 2: of the cave that for the longest he could not access.
Speaker 2: It was like a grueling experience, and you had to
Speaker 2: send like very young specialized scientists down there to physically
Speaker 2: occupy the space. And he was able to send them
Speaker 2: down have them look around, he was able to look
Speaker 2: at video footage from that space, but eventually he could
Speaker 2: not resist. You know, he talks about this at length,
Speaker 2: like he had to go down there. He had to,
Speaker 2: like he lost weight in order to fit through, narrowly
Speaker 2: fit through the passageways that were acquired, a grueling experience
Speaker 2: to actually make it there. So there is something about
Speaker 2: like the human exploratory experience in all of this, Like
Speaker 2: even when you have the technology and the organization, there's
Speaker 2: something about it. You have to be there, you want
Speaker 2: to be there in the space. Man.
Speaker 3: I know I'm not unique in this, but that is
Speaker 3: one of the things that truly gives me the creeps
Speaker 3: to like barely trying to wedge your way through passageways
Speaker 3: inside a cave under the earth that is just ugh.
Speaker 2: Yeah, it gave me the all overs here in some
Speaker 2: of the details in his book, and I think there
Speaker 2: was also covered in a Netflix special if Memory Is Serving,
Speaker 2: And yeah, just the idea of like having to squeeze
Speaker 2: through a space, getting momentarily stuck like the wrench and
Speaker 2: a chimney, and then having to press on potentially like
Speaker 2: you know risk you know, dislocating bones and so forth
Speaker 2: to do it. It's yeah, not for me, but I
Speaker 2: respect anyone who is capable of it totally. Now, the
Speaker 2: most common caves are karstick or limestone. I believe we've
Speaker 2: discussed this before, talking about various geological formations. Limestone, which
Speaker 2: covers roughly fifteen percent of the world above water, is
Speaker 2: highly soluble at least fifty percent calcium carbonate according to Romero,
Speaker 2: so acidic rain water easily modifies it over time, and
Speaker 2: so the world is littered with such cave systems, some
Speaker 2: only partially exposed to the surface world but in important ways.
Speaker 2: Others with key openings that permit light to influence the
Speaker 2: environments of their interior. So you know, some cave systems,
Speaker 2: some rather famous ones even do have sunlight spilling into
Speaker 2: them at least in part, so you end up with
Speaker 2: this kind of like mixed environment. But the lack of
Speaker 2: natural light as a whole is extremely important to understanding
Speaker 2: the individual organisms that make their homes and caves, as
Speaker 2: well as the overall ecosystems in which they thrive.
Speaker 3: Right because when you think about it, basically everywhere else
Speaker 3: on Earth. I mean, there are some extreme exceptions, but
Speaker 3: for most of the world, the food chain is built
Speaker 3: on access to sunlight. To the sunlight powers the powers
Speaker 3: the photosynthesis of the autotrophs that are the base of
Speaker 3: the food chain. Then things start eating them, then other
Speaker 3: things eat those organisms, and on and on up. If
Speaker 3: you don't have sunlight to power the autotrophs at the base,
Speaker 3: what do you do?
Speaker 2: I know, yeah, yeah, we'll get back to this in
Speaker 2: just a minute here, But yeah, humans, of course, we're
Speaker 2: not the first to venture into the subterranean worlds. Countless
Speaker 2: other creatures ventured in, resulting in rich a rich, specialized
Speaker 2: world of troglofauna, and not every creature becomes a permanent
Speaker 2: denizen of the dark, which, if you watch enough sci
Speaker 2: fi and horror movies, you would think, oh, yeah, they're
Speaker 2: just people and creatures fallen in all the time and
Speaker 2: evolving into chuds. But no, that doesn't happen all the time.
Speaker 2: But plenty of creatures have specialized evolution narily often evolving
Speaker 2: defining troglomorphic features that we often associate with these environments,
Speaker 2: namely blindness and loss of pigmentation with the chutification.
Speaker 3: If you will.
Speaker 2: Chutification, I guess yeah. You know, you see movies like
Speaker 2: The Descent and so forth. I love all these movies,
Speaker 2: you know, more Locks and so forth. But maybe we'll
Speaker 2: continue to discuss the possibilities there. But briefly, I want
Speaker 2: to just do a refresher on the various types of
Speaker 2: troglofauna before we continue. We have troglobites. These are obligate
Speaker 2: cave dwellers, strictly bound to the under their underground environments,
Speaker 2: so they are not going to live, they're not going
Speaker 2: to thrive outside of the darkness of the caves. Then
Speaker 2: we have troglasines. These are cave guests, creatures that can
Speaker 2: be found underground sporadically but cannot establish permanent underground populations.
Speaker 2: Then we have troglophiles. These are creatures that live dominantly
Speaker 2: above ground, but can also reside in underground habitats. There
Speaker 2: are two subsets. There's u trogleophiles, surface species that are
Speaker 2: also able to maintain in a permanent subterranean population, and
Speaker 2: then there are subtroglophiles creatures that can and do live
Speaker 2: underground at least temporarily, but are more associated with life
Speaker 2: above ground. So again you can think about the idea
Speaker 2: that there's not just surface creatures and underground creatures. You know,
Speaker 2: there's a lot of back and forth, and there are
Speaker 2: various phases between what we might think of as a
Speaker 2: purely surface creature and a purely subterranean creature.
