Speaker 1: Welcome to Stuff to Blow Your Mind production of iHeartRadio.
Speaker 2: Hey A, you welcome to Stuff to Blow your Mind.
Speaker 2: My name is Robert Lamb.
Speaker 3: And I am Joe McCormick. And we're back with Part
Speaker 3: two in our series on the nineteenth century British naturalist
Speaker 3: Alfred Russell Wallace and on the Wallace Line, the faunal
Speaker 3: boundary in the Malay Archipelago that bears his name. If
Speaker 3: you haven't heard part one yet, I would recommend you
Speaker 3: go back and listen to that one first. But in
Speaker 3: part one we started off with a general character sketch
Speaker 3: of Wallace. He's a man of many adventurers and many opinions,
Speaker 3: best known today for being the other guy who discovered
Speaker 3: evolution by natural selection. He came up with a slightly
Speaker 3: different version of the theory of natural selection around the
Speaker 3: same time Darwin did, just some sort of differences of emphasis. Basically,
Speaker 3: Zarwin's writings ultimately proved more influential in convincing his peers
Speaker 3: on the reality of common descent and on articulating the
Speaker 3: mechanisms by which species evolved. Wallace was also something of
Speaker 3: a celebrity at his time. It's not like one of
Speaker 3: those tragic cases of somebody, you know, somebody else who
Speaker 3: also came across a great idea but was just like
Speaker 3: totally forgotten. Wallace was kind of a celebrity, especially because
Speaker 3: of the quality of his writing, and that would be
Speaker 3: in books like The Malay Archipelago, published in eighteen sixty nine,
Speaker 3: in which he vividly described his earlier travels in that
Speaker 3: region of the world, including lots of very well observed
Speaker 3: biological and cultural detail. And we read a bunch of
Speaker 3: selections from that book in the last episode, and they
Speaker 3: are kind of it's somewhat magical to read.
Speaker 2: Yeah. Absolutely, again, you can see why this book was
Speaker 2: such a success, and it was read by other naturalists,
Speaker 2: but also just members of the general public who are
Speaker 2: interested in the topic, interested in far away lands and
Speaker 2: in the net.
Speaker 3: Yeah. So we read some selections from the chapters where
Speaker 3: he goes exploring, initially against his will, by the way,
Speaker 3: on the islands of Bali and Lombach. And this included
Speaker 3: everything from these evocative descriptions of the land itself, talking
Speaker 3: about the terrorist agriculture and the way the fields are
Speaker 3: irrigated and all that, two discussions about the geographic distribution
Speaker 3: of ghosts to complaints about how hard it is to
Speaker 3: do science when everything smells putrid and discovered in ants.
Speaker 2: Right now, To be clear, it was putrid smelling because
Speaker 2: of all the birds skinning gsty This is not a
Speaker 2: general comment on the region or the people's there. This
Speaker 2: was his nest.
Speaker 3: No, no, it was not like Lombox smells bad. It
Speaker 3: was my room smells bad. Yea yeah. Also we talked
Speaker 3: about the concept of biogeography, the study of what lives,
Speaker 3: where and why. Wallace is considered a very important founding
Speaker 3: figure in biogeography, with one of his most enduring observations
Speaker 3: being what is now called in his honor, the Wallas Line,
Speaker 3: an invisible boundary passing in the ocean between the islands
Speaker 3: of originally Borneo and Sulawesi, and even more astonishingly in
Speaker 3: the tiny narrow strait between Bali and Lombach, which marks
Speaker 3: the westernmost boundary of a lot of characteristic Australasian fauna marsupials,
Speaker 3: cockatoos and things like that. So we are back today
Speaker 3: to talk more about Wallace and the Wallas Line. Now,
Speaker 3: one thing I think we've got to do before we
Speaker 3: move on any further is just a bit of clean up, because,
Speaker 3: as I mentioned in the last episode, our model of
Speaker 3: the Wallas line has undergone some major revisions since Wallace
Speaker 3: first proposed it in the mid nineteenth century. I'm not
Speaker 3: going to do a whole detailed play by play of
Speaker 3: the shifting history of the line, but for a brief
Speaker 3: summary of developments, I was just looking at the abstract
Speaker 3: of a paper called Wallace's Line WALLACEI and Associated Divides
Speaker 3: and Areas History of a tortuous tangle of ideas and labels.
Speaker 3: This is by Ali and Heene, published in the journal
Speaker 3: Biological Reviews in twenty twenty one. So the authors go
Speaker 3: through the concept which we discussed in the last episode,
Speaker 3: the idea of a faunnel boundary being basically a place
Speaker 3: where you find animals mainly of one sort on one
Speaker 3: side of the line and animals mainly of a different
Speaker 3: sort on the other side of the line. And this
Speaker 3: raises questions how did it end up like that? The
Speaker 3: authors write that the earliest dividing lines in this region
Speaker 3: were considered quite sharp, especially the line Wallace drew in
Speaker 3: eighteen sixty three, and this was based on looking at
Speaker 3: the distribution of land mammals and some birds. This is
Speaker 3: the line we've been talking about so far, passing between
Speaker 3: Bali and Lombach, with Asian animals found west from the
Speaker 3: west coast or from the eastern coast of Bali and
Speaker 3: Australasian animals running east from Lombak. This particular theoretical boundary
Speaker 3: proved very influential and it got you know, reproduced in
Speaker 3: a lot of texts. It was baked into maps and
Speaker 3: plates and stuff all over so like it had a
Speaker 3: big cultural footprint, and within the discipline of biogeography it
Speaker 3: came to be seen as something of a fixed marker
Speaker 3: in nature, maybe like the tree line on a mountain.
