September 22, 2026 • 45 mins
In this classic episode of Stuff to Blow Your Mind, Robert and Joe discuss the faunal boundary line between Asia and Australia known as the Wallace Line and the British naturalist it was named for, Alfred Russel Wallace. (orignally published 8/21/2025)
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Speaker 1 (00:06):
Hey, welcome to Stuff to Blow Your Mind. My name
is Robert Lamb. It is a buffer week here on
Stuff to Blow Your Mind. We're gaining strength for our
October Halloween episodes ahead. So today is an episode for
you that originally aired 8-21-2025. It is The Wallace Line
Part 2. Please enjoy. The Wallace Line Part 2.
Speaker 2 (00:29):
Welcome to Stuff to Blow Your Mind, a production of iHeartRadio.
Speaker 1 (00:39):
Hey, welcome to Stuff to Blow Your Mind. My name
is Robert Lamb.
Speaker 2 (00:42):
And I am Joe McCormick. And we're back with part
two in our series on the 19th century British naturalist
Alfred Russell Wallace and on the Wallace Line, the faunal
boundary in the Malay archipelago that bears his name. If
you haven't heard part one yet, I would recommend you
go back and listen to that one first. But in
part one, we started off with a general character sketch
(01:05):
of Wallace. He's a man of many adventures and many opinions,
best known today for being the other guy who discovered
evolution by natural selection. He came up with a slightly
different version of the theory of natural selection around the
same time Darwin did, just some sort of differences of emphasis, basically.
(01:26):
Though Darwin's writings ultimately proved more influential in convincing his
peers on the reality of common descent and on articulating
the mechanisms by which species evolve. Wallace was also something
of a celebrity at his time. It's not like one
of those tragic cases of somebody else who also came
across a great idea but was just totally forgotten. Wallace
(01:47):
was kind of a celebrity, especially because of the quality
of his writings. And that would be in books like
The Malay Archipelago, published in 1869, in which he vividly
described his earlier travels in that region of the world,
including lots of very well-observed biological and cultural detail. And
we read a bunch of selections from that book in
(02:09):
the last episode. And it's somewhat magical to read. Yeah, absolutely.
Speaker 1 (02:14):
Again, you can see why this book was such a success.
And it was read by other naturalists, but also just biologists.
just members of the general public who are interested in
the topic, interested in faraway lands and in the natural world.
Speaker 2 (02:27):
Yeah, so we read some selections from the chapters where
he goes exploring, initially against his will, by the way,
on the islands of Bali and Lombok. And this included
everything from these evocative descriptions of the land itself, talking
about the terrorist agriculture and the way the fields are
irrigated and all that, to discussions about the geographic distribution
(02:48):
of ghosts to complaints about how hard it is to
do science when everything smells putrid and is covered in ants. Right.
Speaker 1 (02:56):
Now, to be clear, it was putrid smelling because of
all the bird skin. This is not a general comment
on on the region or the peoples there. This was
this was his mess. No, no.
Speaker 2 (03:07):
It's not like Lombok smells bad. It was my room
smells bad.
Speaker 1 (03:11):
Yeah. Yeah.
Speaker 2 (03:14):
Also, we talked about the concept of biogeography, the study
of what lives where and why. Wallace is considered a
very important founding figure in biogeography, with one of his
most enduring observations being what is now called, in his honor,
the Wallace Line, an invisible boundary passing in the ocean
between the islands of originally Borneo and Sulawesi, and even
(03:38):
more astonishingly, in the tiny, narrow strait between Bali and Lombok.
which marks the westernmost boundary of a lot of characteristic
Australasian fauna, marsupials, cockatoos, and things like that. So we
are back today to talk more about Wallace and the
Wallace line. Now, one thing I think we've got to
do before we move on any further is just a
(04:01):
bit of cleanup, because as I mentioned in the last episode,
our model of the Wallace line has undergone some some
major revisions since Wallace first proposed it in the mid-19th century.