Speaker 3: I see none of.
Speaker 2: This to be confused with a troglodyte. There are a
Speaker 2: handful of different uses is for this term. It can
Speaker 2: refer either to a human cave dweller or and I believe,
Speaker 2: in one case of an outdated classification of early hominid.
Speaker 2: But there are also a small genus of small birds
Speaker 2: related to rims that are called rolodides. These birds are
Speaker 2: rarely found in caves apparently, but they're so named because
Speaker 2: they'll venture into various crevices and spaces, including caves, in
Speaker 2: search of food.
Speaker 3: So we have these various kinds of spilunking organisms, those
Speaker 3: that do still live life on the surface, but have
Speaker 3: the ability and sometimes the desire to venture into caves
Speaker 3: or subterranean regions for various reasons. Maybe they want to
Speaker 3: shelter there for a certain period, maybe they want to
Speaker 3: go in there to retrieve some kind of resource, who knows.
Speaker 3: But then there's the other kind, the ones that are
Speaker 3: fully adapted to life in the caves and it has
Speaker 3: shaped their evolution, has shaped their morphology. Now their bodies
Speaker 3: are made for the caves. One of the examples that
Speaker 3: people listening are probably thinking of are sightless fish, fish
Speaker 3: that live in cave based in underground waterways and have
Speaker 3: evolved to completely lose their vision or their eyes. And
Speaker 3: this has long been a really captivating image within evolution
Speaker 3: that the idea, you know, people usually think of evolution
Speaker 3: as a constructive process that sort of powers up an organism.
Speaker 3: It gives it a new ability or a new adaptation
Speaker 3: that better suits it to an environment or an environmental
Speaker 3: niche that it's currently occupying, and so it gets upgraded
Speaker 3: in some way. These are these are ways that we
Speaker 3: sometimes discourage people from thinking about evolution. By the way,
Speaker 3: but it is still in the popular consciousness that that's
Speaker 3: how it works. This is evolution by what might be
Speaker 3: thought of as regression, like losing a trait that you
Speaker 3: previously had. So that raises a lot of questions about
Speaker 3: how evolution works and why something like that would happen.
Speaker 2: You know, it's it's fascinating to think about this about
Speaker 2: our our are often human focused bias and considering evolution
Speaker 2: along these lines. Like we think of that classic illustration
Speaker 2: of the ascent of man, you know, the various Homonid
Speaker 2: species evolving towards man, and it's easy to look at
Speaker 2: him and like think, wow, look, with each step he
Speaker 2: just gets hotter. With each step, he just looks more
Speaker 2: like us, he becomes more perfect, like clearly modern Homo
Speaker 2: sapiens are. But even in that, we're seeing the loss
Speaker 2: of various features. We're seeing like the loss of body
Speaker 2: hair and so forth.
Speaker 3: Yeah, totally, And as you know, big asterisk, if you
Speaker 3: see an old illustration like that, there's probably a lot
Speaker 3: of that's inaccurate about it. But some things that broadly
Speaker 3: would be true is that our ancestors do appear to
Speaker 3: have lost adaptations that made them really good tree climbers
Speaker 3: and those adaptations. Those are adaptations that were very useful
Speaker 3: for our ancestors, and we sort of lost them in
Speaker 3: favor of other things.
Speaker 2: Yeah. And it is also interesting again to think about
Speaker 2: the world of the dark, the world of the caves, because,
Speaker 2: like we said, we can have this experience of turning
Speaker 2: out the lights in the cavern and experiencing the true darkness.
Speaker 2: But for the most part, ever since human beings were
Speaker 2: able to capture fire, we have been able to bring
Speaker 2: light into these spaces. And today when we explore caves,
Speaker 2: at least on our own terms, we have that light
Speaker 2: that we bring with us, and we therefore we don't
Speaker 2: have to we change the environment by bringing light into
Speaker 2: it the cavern. Changing us over evolutionary time, of course,
Speaker 2: is not on the table, that's all right.
Speaker 3: But without the ability to modify the features of the
Speaker 3: environment like that other animals have had to adapt evolutionarily.
Speaker 2: Yeah. And in all this we have to again recognize
Speaker 2: just how vital light is for organisms for the evolution
Speaker 2: of life on our planet. Both sunlight and moonlight are
Speaker 2: in play here, as pointed out by Lauren Summer Rooney
Speaker 2: in the Kingdom of the Blind, published in a twenty
Speaker 2: eighteen edition of Integrated and Comparative Biology. Light is a
Speaker 2: fundamental biological cue for almost every animal on Earth, So
Speaker 2: it's not just about oh can I see the inside
Speaker 2: of the cave or not? You know, it plays in
Speaker 2: a navigation foraging, predator avoidance, mate selection, and just like
Speaker 2: general daily rhythms. So to abandon the world of light
Speaker 2: is no small thing, right, and yet we do see
Speaker 2: plenty of organisms that have done just that over time,
Speaker 2: not only cave organisms but certain creatures of the deep ocean.