Speaker 3: But over time it also became obvious to experts in
Speaker 3: the field that there were a lot of exceptions to
Speaker 3: the Wallace line, especially a lot of Asian fauna found
Speaker 3: east of the so called line. And this would not
Speaker 3: have been very surprising to Wallace himself, I think, who
Speaker 3: argued that biogeographic boundaries were all to some extent permeable,
Speaker 3: But it might have been more surprising to people who
Speaker 3: got a you know, a map printed with a line
Speaker 3: on it that says animals do not cross this line. Yeah.
Speaker 3: By later in the nineteenth century, many biogeographers had started
Speaker 3: to think about zones instead of life. For example, there
Speaker 3: is now a biogeographic region known as Wallace Sea. I've
Speaker 3: also heard it pronounced to Wallasia, so Wallasia or Wallacea
Speaker 3: named after again Alfred Russell Wallace. This region was sort
Speaker 3: of proposed in the nineteen twenties, which is a faunal
Speaker 3: transition zone where you essentially have Australasian fauna to the
Speaker 3: east of the zone, Asian fauna to the west of
Speaker 3: the zone, and more of a mix within. Though the
Speaker 3: western boundary of Wallacea is still basically Wallace's line, passing
Speaker 3: between Bali and Lombach and in between Borneo and Sulawesi.
Speaker 3: So we're actually here creeping a bit back toward that
Speaker 3: gradual transition idea of animal ranges that we talked about
Speaker 3: in part one. Though some of the divisions you see,
Speaker 3: especially between Bali and Lombach, are still actually quite striking,
Speaker 3: So Rabbi included for you to look at in our outline.
Speaker 3: Here an illustration from one of the papers I was
Speaker 3: looking at, the region highlighted in red, and all the
Speaker 3: islands within that is what is generally considered Wallacea today.
Speaker 2: Yeah, and on this map here it looks like a
Speaker 2: big old heart, like a big old Valentine. They even
Speaker 2: colored it.
Speaker 3: Red Happy Valentine's Day. You get some marsupials.
Speaker 2: Yeah, and I was in the eastern part of this
Speaker 2: very region.
Speaker 3: Okay. So Ali and Heeney write that in the last
Speaker 3: decade before their paper, even more new regions and boundary
Speaker 3: modifications have been proposed, and the authors paint a truly
Speaker 3: headache inducing picture of the historical understanding of all this.
Speaker 3: As they say in their title, it is a tortuous
Speaker 3: tangle of ideas and labels. For example, I'm just going
Speaker 3: to read one section of their abstract to give you
Speaker 3: an idea quote. Wallace's eighteen sixty three line is not
Speaker 3: the one he finally settled upon in nineteen ten. Its
Speaker 3: path around Sulawesi was transferred from the west to the
Speaker 3: east of the island, ideally Huxley's divide, And that's referring
Speaker 3: to Thomas Henry Huxley, who proposed a modification to this
Speaker 3: to the where the line goes. Hucks divide eighteen sixty
Speaker 3: eight should carry his name rather than Wallace's. The latter
Speaker 3: never accepted the proposition. Lydacer's line of eighteen ninety six
Speaker 3: ought to be labeled the hil Prin Lydacer line in
Speaker 3: recognition of Angelo Hilprin's eighteen eighty seven contribution concerning transition zones. Ideally,
Speaker 3: Wallacea should correspond to its original nineteen twenty four description,
Speaker 3: which incorporated the Philippine Islands bar the Palawan group. Notably,
Speaker 3: though a smaller form introduced by Darlington in nineteen fifty
Speaker 3: seven used frequently from nineteen ninety eight onwards, in which
Speaker 3: all of the Philippine Islands are excluded, is entrenched within
Speaker 3: the recent literature, but this is often without evident justification.
Speaker 3: It should also be recognized that the reduced meaning southern
Speaker 3: Wallacea area was effectively defined by hilprint in eighteen eighty seven,
Speaker 3: but was then labeled the Austro Malaysian transition zone. So
Speaker 3: it's a mess. This is one reason why I've really
Speaker 3: enjoyed researching for the episodes. But like when I was
Speaker 3: reading all this stuff for the last episode, I was
Speaker 3: just like, ah, I can't.
Speaker 2: Yeah, I mean it's again, It's it's not like if
Speaker 2: you take a kangaroo and have it walk over the
Speaker 2: Wallas line, it will explode. Like, so, there's no it
Speaker 2: becomes very difficult to actually test out some of these things.
Speaker 2: It's based on a number of observations and factors.
Speaker 3: Well, these modifications absolutely are based on empirical observations. I mean,
Speaker 3: it's not just people kind of jousting around about absolutely nothing.
Speaker 3: Like there are reasons based on the different kinds of
Speaker 3: fauna that have been observed and the underlying theory about
Speaker 3: like the geologic causes and so forth. But unfortunately, the
Speaker 3: theoretical definition of the Wallace line and Wallace are kind
Speaker 3: of a mess. There have been a few details that
Speaker 3: have persisted, but there has not been clear or consistent
Speaker 3: agreement across time on where to place the boundaries, whether
Speaker 3: it's a line or a zone, to call those lines
Speaker 3: and zones, and what exactly they mean. So you might
Speaker 3: be wondering based on that, like, well, if you know,
Speaker 3: if we can't agree on what we're talking about, is
Speaker 3: the idea of a faunal boundary here just useless or
Speaker 3: is it nonsense? I think the answer is no. I mean,
Speaker 3: for one thing, some of this confusion is historical, Like
Speaker 3: there have been general trends in how the idea is
Speaker 3: getting refined over time. So there's nothing wrong with that.
Speaker 3: I mean, obviously ideas get updated. But some of the
Speaker 3: confusion is also because we're trying to figure out what
Speaker 3: exactly is being proposed here, and so like you have
Speaker 3: gotten away from the idea of a simple boundary line
Speaker 3: on a map that you know, where you have one
Speaker 3: thing on one side and one on another and more
Speaker 3: into say the idea of a transition zone. And it
Speaker 3: is absolutely still safe to say that there is something
Speaker 3: very interesting with reference to biogeography that's happening in the
Speaker 3: islands between Borneo and Bali to the west and Papa
Speaker 3: New Guinea to the east, let's say, and this, this
Speaker 3: boundary line or transition zone can tell us a lot
Speaker 3: about the history of life on the surrounding continents in
Speaker 3: the history of the Earth itself.