I'm not going to do a whole detailed play-by-play of
the shifting history of the line, but for a brief
summary of developments, I was just looking at the abstract
(04:24):
of a paper called Wallace's Line, Wallacea, and Associated Divides
and Areas, History of a Tortuous Tangle of Ideas and Labels.
This is by Ali and Heaney. published in the journal
Biological Reviews in 2021. So the authors go through the concept,
which we discussed in the last episode, the idea of
(04:44):
a faunal boundary being basically a place where you find
animals mainly of one sort on one side of the
line and animals mainly of a different sort on the
other side of the line. And this raises questions, how
did it end up like that? The authors write that
the earliest dividing lines in this region were considered quite sharp,
especially the line Wallace drew in 1863. And this was
(05:08):
based on looking at the distribution of land mammals and
some birds. This is the line we've been talking about
so far, passing between Bali and Lombok, with Asian animals
found west from the western coast of Bali and Australasian
animals running east from Lombok. This particular theoretical boundary proved
(05:30):
very influential and it got, you know, reproduced in a
lot of texts. It was baked into maps and plates
and stuff all over. So like it had a big
cultural footprint and within the discipline of biogeography, it came
to be seen as something of a fixed marker in nature,
maybe like the tree line on a mountain. But over time,
(05:51):
it also became obvious to experts in the field that Yeah.
Speaker 1 (05:54):
Yeah.
Speaker 2 (06:20):
By later in the 19th century, many biogeographers had started
to think about zones instead of lines. For example, there
is now a biogeographic region known as Wallasea. I've also
heard it pronounced Wallasia. So Wallasia or Wallasea, named after, again,
Alfred Russell Wallace. This region was sort of proposed in
(06:40):
the 1920s, which is a faunal transition zone where you
essentially have Australasian fauna. to the east of the zone,
Asian fauna to the west of the zone, and more
of a mix within. Though the western boundary of Wallacea
is still basically Wallace's line, passing between Bali and Lombok,
and in between Borneo and Sulawesi. So we're actually here
(07:03):
creeping a bit back toward that gradual transition idea of
animal ranges that we talked about in part one, though
some of the divisions you see, especially between Bali and Lombok,
are still actually quite striking. So, Rob, I included for
you to look at in our outline here an illustration
from one of the papers I was looking at. The
region highlighted in red and all the islands within that
(07:25):
is what is generally considered Wallasea today.
Speaker 1 (07:27):
Yeah. And on this map here, it looks like a
big old heart, like a big old Valentine. They even
colored it red.
Speaker 2 (07:32):
Happy Valentine's Day. You get some marsupials.
Speaker 1 (07:36):
Yeah. And I was in the eastern part of this
very region.
Speaker 2 (07:39):
Okay. So Ali and Heaney write that in the last
decade before their paper, even more new regions and boundary
modifications have been proposed, and the authors paint a truly
headache-inducing picture of the historical understanding of all this. As
they say in their title, it is a tortuous tangle
of ideas and labels. For example, I'm just going to
(08:01):
read one section of their abstract to give you an idea. Quote,
Wallace's 1863 line is not the one he finally settled
upon in 1910. Its path around Sulawesi was transferred from
the west to the east of the island. Ideally, Huxley's Divide,
and that's referring to Thomas Henry Huxley, who proposed a
(08:22):
modification to where the line goes, Huxley's Divide, 1868, should
carry his name rather than Wallace's. The latter never accepted
the proposition. Leidecker's line of 1896 ought to be labeled
the Heilprin-Leidecker line in recognition of Angelo Heilprin's 1887 contribution.
(08:43):
Concerning transition zones, ideally Wallasea should correspond to its original
1924 description, which incorporated the Philippine islands bar the Palawan group. Notably, though,
a smaller form introduced by Darlington in 1957, used frequently
from 1998 onwards, in which all of the Philippine islands
are excluded, is entrenched within the recent literature, but this
(09:06):
is often without evident justification. It should also be recognized
that the reduced, meaning southern, Wallacea area was effectively defined
by Heilprin in 1887, but was then labeled the Austro-Malaysian
Transition Zone. So it's a mess. This is one reason
why I really enjoyed researching for these episodes, but when
(09:27):
I was reading all this stuff for the last episode,
I was just like, Oh, I can't.