Speaker 2: Though we have to remember that site is still important
Speaker 2: at varying levels of the water column and in the
Speaker 2: presence of phosphorescent glows, so there's a fair amount of
Speaker 2: complexity there. The soil and even interiors of host organisms
Speaker 2: for various parasites are also dark environments that species have
Speaker 2: adapted to.
Speaker 3: Mm hmm.
Speaker 2: Now, as Romero points out, Charles Darwin himself was somewhat
Speaker 2: stumped by the loss of sight and cavefishes, and apparently
Speaker 2: ultimately landed on more of a Lamarchian idea that they
Speaker 2: lost their eyes by simply not using them.
Speaker 3: Oh that's kind of interesting. So Lamarckism or Lamarchianism named
Speaker 3: after a figure named I believe you, Jean Baptist Lamark.
Speaker 3: It was a sort of alternative theory of inheritance to
Speaker 3: what would eventually become Mendalian genetics, which in some ways
Speaker 3: has been partially superseded now. But also but instead of
Speaker 3: the kind of Mendalian genetic inheritance, Lamark would say that,
Speaker 3: like the way that an animal uses its body shapes
Speaker 3: the way its offspring the way their bodies are formed.
Speaker 3: So like if you are reaching up a lot or something,
Speaker 3: you might grow your children might have longer arms, or
Speaker 3: like that a giraffe maybe is reaching its neck up
Speaker 3: to reach higher leaves, and that means it's children will
Speaker 3: be born with longer necks and so forth. For the
Speaker 3: most part, evolution doesn't actually work that way. There are
Speaker 3: some kind of things of gene expression and regulatory functions
Speaker 3: epigenetics that some people have argued in minor ways can
Speaker 3: kind of vindicate Lamarckian thinking, but for the most part,
Speaker 3: that's not really how inheritance works.
Speaker 2: Right right, Yeah, I've been to evolutionary talks, I believe
Speaker 2: the World Science Festival for some biologists have been like, yeah, yeah,
Speaker 2: we're kind of we to start talking about epigenetics, sometimes
Speaker 2: we do get a little Lamarckian in the way we're talking,
Speaker 2: so it is kind of fascinating.
Speaker 3: But the Lamarckian idea would be okay, so fish go
Speaker 3: down in a cave and then they don't use their
Speaker 3: eyes because they're living in the dark. Therefore their children
Speaker 3: or their offspring are born without them or born with
Speaker 3: eyes that regress. We can talk about that difference in
Speaker 3: a minute, and in a way you could say that
Speaker 3: that's loosely kind of right, but it's somewhat different. I
Speaker 3: think think I would say that the important thing is
Speaker 3: not that the fish don't use their eyes, it's that
Speaker 3: they don't need them, so that the eye no longer
Speaker 3: confers a survival or reproductive advantage.
Speaker 2: I think the one of the infectious things about it,
Speaker 2: at least for non scientists, is that it can feel
Speaker 2: right in certain ways. Right. It can sort of match
Speaker 2: up with at least the individual human experience. So we
Speaker 2: might not think, like, man, I better go to the
Speaker 2: gym and need to pump iron so that my offspring
Speaker 2: will will have strong arms. But we might think, oh, well,
Speaker 2: I've got to keep pumping iron otherwise my arms will
Speaker 2: get smaller. You know, I've got to do X so
Speaker 2: that why doesn't happen? And you know, I think we
Speaker 2: do see this infectious idea in some of our fictions
Speaker 2: of subterranean monsters, including the Descent of the Hobbit Smeekeel
Speaker 2: and the Lord of the Rings, who becomes a trogloditic
Speaker 2: Gollum in the course of a single though of course
Speaker 2: magically sustained lifetime. He doesn't really become blind or anything,
Speaker 2: but generally he's depicted as you know, he's changed a lot.
Speaker 2: He's lost his hair, his pigmentation has changed, his eyes
Speaker 2: have gotten larger, I believe.
Speaker 3: Yeah, though I think the morphotypical changes of Gollum could
Speaker 3: be largely due to the corruption of the one Ring.
Speaker 2: That's the thing. The one ring changes everything. You've got
Speaker 2: a huge magical factor to consider in that scenario. But
Speaker 2: sort of just the idea of it is like, yeah,
Speaker 2: people go into caves, they get weird, they turn into monsters,
Speaker 2: or if they're not a monster, you know, weight three generations,
Speaker 2: then just pure monsters.
Speaker 3: Caves will do that.
Speaker 2: Now, Summer Rooney points out that we're still working out
Speaker 2: all the details, but we know much more than Darwin
Speaker 2: did in his time. Of course, the science has come
Speaker 2: quite a way since then in our understanding of how
Speaker 2: creatures evolve and what sort of changes are taking place
Speaker 2: over revolutionary time. The author rights quote eye loss appears
Speaker 2: to be driven at least in part by direct selective pressure,
Speaker 2: but both pliotrophy and genetic are also key influencers. Despite
Speaker 2: substantial recent progress thanks to a combination of developmental and
Speaker 2: molecular techniques, it is not yet clear how these drivers interact,
Speaker 2: or crucially, whether their relationships are similar across taxa and
Speaker 2: habitat types.