Speaker 2: Yeah, and this is what they were trying to figure
Speaker 2: out obviously in the late nineteenth century. But so we'll
Speaker 2: be discussing here they didn't have all the information they
Speaker 2: needed in order to really understand and to make what
Speaker 2: we would think of as a modern theory as to
Speaker 2: why it was like this.
Speaker 3: That's right, and so that brings us to the question
Speaker 3: of what causes the boundary. We raised this in the
Speaker 3: last episode, but didn't really have time to answer it.
Speaker 3: What causes the faunal boundary between the islands of say
Speaker 3: Bali and Lombok. Why do you get mostly one set
Speaker 3: of animals on Lombach and a fairly different set on Bali.
Speaker 3: Even though the strait between these two islands is just
Speaker 3: a few dozen kilometers wide, it's like it's barely any
Speaker 3: ocean at all. And also the environment on the two
Speaker 3: islands is very similar, so it seems like you would
Speaker 3: expect that the animals find on one to be the
Speaker 3: same as the ones you'd find on the other, right, right,
Speaker 3: So to summarize sort of the original mainstream explanation, and
Speaker 3: there will be some additions to this as we move on.
Speaker 3: In this episode, Wallace understood the cause of the apparent
Speaker 3: faunnel boundary between Bali and Lombach to be a result
Speaker 3: of geological history. This is a core insight of biogeography.
Speaker 3: Studying where animals live can tell us not only about
Speaker 3: the animals, but also about the land, and there are
Speaker 3: actually really awesome examples of this. If you get into
Speaker 3: the way paleontology has helped interact with the geohistory of
Speaker 3: Earth like fossil organisms from hundreds of millions of years
Speaker 3: ago that appeared to live only in Africa and South America. Huh.
Speaker 3: One example of this, there's this early Permian reptile called Mesosaurus,
Speaker 3: the sort of shaped like a little crocodile. It lived
Speaker 3: exclusively in freshwater lakes and rivers almost three hundred million
Speaker 3: years ago, and its remains have only been found in
Speaker 3: southern South America and southern Africa. Wait a minute, If
Speaker 3: it only lived in fresh water like lakes and rivers,
Speaker 3: how did it give from Africa to South America and
Speaker 3: as far as we can tell nowhere in between. And
Speaker 3: then you've got other examples like this, like there's a
Speaker 3: Triassic land based therapsid called Syno Nathas, also found in
Speaker 3: the fossil record of South America and Africa, among a
Speaker 3: few other equally baffling locations like Antarctica. These examples showed,
Speaker 3: along with lots of other evidence that we have now,
Speaker 3: that these two land masses used to be one. That
Speaker 3: you know, the range was one continuous range hundreds of
Speaker 3: millions of years ago, and these animals died, their remains
Speaker 3: were posited, they were quickly buried, they became fossilized, and
Speaker 3: then the land split apart and the Atlantic Ocean in
Speaker 3: between was formed. So that's one example of how biogeography
Speaker 3: can tell us not just about the animals, but also
Speaker 3: about the land. Coming back to Wallace. At the time
Speaker 3: Wallace lived, plate tectonics was not yet an accepted theory.
Speaker 3: In fact, it wasn't even really proposed in a form
Speaker 3: that would be recognizable to us until I think, like
Speaker 3: the nineteen tens around them, and it wouldn't be widely
Speaker 3: accepted by geologists until the nineteen sixties. So Wallace in
Speaker 3: his contemporaries did not know that the continents and land
Speaker 3: masses of the Earth moved around, splitting and rejoining on
Speaker 3: the timescale of tens or hundreds of millions of years.
Speaker 3: But Wallace did know that the the surface of the
Speaker 3: Earth could change drastically due to other factors, and one
Speaker 3: of those was climate. Climate patterns determine Earth's average temperature
Speaker 3: and weather, and in instances long ago, when Earth's surface
Speaker 3: was colder and more of Earth's water was locked up
Speaker 3: in polar ice caps, sea levels were lower much lower.
Speaker 3: As a result, many areas that are now ocean were
Speaker 3: once dry land, and many parts of the world that
Speaker 3: are now islands surrounded by shallow seas were once continuous
Speaker 3: land masses, allowing land animals to cross freely between them,
Speaker 3: which is how you end up with large land mammals
Speaker 3: like tigers and elephants living on what appeared to be
Speaker 3: remote islands. In most cases, they didn't swim there, certainly
Speaker 3: not in stable breeding populations. They crossed on land at
Speaker 3: a time when there was a land bridge, and for
Speaker 3: the most part, Wallace correctly identified that many islands east
Speaker 3: of the line of the Wallas line were once joined
Speaker 3: as a single land mass with Australia, this land mass
Speaker 3: now known as the paleocontinent Sahul, with islands west of
Speaker 3: the line once being part of a more continuous land
Speaker 3: mass with Asia, now known as Sunda. However, just because
Speaker 3: islands are close to each other does not mean a
Speaker 3: land bridge always forms between them when sea levels drop.
Speaker 3: What Wallace did not know for sure, but what later
Speaker 3: proved perfectly consistent with his observation of the faunal boundary,
Speaker 3: is that between Bali and Lombach there is a deep
Speaker 3: ocean trench. The Strait of Lombach is not wide again,
Speaker 3: it's just a few kilometers wide at the narrowest point,
Speaker 3: but it is very deep, meaning that even when sea
Speaker 3: levels were at their lowest, a land bridge between the
Speaker 3: two islands never formed, and thus there was much less
Speaker 3: opportunity for land based animals to colonize one island from
Speaker 3: the other. The reason for the existence of this deep
Speaker 3: ocean trench, by the way, brings us back once again
Speaker 3: to plate tectonics. This trench is the meeting point between
Speaker 3: two plates where one is undergoing subduction, meaning it's being
Speaker 3: driven down underneath the other plate at the meeting point.