Speaker 1 (09:32):
Yeah. I mean, it's again, it's it's not like if
you take a kangaroo and have it walk over the
Wallace line, it will explode. So there's no it becomes
very difficult to actually test out some of these things.
It's based on a number of observations and vectors.
Speaker 2 (09:48):
Well, these modifications absolutely are based on empirical observations. And
it's not just people kind of jousting around about absolutely
nothing There are reasons based on the different kinds of
fauna that have been observed and the underlying theory about
the geologic causes and so forth. But unfortunately, the theoretical
(10:10):
definition of the Wallace line and Wallacea are kind of
a mess. There have been a few details that have persisted,
but there has not been clear or consistent agreement across
time on where to place the boundaries, whether it's a
line or a zone, what to call those lines and zones,
and what exactly they mean. So you might be wondering
(10:31):
based on that, like, well, if we can't agree on
what we're talking about, is the idea of a faunal
boundary here just useless or is it nonsense? I think
the answer is no. I mean, for one thing, Some
of this confusion is historical, like there have been general
trends in how the idea is getting refined over time.
So there's nothing wrong with that. I mean, obviously ideas
(10:51):
get updated, but some of the confusion is also because
we're trying to figure out what exactly is being proposed here.
And so like you have gotten away from the idea
of a simple boundary line on a map that, you know,
where you have one thing on one side and one
on another and more into say the idea of a
transition zone. And it is absolutely still safe to say
(11:12):
that there is something very interesting with reference to biogeography
that's happening in the islands between Borneo and Bali to
the west and Papua New Guinea to the east, let's say.
And this boundary line or transition zone can tell us
a lot about the history of life on the surrounding
continents and the history of the Earth itself.
Speaker 1 (11:32):
Yeah, and this is what they were trying to figure out, obviously,
in the late 19th century. But as we'll be discussing here,
they didn't have all the information they needed in order
to really understand and to make what we would think
of as a modern theory as to why it was
like this.
Speaker 2 (11:52):
That's right. And so that brings us to the question
of what causes the boundary. We raised this in the
last episode, but we didn't really have time to answer it.
What causes the faunal boundary between the islands of, say,
Bali and Lombok? Why do you get mostly one set
of animals on Lombok and a fairly different set on Bali,
(12:12):
even though the straight between these two islands is just
a few dozen kilometers wide. It's like it's barely any
ocean at all. And also the environment on the two
islands is very similar. So it seems like you would
expect that the animals you find on one to be
the same as the ones you'd find on the other.
Speaker 1 (12:30):
Right, right.
Speaker 2 (12:31):
So to summarize sort of the original mainstream explanation, and
there will be some additions to this as we move
on in this episode, Wallace understood the cause of the
apparent faunal boundary between Bali and Lombok to be a
result of geological history. This is a core insight of biogeography.
(12:52):
Studying where animals live can tell us not only about
the animals, but also about the land. And there are
actually really awesome examples of this. If you get into
the way paleontology has helped interact with the geohistory of Earth,
like fossil organisms from hundreds of millions of years ago
(13:17):
that appeared to live only in Africa and South America.
Speaker 1 (13:21):
Huh?
Speaker 2 (13:23):
Well, one example of this, there's this early Permian reptile
called Mesosaurus that sort of shaped like a little crocodile.
It lived exclusively in freshwater lakes and rivers almost 300
million years ago, and its remains have only been found
in southern South America and southern Africa. Wait a minute.