Speaker 3: Yeah, so this was really interesting, and I ended up
Speaker 3: going kind of deep on what we do know about
Speaker 3: eye loss and a related issue of pigmentation loss in
Speaker 3: cave fish. But I wanted to break down some of
Speaker 3: the terms that this author uses in that paragraph you
Speaker 3: just read. So selective pressure, of course would mean an
Speaker 3: advantage in terms of survival or reproduction, a direct advantage.
Speaker 3: That's the kind of evolution we think about most often.
Speaker 3: But there are two other factors that could be influencing
Speaker 3: the loss of eyes or the loss of pigmentation in
Speaker 3: fish that the author mentions, and those are plyotrophy and
Speaker 3: genetic drift. Plyotropy, to sort of oversimplify, means when one
Speaker 3: gene determines multiple outcomes in the phenotype, and the phenotype
Speaker 3: is like the body or the behavior of the organism.
Speaker 3: It's what the genes produce in the animal. So, for example,
Speaker 3: imagine there is a mutation at one locust, and that
Speaker 3: one mutation happens to both give you smaller eyes but
Speaker 3: also longer arms. Or maybe there's a mutation that happened
Speaker 3: to give you both higher rates of cancer and higher
Speaker 3: average fertility. And pliotropies like these can They're very interesting
Speaker 3: because they can sometimes explain the evolution of traits that
Speaker 3: would seem to be disadvantageous because the seemingly bad trait
Speaker 3: is actually part of a hidden package deal, a genetic
Speaker 3: package deal with an even more advantageous trait balancing it out.
Speaker 3: You can get either both or neither, but not one
Speaker 3: by itself. So in the case of cavefish, it's possible
Speaker 3: there is a single mutation that both causes the loss
Speaker 3: of eyes but also does something else that helps the
Speaker 3: fish survive and reproduce. And since there's little to no
Speaker 3: penalty attending the loss of vision in the pitch black
Speaker 3: cave environment, the helpful half of that pliotropy wins out
Speaker 3: and the two for one version of the gene is
Speaker 3: selected for. And then the final factor mentioned here is
Speaker 3: genetic drift. Genetic drift is change in the frequency of
Speaker 3: gene variants within a population due to random chance. So
Speaker 3: genes becoming more or less prevalent within a group of organisms,
Speaker 3: not for selective reasons, not because they help or hurt
Speaker 3: the organism, but just randomly, you know. And this happens
Speaker 3: all the time. Gene variants just become more common less
Speaker 3: common within a population, and it happens due to random factors.
Speaker 3: Though sometimes if a population is small enough. An interesting
Speaker 3: thing is that traits result from random genetic drift can
Speaker 3: become what's called fixed, meaning that you kind of hit
Speaker 3: a genetic tipping point and a trait that was randomly
Speaker 3: fluctuating up and down in the population it becomes prevalent
Speaker 3: enough that suddenly it's fixed, and then all the individuals
Speaker 3: within the population have the gene for that trait. Going forward.
Speaker 2: Yeah, now, now, going back to what you mentioned about
Speaker 2: the idea of there possibly being like an advantage and
Speaker 2: a disadvantage tied up in one change, but The thing is,
Speaker 2: you find yourself, if you're this fish undergoing this change
Speaker 2: or some other organism, you find yourself in a lightless environment.
Speaker 2: And so in this you dip over into the neutral
Speaker 2: mutation hypothesis, This idea that while naturally occurring mutations for
Speaker 2: in this case blindness would otherwise spell doom for a
Speaker 2: fish in a typical environment, in a lighted environment, it
Speaker 2: doesn't matter in the darkness, and therefore these mutations can
Speaker 2: flirt along with whatever other changes they might be bringing.
Speaker 2: That's right.
Speaker 3: Then they can flourish for multiple reasons. They could flourish
Speaker 3: because they're part of a plotropy that has an advantage
Speaker 3: on the other half, or they can just flourish randomly
Speaker 3: through genetic drift because there's no penalty to the loss
Speaker 3: of sight in the in the darkness.
Speaker 2: Yeah, this I can't help be reminded of various like
Speaker 2: gaming rules and gaming systems and all of this. Like
Speaker 2: it's it's like, imagine that you're playing Dungeons and Dragons
Speaker 2: and let's say you're venturing into the under dark. You're
Speaker 2: venturing into a lightless environment, and you have a weapon
Speaker 2: that gives you, say a plus two on attack. But
Speaker 2: if you're in the sunlight, it has your hit points
Speaker 2: or something like something that would make this all right.
Speaker 2: It has advantages to it, but the disadvantage is far
Speaker 2: outweigh it in a different environment, but you're not in
Speaker 2: that environment. Now you're in an environment where the disadvantage
Speaker 2: does not matter.
Speaker 3: Right. Yes, that's analogy, and it's also a good reason
Speaker 3: to remember to check your build when your environment changes.