Speaker 3: Subduction zones tend to create deep ocean trenches along with
Speaker 3: other geologic activity like earthquakes and volcanic eruptions, so plate
Speaker 3: tectonics also it explains that the deep ocean trench that
Speaker 3: keeps the islands apart and never forming a land bridge,
Speaker 3: but it Plate tectonics also helps explain why there were
Speaker 3: such different collections of animals on Sunda and so Whul.
Speaker 3: To begin with, they were once separated by much more
Speaker 3: ocean than they are now Sahul And again, this is
Speaker 3: modern day Australia and New Guinea and some other associated islands.
Speaker 3: This was one part of the same land mass as
Speaker 3: what is now Antarctica, and over tens of millions of
Speaker 3: years in isolation from the Afro Eurasian and American land masses,
Speaker 3: this ancient continent experienced its own distinct branch of evolution,
Speaker 3: giving us, for example, marsupials instead of the placental mammals
Speaker 3: that you would find on the rest of the globe.
Speaker 3: It was only a little more than thirty million years
Speaker 3: ago that sa who will actually broke away from the
Speaker 3: land mass that would become Antarctica, and so who will?
Speaker 3: After that, slowly drifted north and then collided with the
Speaker 3: continental plates of Southeast Asia, giving us the arrangement of
Speaker 3: islands that we see today in Indonesia and the Malay Archipelago.
Speaker 3: And some of the islands we see in that archipelago
Speaker 3: were created by the collision. They were raised up. But
Speaker 3: again at the place where the plates collide we get
Speaker 3: a subduction zone and a deep ocean trench, which means
Speaker 3: even though the islands are now close, land bridge is
Speaker 3: never formed between Bali and Lombach, So even though they
Speaker 3: have been close, now for millions of years. There is
Speaker 3: relatively little interchange of large land animals back and forth. However,
Speaker 3: there is some interchange, and as we've discussed already, all
Speaker 3: faunnel boundaries are to some extent permeable, and studying those
Speaker 3: crossover examples can tell us even more about how this
Speaker 3: boundary works. I'm going to come back to that in
Speaker 3: a bit, but Rob, I think you also had some
Speaker 3: good stuff about how this interacts with plate tectonics.
Speaker 2: Yeah, and just getting a little bit into a little
Speaker 2: more into just sort of the history of Wallace and
Speaker 2: Darwin in trying to figure out why the Wallace line
Speaker 2: seemed to be a thing, how this boundary worked, and
Speaker 2: what were the factors behind it. Again, as you mentioned,
Speaker 2: you know, we didn't have continental drift in a sort
Speaker 2: of crystallized form until nineteen twelve, and we didn't have
Speaker 2: plate tectonics until the nineteen sixties, again in its more
Speaker 2: crystallized accepted form. At the time, a leading approach to
Speaker 2: understanding all this was a theory of continental extensionism, which
Speaker 2: Radical by Nature author James T. Costa describes as land
Speaker 2: bridges on steroids. So you know, we've been talking here
Speaker 2: about the concept of land bridges and how they work,
Speaker 2: and again they're certainly real. The Isthmus of Panama is
Speaker 2: a current land bridge, lost land bridges once connected India
Speaker 2: and Sri Lanka, and a land mass known as the
Speaker 2: Basian Plaine once connected Tasmania to mainland Australia. And of
Speaker 2: course we've also talked about the now submerged north Sea
Speaker 2: land mass of Doggerland.
Speaker 3: Ah. Yeah, we did a couple of episodes on that.
Speaker 3: Go back and look them up. Yeah, sometime last year.
Speaker 2: So land bridges as an explanation for certain regional slash
Speaker 2: local examples of geological distribution, that's all good. The problem
Speaker 2: with continental extensionism as it was used at the time
Speaker 2: was it leaned heavily on this as the prime, or
Speaker 2: even the only way that many forms of fauna had spread,
Speaker 2: employing actual and likely examples of land bridges, you know,
Speaker 2: sunken land masses and shallow seas, but then adding in
Speaker 2: other necessary lost land masses and land bridges as required
Speaker 2: to fill in the gaps, often with little or no
Speaker 2: actual evidence, and even dipping into lost continent myth making
Speaker 2: and pseudoscience in order to get the job done.
Speaker 3: God love a lost continent.
Speaker 2: I mean, we all do. It's a it's a wonderful
Speaker 2: concept in myth and in fiction and science fiction. But yeah,
Speaker 2: this was especially the case when trying to connect the
Speaker 2: dots across like Abystle Seas, so you'd have respected naturalists
Speaker 2: of the days suddenly leaning into well, you know, the
Speaker 2: idea of Lemuria, but even Atlantis.
Speaker 3: We may have also talked about them in a series
Speaker 3: of episodes we did, I think starting with the Eltannan Antenna,
Speaker 3: where we were talking about how people take anomalous underwater
Speaker 3: imagery that kind of looks interesting and then kind of
Speaker 3: run wild with it, deciding that oh, this thing under
Speaker 3: water has got to be an alien antenna, and actually
Speaker 3: it's probably a sponge, and this other thing has got
Speaker 3: to be evidence of Atlantis, when actually it's probably a rock.
Speaker 2: That's right. And we definitely talked about Lemurria. This was
Speaker 2: This one's a little different compared to Atlantis. It was
Speaker 2: proposed in eighteen sixty four by zoologists Phillip Sklater to
Speaker 2: explain the presence of lemur fossils, thus Lmuria on Madagascar
Speaker 2: and in India, but not in Africa or the Middle East.