(13:44):
If it only lived in freshwater, like lakes and rivers,
how did it get from Africa to South America and,
as far as we can tell, nowhere in between? And
then you've got other examples like this, like there's a
Triassic land-based therapsid called Sinonathus, also found in the fossil
record of South America and Africa, among a few other
(14:07):
equally baffling locations like Antarctica. These examples showed, along with
lots of other evidence that we have now, that these
two land masses used to be one, that the range
was one continuous range hundreds of millions of years ago.
and these animals died, their remains were deposited, they were
quickly buried, they became fossilized, and then the land split apart,
(14:31):
and the Atlantic Ocean in between was formed. So that's
one example of how biogeography can tell us not just
about the animals, but also about the land. Coming back
to Wallace, at the time Wallace lived plate tectonics was
(14:55):
not yet an accepted theory. In fact, it wasn't even
really proposed in a form that would be recognizable to
us until I think like the 1910s around then. And
it wouldn't be widely accepted by geologists until the 1960s.
So Wallace and his contemporaries did not know that the
continents and land masses of the earth moved around, splitting
(15:18):
and rejoining on the timescale and of tens or hundreds
of millions of years. But Wallace did know that the
surface of the Earth could change drastically due to other factors,
and one of those was climate. Climate patterns determine Earth's
average temperature and weather, and in instances long ago when
(15:40):
Earth's surface was colder and more of Earth's water was
locked up in polar ice caps, sea levels were lower,
much lower. As a result, many areas that are now
ocean were once dry land, and many parts of the
world that are now islands surrounded by shallow seas were
once continuous land masses, allowing land animals to cross freely
(16:04):
between them, which is how you end up with large
land mammals like tigers and elephants living on what appear
to be remote islands. In most cases, they didn't swim there,
certainly not in stable breeding populations. They crossed on land
at a time when there was a land bridge. And
for the most part, Wallace correctly identified that many islands
(16:27):
east of the line, of the Wallace line, were once
joined as a single land mass with Australia. This land
mass now known as the Paleocontinent Sahul, with islands west
of the line being once being part of a more
continuous landmass with Asia now known as Sunda. However, just
(16:48):
because islands are close to each other does not mean
a land bridge always forms between them when sea levels drop.
What Wallace did not know for sure, but what later
proved perfectly consistent with his observation of the faunal boundary,
is that between Bali and Lombok, there is a deep
(17:10):
ocean trench. The Strait of Lombok is not wide. Again,
it's just a few kilometers wide at the narrowest point,
but it is very deep, meaning that even when sea
levels were at their lowest, a land bridge between the
two islands never formed, and thus there was much less
opportunity for land-based animals to colonize one island from the other.
(17:33):
The reason for the existence of this deep ocean trench,
by the way, brings us back once again to plate tectonics.
This trench is the meeting point between two plates where
one is undergoing subduction, meaning it's being driven down underneath
the other plate at the meeting point. Subduction zones tend
to create deep ocean trenches along with other geologic activity
(17:54):
like earthquakes and volcanic eruptions. So plate tectonics also, it
explains that, the deep ocean trench that keeps the islands apart,
never forming a land bridge. But plate tectonics also helps
explain why there were such different collections of animals on
Sunda and Sahul to begin with. They were once separated
(18:17):
by much more ocean than they are now. Sahul, and again,
this is modern-day Australia and New Guinea and some other
associated islands, this was once part of the same landmass
as what is now Antarctica. And over tens of Millions
of years in isolation from the Afro-Eurasian and American landmasses,
(18:38):
this ancient continent experienced its own distinct branch of evolution,
giving us, for example, marsupials instead of the placental mammals
that you would find on the rest of the globe.
It was only a little more than 30 million years
ago that Sahul actually broke away from the landmass that
would become Antarctica. And so who will, after that slowly
(18:58):
drifted north and then collided with the continental plates of
Southeast Asia, giving us the arrangement of islands that we
see today in Indonesia and the Malay archipelago. And some
of the islands we see, uh, in that archipelago were, uh,
were created by the collision. They were raised up. Um,
but again, at the place where the plates collide, we
(19:19):
get a subduction zone and a deep ocean trench, which
means even though the islands are now close. Land bridges
never form between Bali and Lombok, so even though they
have been close now for millions of years, there is
relatively little interchange of large land animals back and forth. However,
there is some interchange, and as we've discussed already, all
(19:42):
faunal boundaries are to some extent permeable, and studying those
crossover examples can tell us even more about how this
boundary works. I'm going to come back to that in
a bit. But Rob, I think you also had some
good stuff about how this interacts with plate tectonics.