Speaker 3: But yeah, so there is, as we're saying, some possibility
Speaker 3: for all of these options in explaining the evolution of
Speaker 3: cavefish blindness. There could be direct selective pressure, there could
Speaker 3: be indirect selective pressure through pliotropy or the package deal,
Speaker 3: and there could be random genetic drift. However, it seems
Speaker 3: that there is some pretty good evidence for the presence
Speaker 3: of a direct selective pressure for fish to lose their
Speaker 3: eyes when they live in a lightless environment, and there
Speaker 3: might be some evidence for plotropy as well. So to
Speaker 3: get a more concrete idea of what we're talking about,
Speaker 3: I wanted to focus on a particular species of fish
Speaker 3: that has been studied a lot, and that species is
Speaker 3: called Astianax mexicanus, also known as the blind Mexican cave fish,
Speaker 3: also known as the blind cave tetra. These are small
Speaker 3: like fish with silver scales. I saw different size estimates
Speaker 3: for them. Some said they grow to a maximum length
Speaker 3: of about nine centimeters. Other sources said about twelve centimeters,
Speaker 3: but they're relatively small in any case. One thing that
Speaker 3: makes these Astianax fish very interesting is that there exist
Speaker 3: both surface or epigean and cave or hypogean variants of
Speaker 3: the same fish species, and they have different body features.
Speaker 3: And scientists can learn a lot by comparing these extremely
Speaker 3: closely related populations of animals to one another, cross checking
Speaker 3: genomic information back and forth, and seeing what that correlates
Speaker 3: to in terms of their phenotypic traits. So the cave
Speaker 3: variant of Astianax lives in this big network of underground
Speaker 3: waterways in the mountainous areas of northeastern Mexico, and they've
Speaker 3: been in these caves since. One source I was reading
Speaker 3: said they colonize them probably around the end of the
Speaker 3: Pleistocene epic, so you know, roughly twelve thousand years ago
Speaker 3: or so. There are photos you can find online which
Speaker 3: I'd recommend looking up, which not only show the cave
Speaker 3: variant of Astianax, but They depict the cave variant and
Speaker 3: the surface variant side by side to highlight the differences
Speaker 3: in their bodies. So the cave version is much more
Speaker 3: pale and translucent. And indeed, in the places where the
Speaker 3: surface variant has a big, old, bulging fish eye on
Speaker 3: either side of its skull, the cave variant has only
Speaker 3: a strange, fatty looking lump underneath its scales, just in
Speaker 3: front of the red flap of the gill.
Speaker 2: Yeah, these are haunting images to look at. Again, we
Speaker 2: can't help but think of like various sneakels and so
Speaker 2: forth when looking at it.
Speaker 3: Yeah, exactly, And I bet these fish are good at
Speaker 3: riddle games. Now. One thing that I was reading this
Speaker 3: kind of interesting about these the eyeless variant of the
Speaker 3: Asti annex fish, is that they undergo what's called eye regression.
Speaker 3: So I don't know if this is true of all populations,
Speaker 3: because they're like different cave populations of this fish that
Speaker 3: have been studied, and they have some differences between them.
Speaker 3: But I was reading at least in some populations what
Speaker 3: happens is not that these fish never have eyes, but
Speaker 3: they begin to grow eyes during embryonic development and then
Speaker 3: as they develop. They they acquire eyes and then lose
Speaker 3: the eyes in development, and the eye sockets are paved
Speaker 3: over by skin and scales, and they there remain these
Speaker 3: little fatty lumps where the sockets used to be, and
Speaker 3: the bone kind of collapses around it. So I thought
Speaker 3: this was kind of interesting. It's not just that they
Speaker 3: never have eyes. They get eyes and then they lose them,
Speaker 3: all in the process of a single animal growing up.
Speaker 2: Fascinating, you know. I also have to throw in, okay,
Speaker 2: ASTI annex part of the scientice name for the creature.
Speaker 2: This is the son of Hector in the Iliad, and
Speaker 2: I'm guessing here. I couldn't find anything just definitive offhand,
Speaker 2: but I'm assuming the naming here is because Astianax, the
Speaker 2: child of Hector, is hidden in the tombs. That is
Speaker 2: the way that they try to ensure his survival. And
Speaker 2: so you know, we see some version of that here,
Speaker 2: a survival in the underworld of the caves as opposed
Speaker 2: to the tomb.
Speaker 3: I did not make that connection. That is interesting. So
Speaker 3: I mentioned that there was some evidence that actual selection pressure,
Speaker 3: a direct selection pressure, drove the evolution of eyelessness in
Speaker 3: these fish, as well as some possible evidence for pliotropy.
Speaker 3: Let's look at a paper to learn more.
Speaker 1: So.