Speaker 2: So you can you can see how Lumurria is very
Speaker 2: much about filling in that gap, saying, these creatures had
Speaker 2: to get from one side of the world as we
Speaker 2: know it to another. How did they do it? There
Speaker 2: had to be a land bridge, but not just any
Speaker 2: land bridge. There had to be a lost continent right
Speaker 2: there in the middle.
Speaker 3: That's funny because it would be trying to explain, actually,
Speaker 3: what is evidence for continental drift or plate tectonics.
Speaker 2: Right right, So initially it's just it was an idea
Speaker 2: to Lemuria was just a way to hypothetically fill in
Speaker 2: the blanks here continental extensionism to explain everything into other
Speaker 2: Individuals like biologist Ernst Haekel, considered the idea that human
Speaker 2: origins might tie to this lost continent as well. But
Speaker 2: then in the late nineteenth century, occultists began to appropriate
Speaker 2: the concept, and Lumiria of course dies as a scientific
Speaker 2: hypothesis in the nineteen sixties, but refuses to die in occultism,
Speaker 2: conspiracy theory, and of course fiction. You can look to
Speaker 2: various examples such as the fiction of Roberty Howard the
Speaker 2: fiction of Lynn Carter. You'll find Lumiria showing up as
Speaker 2: a location for various fantastic magical adventures.
Speaker 3: It's a great place to have a leather diaper, barbarian face,
Speaker 3: some psychic monsters.
Speaker 2: Yes, so continental extensionism. This was a concept that Wallace
Speaker 2: himself was rather taken with early on. Again, it seemed
Speaker 2: a possible way to explain geological distribution of fauna and
Speaker 2: explain some of what he was seeing. Though obviously, as
Speaker 2: we've also discussed, he clearly wasn't opposed to viewpoints outside
Speaker 2: of the scientific mainstream. Again, he was very interested in
Speaker 2: spiritualism and defended spiritualism and so forth, So it's not
Speaker 2: a stretch to imagine that he, you know, wouldn't have
Speaker 2: strong objections to some of these concepts, which again at
Speaker 2: the time were not like firmly wrapped up in a
Speaker 2: lot of occultist dreaming, and was more firmly within the
Speaker 2: realm of possible scientific explanations for the world.
Speaker 3: That's right, And again to clarify we talked about this
Speaker 3: last time, but spiritualism is different than what people mean
Speaker 3: when they say like I'm spiritual today. Spiritualism then meant
Speaker 3: the belief that you could make contact with and communicate
Speaker 3: with the dead or with spirits other than living humans,
Speaker 3: and that kind of contact was exciting to a lot
Speaker 3: of people who were in the sciences. They were like, hey,
Speaker 3: look here's something we can study empirically. Let's you know,
Speaker 3: let's study it, let's take notes.
Speaker 2: And this is another place where we kind of get
Speaker 2: into the the butting heads of Darwin and Wallace, who again,
Speaker 2: these two were not enemies. They rode to each other,
Speaker 2: they seemed, you know, friendly with each other, and Wallace
Speaker 2: looked up to Darwin. But Darwin really hated all of this.
Speaker 2: He hated that naturalists would even flirt with the idea
Speaker 2: of atlantis, and Darwin believed that over the course of
Speaker 2: life's history on Earth, wind wing flotation, these were sufficient
Speaker 2: to explain everything that we saw. In fact, Darwin favored
Speaker 2: the permanence of ocean basins and continents in his calculations,
Speaker 2: so not to say he thought everything was set in stone.
Speaker 2: You know, he recognized that there would be regional uplift
Speaker 2: and subsidence, for example. This was a part of a
Speaker 2: key part of his understanding how reefs and atolls formed.
Speaker 2: We've talked about that on the show before. But you know,
Speaker 2: he was not keen on the idea that there were
Speaker 2: lost continents that would have connected one land mass to
Speaker 2: another and didn't think they were necessary. More the point,
Speaker 2: and this is all interesting because everyone here had some
Speaker 2: really good ideas going on, but those ideas had to
Speaker 2: ultimately be overclocked to make up for what we had
Speaker 2: yet to settle on regarding continental drift and plate tectonics.
Speaker 2: So again, land bridges are a local slash regional reality,
Speaker 2: but you can't apply them globally. It's not a solution
Speaker 2: to every problem in the distribution of fauna.
Speaker 3: Right, Land bridges explain a lot, but they don't explain
Speaker 3: how life forms got to Easter Island, or to Hawaii
Speaker 3: for that matter.
Speaker 2: Right, right and wing, wind and flotation can get the
Speaker 2: job done over the fullness of time, sometimes in remarkable ways.
Speaker 2: But there are limitations to that as well. But there
Speaker 2: are also some pretty amazing examples. I'm reminded of my
Speaker 2: recent interview with Tom Lathan about his book Lost Wonders
Speaker 2: regarding a number of species that wind up in rare,
Speaker 2: far flung places, including both the Galapagos tortoise, which that's,
Speaker 2: as we've discussed on the show before, that's an example
Speaker 2: of floatation in action. But then you have the now
Speaker 2: extinct Saint Helena Olive of Saint Helena Island in the
Speaker 2: South Atlantic. The olive, as listeners might remember, possibly became
Speaker 2: established there by dropping off of an albatross. So it's
Speaker 2: one of these things that was extremely unlikely to occur,
Speaker 2: but unlikely rare events do occur over the course of
Speaker 2: geological time.
Speaker 3: Yeah.
Speaker 2: So Wallace did eventually come around to a different understanding
Speaker 2: and wrote the eighteen eighty book Island Life, in which
Speaker 2: he sets aside really any thought of lost continence in
Speaker 2: favor of kind of a combo of both land bridges
Speaker 2: and long distance dispersal via wind wing and floatation. And
Speaker 2: so in this it brought his line of thinking closer
Speaker 2: to Darwin's own, but ultimately leaning on a combination of factors.