Speaker 1 (19:56):
Yeah, and just getting a little bit into a little
more into just sort of the history of Wallace and
Darwin in trying to figure out why the Wallace line
seemed to be a thing. How this boundary worked and
what were the factors behind it. Again, as you mentioned,
we didn't have continental drift in a sort of crystallized
form until 1912, and we didn't have plate tectonics until
(20:18):
the 1960s, again, in its more crystallized accepted form. At
the time, a leading approach to understanding all this was
a theory of continental extensionism, which Radical by Nature author
James T. Costa describes as land bridges on steroids. So,
you know, we've been talking here about the concept of
(20:40):
land bridges and how they work. And again, they're certainly real.
The Isthmus of Panama is a current land bridge. Lost
land bridges once connected India and Sri Lanka. And a
landmass known as the Bassian Plain once connected Tasmania to
mainland Australia. And of course, we've also talked about the
now submerged North Sea landmass of Doggerland.
Speaker 2 (21:01):
Yeah, we did a couple of episodes on that. Go
back and look them up. They're from sometime last year.
Speaker 1 (21:06):
So land bridges as an explanation for certain regional slash
local examples of geological distribution, that's all good. The problem
with continental extensionism as it was used at the time
was that it leaned heavily on this as the prime
or even the only way that many forms of fauna
had spread, employing actual and likely examples of land bridges,
(21:28):
you know, sunken land masses and shallow seas and but
then adding in other necessary lost land masses and land
bridges as required to fill in the gaps, often with
little or no actual evidence, and even dipping into lost
continent myth-making and pseudoscience in order to get the job done.
Speaker 2 (21:47):
Gotta love a lost continent.
Speaker 1 (21:49):
I mean, we all do. It's a wonderful concept in
myth and in fiction and science fiction. But yeah, this
was especially the case when trying to connect the dots
across abyssal seas. So you'd have respected naturalists of the
day suddenly leaning into the idea of Lemuria, but even Atlantis.
Speaker 2 (22:09):
We may have also talked about them in a series
of episodes we did, I think, starting with the El
Tannin antenna, where we were talking about how people take
anomalous underwater imagery that kind of looks interesting and then
kind of run wild with it, deciding that, oh, this
thing underwater has got to be an alien antenna. And actually,
(22:29):
it's probably a sponge. And this other thing has got
to be evidence of Atlantis when actually it's probably a rock.
Speaker 1 (22:36):
That's right. And we definitely talked about Lemuria. This one's
a little different compared to Atlantis. It was proposed in
1864 by zoologist Philip Sclater to explain the presence of
lemur fossils, thus Lemuria, on Madagascar and in India, but
not in Africa or the Middle East. So you can
see how Lemuria is very much about filling in that gap.
(22:59):
saying these creatures had to get from one side of
the world as we know it to another. How did
they do it? There had to be a land bridge,
but not just any land bridge. There had to be
a lost continent right there in the middle.
Speaker 2 (23:11):
That's funny because it would be trying to explain actually
what is evidence for continental drift or plate tectonics.
Speaker 1 (23:18):
Right, right. So initially it's just, it was an idea to,
Lemuria was just a way to hypothetically fill in the
blanks here using continental extensionism. to explain everything. And other
individuals like biologist Ernst Haeckel considered the idea that human
origins might tie to this lost continent as well. But
(23:38):
then in the late 19th century, occultists began to appropriate
the concept. And Lemuria, of course, dies as a scientific
hypothesis in the 1960s, but refuses to die in occultism,
conspiracy theory, and of course, fiction. You can look to
various examples, such as the fiction of Robert E. Howard,
the fiction of Lynn Carter. You'll find Lemuria showing up
(24:01):
as a location for various fantastic magical adventures.