Speaker 3: The paper I was reading is called Cavefish and the
Speaker 3: Basis for Eye Loss by Jaya Krishnan and Nicholas Rohner,
Speaker 3: published in Philosophical Transactions of the Royal Society b Biological
Speaker 3: Sciences twenty seventeen, and in the section of their paper
Speaker 3: discussing the reasons for eye loss, the authors note that
Speaker 3: it is not unique to the example of these blind
Speaker 3: Mexican cavefish for an animal to evolve by losing a
Speaker 3: trait or characteristic that was once positively selected for in
Speaker 3: that animal's ancestors. Examples include the loss of tails in
Speaker 3: some primates, including us, the loss of teeth in birds,
Speaker 3: and the loss of legs in Wales. All of these
Speaker 3: are examples where an animal lineage acquired an adaptation that
Speaker 3: was useful long ago, and then some branch of that
Speaker 3: lineage entered a new ecological niche in which that trait
Speaker 3: was no longer useful or was actively harmful, and evolved
Speaker 3: once again to lose the trait that its ancestor bodies
Speaker 3: had constructed so long ago. And the example of cavefish
Speaker 3: that lost their eyes was long simply assumed to be
Speaker 3: a result of genetic drift. Eyelessness appears in the gene pool,
Speaker 3: and then with no selection pressure favoring eyes to keep
Speaker 3: the eyelessness suppressed, it would just randomly fluctuate and frequency
Speaker 3: in the gene pool until at some point it became fixed. However,
Speaker 3: the authors say that in roughly the last decade before
Speaker 3: this paper, and this was again twenty seventeen, there had
Speaker 3: been more attention to the possibility of a positive selection
Speaker 3: pressure favoring the loss of eyes. There's a reason to
Speaker 3: lose the eyes, either directly or through pliotropy, So let's
Speaker 3: talk direct selection first. The main explanation offered for direct
Speaker 3: selection against eyes is energy conservation, also sometimes expressed as
Speaker 3: metabolic cost. It costs energy the energy that animals get
Speaker 3: from food food to have functioning organs, and some types
Speaker 3: of organs are more energy hungry than others. In fact,
Speaker 3: the authors demonstrate that neural tissue, which includes brain cells
Speaker 3: and also eyes and nerves, that type of tissue is
Speaker 3: one of the most expensive types that an animal can
Speaker 3: grow and maintain. So any animal's evolution is guided by
Speaker 3: a balancing of interests. The benefits provided by sensory awareness
Speaker 3: and the high metabolic cost of having this neural tissue
Speaker 3: of having these systems. This is true in any animal
Speaker 3: in any environment, but the balance is especially crucial in
Speaker 3: harsh environments like caves, where food energy is more scarce
Speaker 3: than on the surface. It's harder to come by food
Speaker 3: energy in a cave than it is in the surface
Speaker 3: waters where the surface variant of the Mexican cavefish lives.
Speaker 3: And in favor of this explanation the energy cost explanation
Speaker 3: the author's side of paper from twenty fifteen by Moran
Speaker 3: and Softly and Warrant published in Science Advances called the
Speaker 3: Energetic Cost of Vision and Evolution of Eyeless Mexican Cavefish,
Speaker 3: summarizing the findings of Meranatol, Krishnan and Rohner write quote,
Speaker 3: this study calculated significant metabolic costs for the optic tectum,
Speaker 3: and the tectum is the part of the fish's brain
Speaker 3: that is used to process visual information going on with
Speaker 3: the quote and the eyes adding up to fifteen percent
Speaker 3: of the resting metabolism needs in juvenile fish. Such a
Speaker 3: high cost of vision almost reaching the cost of the
Speaker 3: human brain at twenty to twenty five percent makes an
Speaker 3: adaptive loss of eyes owing to energy constraints highly probable.
Speaker 3: So usually there's a high energy cost you need to
Speaker 3: eat a lot of food to grow and sustain the
Speaker 3: sensory organs and visual processing in the brain tissue. But
Speaker 3: on the surface, evolution is usually willing to pay that
Speaker 3: cost because it's really beneficial to get visual information about
Speaker 3: your surroundings that really helps you survive and reproduce. If
Speaker 3: getting visual information about your surroundings becomes impossible because you
Speaker 3: live in total darkness, it's time to make budget cuts.
Speaker 3: Another possible direct selection factor I haven't come across direct
Speaker 3: evidence for this, but it's just mentioned as a possibility
Speaker 3: by several sources, and that is that eyes are also
Speaker 3: a liability in that they are prone to injury and
Speaker 3: an entry way for infection. So it's kind of like
Speaker 3: why would you put a windshield on your armored vehicle
Speaker 3: if you're driving in pitch black with no headlights. It
Speaker 3: would just be a functionless weak point in the armor
Speaker 3: of the vehicle that provides no benefit. And it's possible
Speaker 3: that eyes are like this also. Once there's nothing to see,
Speaker 3: all they would be is a soft spot to let
Speaker 3: germs and sharp objects in.
Speaker 2: Yeah, kind of like with sci fi visions of spaceships, right,
Speaker 2: like why have an actual observation lounge or observation bubble
Speaker 2: or so forth, Why have actual glass there or any
Speaker 2: kind of like transparent wall when you can if you
Speaker 2: have like ubiquitous abilities to have video footage and screens
Speaker 2: around you. Basically the star.
Speaker 3: Trek principle exactly, if it's not actually providing you a benefit,
Speaker 3: then it's just a liability.
Speaker 2: Yeah.