Speaker 3: And again he was aware of the idea of past
Speaker 3: sea level changes due to changes in how much of
Speaker 3: Earth's water is locked up in ice.
Speaker 2: Absolutely and again those deep ocean trenches, though, would have
Speaker 2: just been too deep to have been impacted by these
Speaker 2: sea level drops, you know, I mean, sea level can
Speaker 2: only drop so much and you're still going to have
Speaker 2: to contend with trenches at that point.
Speaker 3: So I was looking for some more recent papers on
Speaker 3: the Wallace line, and I came across one that I
Speaker 3: thought was pretty interesting. This was a twenty twenty three
Speaker 3: study published in the journal Science by Skills at All
Speaker 3: called paleo Environments Shaped by Exchange of terrestrial vertebrates across
Speaker 3: Wallace's Line. The authors in this paper start off by
Speaker 3: establishing a concept called biotic interchange. This is what happens
Speaker 3: when the flora and fauna of two previously separate regions
Speaker 3: are suddenly allowed to move into each other. They're allowed
Speaker 3: to freely colonize one another's regions. Examples of this usually
Speaker 3: occur when a barrier to travel between two places is
Speaker 3: suddenly removed, and suddenly here would be a relative modifier
Speaker 3: suddenly on geologic time. One example of this is the
Speaker 3: American interchange, where land and freshwater animals from North and
Speaker 3: South America were suddenly able to migrate across into each
Speaker 3: other's territory. After Rob you mitched this earlier. The idea
Speaker 3: of the Panama is isthmus. Oh Man, that word is
Speaker 3: so hard to say isthemus for me, It's difficult. After
Speaker 3: the Panama Isthmus was raised by volcanic activity, so that
Speaker 3: came up from the seafloor. And previously there had been
Speaker 3: a saltwater barrier between North and South America for some time.
Speaker 3: But you have this volcanic activity. Now there's a land
Speaker 3: bridge in Central America that happens between two and three
Speaker 3: million years ago, and now there's all this exchange of
Speaker 3: life forms across the barrier or across the former barrier.
Speaker 3: Another one of these great biotic mingling events is the
Speaker 3: Indo Australian Interchange, which arose when the Australian and Eurasian
Speaker 3: techtonic plates smashed together, forming some islands in between within
Speaker 3: the region we were talking about earlier known as Wallacea.
Speaker 3: Now we've already talked about how there are some barriers
Speaker 3: to exchange along this connection point. The extent of that
Speaker 3: barrier to cross colonization was the original observation of the
Speaker 3: Wallace line. It's like what Wallace observed is, hey, there's
Speaker 3: not a lot of animal species going across here. Unlike
Speaker 3: the American Interchange. There is not and was never a
Speaker 3: full land bridge between Sunda and Sahul. But as Wallace
Speaker 3: also observed, all boundaries are permeable, and we now know
Speaker 3: a good bit of exchange does happen across this line,
Speaker 3: and there are some more faunally mixed transition zones in
Speaker 3: between in the islands of Wallacea. But the authors of
Speaker 3: this paper point out something interesting. Biotic interchanges are rarely symmetrical.
Speaker 3: They're rarely totally even. Usually you have more life flowing
Speaker 3: in one direction across the new corridor than flowing in
Speaker 3: the other direction. Why would that be? Why the asymmetry.
Speaker 3: Some answers lie in characteristics of the organisms. Maybe some
Speaker 3: organisms are just more adaptable, they're better at dispersing into
Speaker 3: new environments. Maybe they're better at evolving more quickly and
Speaker 3: changing what they're adapted to. But some answers, on the
Speaker 3: other hand, might lie in characteristics of the land and
Speaker 3: environment on either side of the new corridor, or within
Speaker 3: the corridor itself. What if something about the geography favors
Speaker 3: dispersal one way but not the other. And to be
Speaker 3: clear here, the authors do not argue that Wallace was
Speaker 3: wrong about the deep ocean trench in the Lombock Strait
Speaker 3: being a barrier to dispersal. It is, as it does
Speaker 3: stop a lot of exchange, and they note that the
Speaker 3: only mammals which seem to have successfully crossed that boundary.
Speaker 3: Naturally are bats and rodents. Big mammals seemingly do not cross.
Speaker 3: But of the animals that have crossed, one thing that
Speaker 3: is clear is which way the asymmetry goes. Over the
Speaker 3: last twenty million years, way more animals from Asia have
Speaker 3: successfully crossed the barrier and made their way into New
Speaker 3: Guinea and onto the Australian continent. Relatively little dispersal has
Speaker 3: gone the other direction, from Australia and New Guinea to
Speaker 3: the west into Asia. Why would that be well, The
Speaker 3: authors argue, based on analysis of a huge data set
Speaker 3: and a sophisticated computer model, that the answer has to
Speaker 3: do with the climate of the islands within and on
Speaker 3: either side of Wallacea, especially how wet or dry those
Speaker 3: islands are, and the precipitation tolerance of the animals that
Speaker 3: evolved on either side. So to quote the authors quote here,
Speaker 3: analysis of more than twenty thousand vertebrate species with a
Speaker 3: model of geoclimate and biological diversification shows that broad precipitation
Speaker 3: tolerance and dispersal ability were key for exchange across the
Speaker 3: deep time precipitation gradients spanning the region Sundanian meaning Southeast
Speaker 3: Asian lineages evolved in a climate similar to the humid
Speaker 3: quote stepping stones of Wallacea, facilitating colonization of the Sahulian
Speaker 3: meaning Australian continental shelf. By contrast, Sahulian lineages predominantly evolved
Speaker 3: in drier conditions, hampering establishment in sunda and shaping faunal distinctiveness. So,
Speaker 3: in other words, the islands in the transition zone here
Speaker 3: are typically full of tropical rainforests like you would find
Speaker 3: further up on the Malay Peninsula and the Asian Maine land.