Speaker 2 (24:05):
It's a great place to have a leather diaper barbarian
face some psychic monsters.
Speaker 1 (24:10):
Yes. So continental extensionism, this was a concept that Wallace
himself was rather taken with early on. Again, it seemed
a possible way to explain geological distribution of fauna and
explain some of what he was seeing. Though obviously, as
we've also discussed, he clearly wasn't opposed to viewpoints outside
(24:31):
of the scientific mainstream. Again, he was very interested in
spiritualism and defended spiritualism and so forth. So it's not
a stretch to imagine that he wouldn't have strong objections
to some of these concepts, which again, at the time,
were not like firmly wrapped up in a lot of
occultist dreaming and was more firmly within the realm of
(24:54):
possible scientific explanations for the world.
Speaker 2 (24:57):
That's right. And again, to clarify, we talked about this
last time, but spiritualism is different than what people mean
when they say, like, I'm spiritual today. Spiritualism then meant
the belief that you could make contact with and communicate
with the dead or with spirits other than living humans.
And that kind of contact was exciting to a lot
of people who were in the sciences. They were like, hey, look,
(25:18):
here's something we can study empirically. Let's study it. Let's
take notes.
Speaker 1 (25:23):
And this is another place where we kind of get
into the butting heads of Darwin and Wallace. So again,
these two were not enemies. They wrote to each other.
They seemed friendly with each other. And Wallace looked up
to Darwin. But Darwin really hated all of this. He
hated that naturalists would even flirt with the idea of
Atlantis and And Darwin believed that over the course of
(25:46):
life's history on Earth, wind, wing, flotation, these were sufficient
to explain everything that we saw. In fact, Darwin favored
the permanence of ocean basins and continents in his calculations.
So not to say he thought everything was set in stone.
You know, he recognized that there would be regional uplift
and subsidence. For example, this was a key part of
(26:08):
his understanding of how reefs and atolls formed. We've talked
about that on the show before. But, you know, he
was he was not keen on the idea that there
were lost continents that would have connected one landmass to
another and didn't think they were necessary. More to the point.
(26:29):
And this is all interesting because everyone here had some
really good ideas going on. But those ideas had to
ultimately be overclocked to make up for what we had
yet to settle on regarding continental drift and plate tectonics.
So again, land bridges are a local slash regional reality,
but you can't apply them globally. It's not a solution
(26:52):
to every problem in the distribution of fauna. Right.
Speaker 2 (26:56):
Land bridges explain a lot, but they don't explain how
life forms got to Easter Island or to Hawaii, for
that matter.
Speaker 1 (27:02):
Right, right. And wing wind and flotation can get the
job done over the fullness of time, sometimes in remarkable ways,
but there are limitations to that as well. But there
are also some pretty amazing examples. I'm reminded of my
recent interview with Tom Lathan about his book, Lost Wonders,
regarding a number of species that wind up in rare,
far-flung places, including both the Galapagos tortoise, which that's, as
(27:27):
we've discussed in the show before, that's an example of
flotation in action. But then you have the now extinct St.
Helena olive of St. Helena Island in the South Atlantic.
The olive, as listeners might remember, possibly became established there
by dropping off of an albatross. So it's one of
these things that was extremely unlikely to occur, but unlikely
(27:50):
rare events do occur over the course of geological time.
So Wallace did eventually come around to a different understanding
and wrote the 1880 book, Island Life, in which he
sets aside really any thought of lost continents in favor
of kind of a combo of both land bridges and
long-distance dispersal via wind, wing, and flotation. And so in this,
(28:14):
it brought his line of thinking closer to Darwin's own,
but ultimately leaning on a combination of factors.