Speaker 3: Also, the authors of this paper mention some evidence for
Speaker 3: indirect selection or pliotropy. Specifically, they bring up a signaling
Speaker 3: pathway identified in the scientific literature as SHH. And do
Speaker 3: you want to guess what SHH stands for?
Speaker 2: Now?
Speaker 3: Tell me what stands for sonic hedgehog. That's really what
Speaker 3: they say, and that's what it is, really, Yes, scientific papers,
Speaker 3: this is the sonic hedgehog signaling pathway. There's also just
Speaker 3: an HH or hedgehog pathway, so I don't know attacking
Speaker 3: on maybe that was discovered first, and then they tacked
Speaker 3: on sonic to be funny for this other pathway. I
Speaker 3: don't know for sure, but it is called the sonic
Speaker 3: hedgehog signaling pathway and the signaling pathway, by the way,
Speaker 3: is it's like a series of chemical reactions in the
Speaker 3: body that controls and coordinates the activity of cells in
Speaker 3: order to do things like grow new tissue during development,
Speaker 3: or cause an immune system response or something like that.
Speaker 3: The sonic hedgehog pathway in particular seems important during embryonic development.
Speaker 3: It sort of guides growth and plays a role in
Speaker 3: the patterning of tissues that emerge from the growth process.
Speaker 3: So Krishnan and Rohner point to a study by Yamamoto
Speaker 3: at all from twenty nine, not twenty nineteen, from two
Speaker 3: thousand and nine, sorry, in Developmental biology, which found that
Speaker 3: by altering expression of the sonic hedgehog signaling pathway in
Speaker 3: embryos of the surface dwelling relative of the blind Mexican cavefish,
Speaker 3: you would actually get multiple changes together. This is our
Speaker 3: package deal. You would get regression of the eyes like
Speaker 3: we talked about, so the eyes would sort of like
Speaker 3: be absorbed and you would no longer have functioning eyes
Speaker 3: as an adult fish. But you would also get enhancement
Speaker 3: of taste, buds and jaws. So that's your package deal.
Speaker 3: And in the dark you can imagine why that would
Speaker 3: be a good package deal. The author's write quote SAHH
Speaker 3: has wide range effects on the development of various organs,
Speaker 3: and it has been identified as a strong candidate for
Speaker 3: eye regression. Its elevated expression in the oral pharyngial region
Speaker 3: in cavefish taste buds, and the fact that eye size
Speaker 3: and number of taste buds are correlated in hybrids makes
Speaker 3: SAHH a promising candidate for functional studies. So that's another
Speaker 3: factor possibly at work here. So to review, you got
Speaker 3: eyes and visual tissue in the brain require a lot
Speaker 3: of food to sustain, and food is especially scarce in
Speaker 3: the cave. Also, the eyes are not really useful in
Speaker 3: the cave, so you know, the trade off there seems
Speaker 3: pretty obvious. There's less evidence for this, but it has
Speaker 3: been suggested that eyes are also a liability in terms
Speaker 3: of infection and injury, and it seems there are certain
Speaker 3: genetic and developmental changes that offer a package deal where
Speaker 3: you get eye regression, your eyes are absorbed, you have
Speaker 3: no eyes as an adult fish, but more taste buds
Speaker 3: and in the dark. That's a good deal to take
Speaker 3: because remember taste buds. We think of them mainly in
Speaker 3: terms of like providing pleasure, but taste buds give you
Speaker 3: important survival information. They are your your body's chemistry set,
Speaker 3: along with your ural factory, you know, smell and taste
Speaker 3: together help you sort of test incoming materials for safety
Speaker 3: and nutritional value. And that is actually a very valuable
Speaker 3: thing in the wild, especially if you can't look at
Speaker 3: stuff you're going to eat.
Speaker 2: Yeah. Yeah, we have to be reminded of that because
Speaker 2: we are such site dependent organists that we easily think
Speaker 2: of like the world we can see and then the
Speaker 2: other senses that provides sort of backup information, right, or
Speaker 2: at least it's easy to think about that in terms
Speaker 2: of a lot of stimuli. Obviously it's different once you
Speaker 2: get into, say, you know, the actual experiences of consuming food.
Speaker 2: But even that, the visual factor is a huge part
Speaker 2: of it totally. When you pay for a nice meal,
Speaker 2: when you pay for a nice cocktail or zero proof cocktail,
Speaker 2: whatever floats your boat, you know, I mean presentation, visual
Speaker 2: presentation is a huge part of what you're signing up for.
Speaker 3: It absolutely is. Even I would say this, even if
Speaker 3: you think you quote know better, do you know what
Speaker 3: I mean about that rot like that? Some people can
Speaker 3: think like, oh, you know, I'm just in it for
Speaker 3: the tasty food. I don't really care if it looks good.
Speaker 3: But actually you do care in subconscious ways that you're
Speaker 3: not admitting to yourself.
Speaker 2: Yeah, on some level, you were still anticipating the flavor,
Speaker 2: anticipating the taste experience based on to a large degree,
Speaker 2: on visual data.