Speaker 3: New Guinea, on the same tectonic plate with Australia, is
Speaker 3: also dominated with tropical rainforest. So animals adapted to Asian
Speaker 3: tropical rainforests would find generally familiar and tolerably wet climates
Speaker 3: throughout most of the islands that formed these stepping stones
Speaker 3: leading to Australia and New Guinea, and once they reached
Speaker 3: Australia there would be different conditions. The continent would be
Speaker 3: much drier, but they could have more evolutionary time to
Speaker 3: adapt by colonizing the islands along the way or by
Speaker 3: landing in the friendly climate of New Guinea. Meanwhile, animals
Speaker 3: adapted to the dry climate of Australia would have a
Speaker 3: quite difficult time finding hospitable conditions along the humid stepping stones,
Speaker 3: and once you get to the Asian mainland, it's still
Speaker 3: just humid and dominated by tropical rainforest. So because the
Speaker 3: transitional islands in between were more easily tolerated by the
Speaker 3: Asian animal, more Asian animals flowed into the land masses
Speaker 3: of the Australian Plate than the other way around. And
Speaker 3: I thought that was really interesting because it I don't
Speaker 3: know if it often occurs to us to think when
Speaker 3: we think about like animals dispersing from one region into another,
Speaker 3: we think about the sort of end stage reach, like
Speaker 3: they start here and then they end up here, But
Speaker 3: the place that they're having to move through as they
Speaker 3: migrate also plays a big role in whether that migration
Speaker 3: can take place at all.
Speaker 2: Hmm. Yeah, that's a great point.
Speaker 3: I was trying to think of a good analogy, and
Speaker 3: this doesn't exactly work, but to grossly oversimplify. What if
Speaker 3: there were a highway suddenly opened across the Atlantic Ocean
Speaker 3: between Great Britain and the United States and you could
Speaker 3: cross it by car, do you think you'd have more
Speaker 3: people crossing one way or another. I mean, in reality,
Speaker 3: there would be a lot of things influencing this, but
Speaker 3: you could imagine that which side of the highway you
Speaker 3: have to drive on on the highway going in between
Speaker 3: might determine a lot about who decides to make the
Speaker 3: journey more often, like if you got to be if
Speaker 3: you got to be on the left side of the road,
Speaker 3: I can imagine a lot of Americans saying, I'm not
Speaker 3: driving that far on the left side. I don't know
Speaker 3: how to do that. So if we were driving on
Speaker 3: the left side, maybe it would be a motorway and
Speaker 3: not a highway. I'm not sure.
Speaker 2: Yeah, yeah, but I see what you're going for here,
Speaker 2: Like the side of the car that you drive upon
Speaker 2: is kind of like the environmental conditions of the passage. Yeah. Yeah.
Speaker 2: And how conducive drivers on either side or going to
Speaker 2: be to that passage.
Speaker 3: Yeah. It's not just how well adapted you already are
Speaker 3: to the end point. It's like how well adapted you
Speaker 3: are to each little step along the way.
Speaker 2: Yeah.
Speaker 3: And in the case of Wallacea, it proved easier for
Speaker 3: the humid adapted Asian mainland fauna to to make the
Speaker 3: journey into especially into New Guinea.
Speaker 2: Now, as we begin to reach the end of this episode,
Speaker 2: I want to bring it back a little bit to
Speaker 2: my travel experience in Indonesia where I actually visited these
Speaker 2: stepping stones or some of these stepping stones we've been
Speaker 2: talking about. So again, I was east of the wallace
Speaker 2: line in Raja Ampat, which situated is also in an
Speaker 2: area that is often called the coral triangle. Joe, I've
Speaker 2: included a map of the coral triangle for you here.
Speaker 2: This one isn't heart shaped and it's also not really
Speaker 2: triangle shaped. It's located between the Pacific and Indian Oceans
Speaker 2: and encompasses portions of two biogeographic regions, so the Indonesian
Speaker 2: Philippines region and the Far Southwestern Pacific region. If it
Speaker 2: looks like anything, it looks more like a rough sketch
Speaker 2: of a fish.
Speaker 3: Yeah, I guess see, it's a psilocybin triangle.
Speaker 2: Yeah yeah. And so why do we call it the
Speaker 2: coral triangle? Well, because it is super rich in coral.
Speaker 2: According to the Coral Reef Alliance, this region contains over
Speaker 2: seven twenty six percent of the world's coral species and
Speaker 2: thirty seven percent of reef fish species. So we're talking
Speaker 2: about somewhere around five hundred of the eight hundred and
Speaker 2: forty coral species that we know of. So just a
Speaker 2: rich hot bed of marine biodiversity, and there's a reason
Speaker 2: that snorkelers and divers from around the world seek it out.
Speaker 2: There's just breathtaking abundance there. I've never experienced snorkeling like
Speaker 2: this before, and you know, unless I go back there,
Speaker 2: I'm not sure I will again.
Speaker 3: Well, as as I've said, I have no experience snorkeling myself,
Speaker 3: but I'm still envious of what you got to see there.
Speaker 3: But this does raise the question does the Wallis line
Speaker 3: hold at all for underwater fauna. We know that it
Speaker 3: holds to some extent for like land mammals. But you
Speaker 3: might think intuitively, well, surely there would be no way
Speaker 3: that the Wallas line would have any impact on say,
Speaker 3: fish or other things that I can swim through. They
Speaker 3: can just swim right across the strait.
Speaker 2: Right, you would think of the fish or just have
Speaker 2: a free for all, and so they don't have to
Speaker 2: obey the rules of the Wall's line, and neither do
Speaker 2: the birds. But this is not the case. Despite the
Speaker 2: fact that we're under the water here, we're still subject
Speaker 2: to some of the boundary effects of deep ocean channels.
Speaker 2: So these deep trenches seem to act as persistent barriers
Speaker 2: that prevent the widespread migration of many coral larvae and
Speaker 2: many other marine invertebrates as well. Additionally, there's the Indonesian
Speaker 2: through flow, which moves massive amounts of water from the
Speaker 2: Pacific to the Indian Ocean, and so it plays a
Speaker 2: crucial role here as well. It'll transport some larvae, but
Speaker 2: also acts as a barrier to others, influencing the genetic
Speaker 2: flow between populations of coral on either side of the line.
Speaker 2: So some coral species have wide Indo Pacific ranges and
Speaker 2: can be found on both sides, but others follow distribution
Speaker 2: patterns that are more in line with some of the
Speaker 2: terrestrial animals that are impacted by the Wallas line. And
Speaker 2: we also see this play into distinct species. For instance,
Speaker 2: there are two distinct subpopulations of the Indo Pacific leopard shark,
Speaker 2: and they are separated by the Indonesian through flow current,
Speaker 2: which again coincides roughly with the Wallas line. Here. For instance,
Speaker 2: the Missoul Foundation, which runs the Missoul resort in raja Ampat.
Speaker 2: There's they are part of a Star Project Reshark program
Speaker 2: to help re establish the eastern subpopulation of the species
Speaker 2: in raja Ampat. Meanwhile, there's another program Star Project Thailand
Speaker 2: focused on the western variety. So one variety of Indo
Speaker 2: Pacific leopard shark on one side and then another variety
Speaker 2: on the other side.
Speaker 3: Wow, I mean, even though we've established that the line
Speaker 3: is not impermeable and sometimes animals do cross, I don't know,
Speaker 3: it's fascinating how just like how much of a barrier
Speaker 3: there could be that's just totally invisible to us and
Speaker 3: and even even affects creatures. You would think that it
Speaker 3: that it couldn't like many underwater creatures.
Speaker 2: Yeah, that we can read it in the coral, we
Speaker 2: can read it in the leopard sharks, and yeah, it's
Speaker 2: it's fascinating. Yeah, just the power the powerful interplay of
Speaker 2: division and mixing between rich ecosystems. Here. Another interesting thing
Speaker 2: about the leopard shark program there is that they uh,
Speaker 2: they were having to get you know, you wanted to
Speaker 2: make sure that you were using the right variety of
Speaker 2: into Pacific leopard shark to repopulate the region. There in
Speaker 2: raja Ampat. So they had to reach out to various
Speaker 2: aquariums to get surplus eggs from into Pacific leopard sharks
Speaker 2: of the right variety, and one of the aquariums is
Speaker 2: actually the Georgia Aquarium here in Atlanta. Uh So it
Speaker 2: was entirely I don't know for certain, but it was
Speaker 2: entirely possible that some of the the the baby leopard
Speaker 2: sharks that I that I saw in the tanks that
Speaker 2: were then going to be gradually re produced into the wild.
Speaker 2: Perhaps we're descendants of eggs that were produced all the
Speaker 2: way back home in Atlanta. The circle of life, Yeah,
Speaker 2: circle of life with leopard shark eggs, you know, taking
Speaker 2: international flights for days across the Earth, which in a
Speaker 2: way is entering because it brings it back into the
Speaker 2: like the vast way that humans end up moving around.
Speaker 2: And of course we've talked before in the show about
Speaker 2: how the movements of humans has of course been another
Speaker 2: huge factor in the way other organisms are intentionally and
Speaker 2: unintentionally spread around the world. Yeah, of course, but occasionally
Speaker 2: we get strategic about it. We attempt to fix things
Speaker 2: that we had broken before, because you know, there's a
Speaker 2: reason that sharks are missing, as we've discussed before, or
Speaker 2: shark populations are greatly decreased in various areas, and it's
Speaker 2: because we have hunted them, we fish for them, we
Speaker 2: have feared them, and so forth, and in doing so,
Speaker 2: you know, we end up damaging the ecosystem that they
Speaker 2: were a part of, because those apex predators are essential
Speaker 2: to the overall structure of things. And also, as we've
Speaker 2: discussed in the show, their place is fragile.
Speaker 1: At the top.
Speaker 3: Yeah, all right, Well, I have enjoyed this journey with
Speaker 3: Alfred Russell Wallace.
Speaker 2: Yeah, like I said, a fascinating individual. If you want
Speaker 2: to read more about him, that book Radical by Nature
Speaker 2: by James T. Costa is quite good, and there are
Speaker 2: a number of other books as well that have come
Speaker 2: out in over the recent decades about his work. There's
Speaker 2: kind of been a resurgence of interest in Alfred Russell Wallace.
Speaker 3: I enjoyed so much just reading Wallace's own writing in
Speaker 3: the Malay Archipelago that I may want to come back
Speaker 3: and plug that plum that maybe for some future topics.
Speaker 2: Yeah, yeah, Plus, yeah, he wrote many, many books on
Speaker 2: a number of topics, so yeah, he may pop up
Speaker 2: again in a future episode of Stuff Blear Your Mind.
Speaker 2: All right, we're going to go ahead and close it
Speaker 2: up there, but we'd love to hear from everyone out
Speaker 2: there if you have thoughts about the Wallace line, about
Speaker 2: some of the organisms that we've discussed here. Just a
Speaker 2: reminder of Stuff to Blow Your Mind is primarily a
Speaker 2: science and culture podcast, with core episodes on Tuesdays and Thursday,
Speaker 2: short form episode on Wednesdays and on Fridays. We set
Speaker 2: aside most serious concerns to just talk about a weird
Speaker 2: film on Weird House Cinema.
Speaker 3: Huge thanks as always to our excellent audio producer JJ Posway.
Speaker 3: If you would like to get in touch with us
Speaker 3: with feedback on this episode or any other, to suggest
Speaker 3: a topic for the future, or just to say hello,
Speaker 3: you can email us at contact, stuff to Blow your
Speaker 3: Mind dot com.
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