Speaker 3: Totally. So one more thing I want to say about
Speaker 3: these the blind Mexican cavefish, because I was looking into
Speaker 3: this cavefish species and it seems that eye loss or
Speaker 3: I regression is strongly associated with a parallel morphotype, which
Speaker 3: is pigmentation loss. So a source I was using on
Speaker 3: pigmentation loss in cavefish is a chapter in a book.
Speaker 3: The chapter is called Evolutionary Genetics of Pigmentation Loss in
Speaker 3: Blind Mexican Cavefish by Joshua B. Gross and Clifford J. Tabin.
Speaker 3: It is from the book In Search of the Causes
Speaker 3: of Evolution, From Field Observations to Mechanisms, Princeton University Press,
Speaker 3: twenty eleven, edited by Peter and Rosemary Grant. So most
Speaker 3: of this chapter is focused on the specific genetic mutations
Speaker 3: and regulatory changes that result in the reduction in or
Speaker 3: total loss of pigment in blind Mexican cavefish, the reduction
Speaker 3: in the melanophores and color color molecules in the fish's body,
Speaker 3: and they find that in different populations they're actually you
Speaker 3: get these similar, similar adaptations of the loss of pigment
Speaker 3: in the body, but they have different underlying genetic or
Speaker 3: regulatory mechanisms, so it's evolved in different ways to get
Speaker 3: sort of the same result in these different cave fish populations.
Speaker 3: But also this chapter gets a little bit into what
Speaker 3: the environmental selection pressure is related to, or the lack
Speaker 3: of selection pressures related to pigmentation might be. So, for example,
Speaker 3: on the surface, biological pigments like melanin help protect the
Speaker 3: cells of animals from ultraviolet radiation in the sunlight. In
Speaker 3: the cave, there's no light, so this radiation shielding effect
Speaker 3: is useless on the surface. Pigments are also arranged in
Speaker 3: patterns on the body as ornamentation, which plays a role
Speaker 3: in sexual selection. So a fish with certain types of
Speaker 3: pigmentation patterns might be seen as a more desirable mate,
Speaker 3: but in the cave, your mate can't see you, so
Speaker 3: it doesn't matter. On the surface, pigmentation can also play
Speaker 3: a role in camouflage concealing your body within the environment,
Speaker 3: but in the cave. Once again, this doesn't make any difference.
Speaker 3: So much like with eyes, pigmentation would be a phenotypic
Speaker 3: trait that confers huge advantages on the surface where there's light,
Speaker 3: and it suddenly confers little to no advantage underground. So
Speaker 3: even if it's just subject to genetic drift, pigmentation, like eyes,
Speaker 3: could fluctuate out of the population. But are there any
Speaker 3: reasons to think it is actively selected against? It seems
Speaker 3: like there is less evidence for this than there is
Speaker 3: in the case of eyes. There's probably good reason to
Speaker 3: think that eyes are being selected against in the cave,
Speaker 3: less so for pigmentation. The authors here cite research by
Speaker 3: protests at all from two thousand and seven than indicating
Speaker 3: it's more likely a result of drift, though some amount
Speaker 3: of indirect selection through pleiotropy is possible.
Speaker 2: Okay, all right, we have more complex understanding, perhaps than
Speaker 2: of what might be going on here with these changes.
Speaker 2: All Right, We're going to go ahead and close out
Speaker 2: this episode of Stuff to Blow your mind, but we'll
Speaker 2: be back with part two in this series on Thursday,
Speaker 2: and don't worry, we will get into the guano. We
Speaker 2: will get into that back guano because it is vitally
Speaker 2: important to this discussion. And also I think we're going
Speaker 2: to discuss guano in a way that may turn the
Speaker 2: concept on its head for you. I know when I
Speaker 2: was reading about it in the way that we're going
Speaker 2: to discuss it, it made me think about it in
Speaker 2: a new light.
Speaker 3: I can't wait.
Speaker 2: In the meantime, we'd love to hear from you if
Speaker 2: you have thoughts about particular caves that you've been to,
Speaker 2: cave environments that you're familiar with, If you have thoughts
Speaker 2: on scientists naming things after everything from the Iliad to
Speaker 2: Sonic the Hedge. We'd also love to hear from you
Speaker 2: on that point as well. But yeah, yeah, particular cave
Speaker 2: environments that you've really enjoyed. Like I say, I always
Speaker 2: enjoy checking out caves. I believe Colossal Cave in Tucson,
Speaker 2: Arizona is one that I visited many years back, and
Speaker 2: I really want to go back there in the future
Speaker 2: because this is one if memory serves was discovered late
Speaker 2: enough that they were able to preserve the cave environment
Speaker 2: to a large degree by use of essentially like an
Speaker 2: air lock system. So that they're not just opening up
Speaker 2: this environment in a way that destabilizes what has evolved
Speaker 2: beneath the surface. But anyway, more on this sort of
Speaker 2: thing in the next episode. In the meantime, we'll remind
Speaker 2: you once more that Stuff to Blow your Mind is
Speaker 2: primarily a science podcast Science and culture podcasts, with core
Speaker 2: episodes on Tuesdays and Thursdays. Mondays we do listener mail.
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