Speaker 2 (28:21):
And again, he was aware of the idea of past
sea level changes due to changes in how much of
Earth's water is locked up in ice.
Speaker 1 (28:29):
Absolutely. And again, those deep ocean trenches, though, would have
just been too deep to have been impacted by these
sea level drops, you know, I mean, sea level can
only drop so much and you're still going to have
to contend with trenches at that point.
Speaker 2 (28:51):
So I was looking for some more recent papers on
the Wallace line, and I came across one that I
thought was pretty interesting. This was a 2023 study published
in the journal Science by Skeels et al. called Paleoenvironments
Shaped by Exchange of Terrestrial Vertebrates Across Wallace's Line. The
authors in this paper start off by establishing a concept
(29:15):
called biotic interchange. This is what happens when the flora
and fauna of two previously separate regions are suddenly allowed
to move into each other. They're allowed to freely colonize
one another's regions. Examples of this usually occur when a
barrier to travel between two places is suddenly removed, and
(29:37):
suddenly here would be a relative modification. suddenly on geologic time.
One example of this is the American Interchange, where land
and freshwater animals from North and South America were suddenly
able to migrate across into each other's territory and After, Rob,
you mentioned this earlier, the idea of the Panama Isthmus. Oh, man,
(29:58):
that word is so hard to say. Isthmus for me.
It's difficult. After the Panama Isthmus was raised by volcanic activity.
So that came up from the seafloor. And previously there
had been a saltwater barrier between North and South America
for some time. But you have this volcanic activity. Now
there's a land bridge in Central America that happens between
(30:19):
two and three million years ago. And now there's all
this exchange of life forms across the barrier, or across
the former barrier. Another one of these great biotic mingling
events is the Indo-Australian Interchange, which arose when the Australian
and Eurasian tectonic plates smashed together, forming some islands in
(30:41):
between within the region we were talking about earlier, known
as Wallasea. Now, we've already talked about how there are
some barriers to exchange along this connection process. The extent
of that barrier to cross colonization was the original observation
of the Wallace line. It's like what Wallace observed is, hey,
there's not a lot of animal species going across here.
(31:02):
Unlike the American interchange, there there is not and was
never a full land bridge between Sunda and Sahul. But
as Wallace also observed, all boundaries are permeable. And we
now know a good bit of exchange does happen across
this this line. And there are some more faunally mixed
(31:23):
transition zones in between in the islands of Wallacea. But
the authors of this paper point out something interesting. Biotic
interchanges are rarely symmetrical. They're rarely totally even. Usually you
have more life flowing in one direction across the new
corridor than flowing in the other direction. Why would that be?
(31:44):
Why the asymmetry? Some answers lie in characteristics of the organisms.
Maybe some organisms are just more adaptable. They're better at
dispersing into new environments. Maybe they're better at evolving more
quickly and changing what they're adapted to. But some answers,
on the other hand, might lie in characteristics of the
land and environment on either side of the new corridor
(32:08):
or within the corridor itself. What if something about the
geography favors dispersal one way but not the other? And
to be clear here, the authors do not argue that
Wallace was wrong about the deep ocean trench in the
Lombok Strait being a barrier to dispersal.
Speaker 1 (32:26):
It is.
Speaker 2 (32:26):
It does stop a lot of exchange. And they note
that the only mammals which seem to have successfully crossed
that boundary naturally are bats and rodents. Big mammals seemingly
do not cross. But of the animals that have crossed,
one thing that is clear is which way the asymmetry goes.
Over the last 20 million years, way more animals from
(32:50):
Asia have successfully crossed the barrier and made their way
into New Guinea and onto the Australian continent. Relatively little
dispersal has gone the other direction, from Australia and New
Guinea to the west into Asia. Why would that be? Well,
the authors argue, based on analysis of a huge dataset
(33:11):
and a sophisticated computer model, that the answer has to
do with the climate of the islands within and on
either side of Wallacea, especially how wet or dry those
islands are and the precipitation tolerance of the animals that
evolved on either side. So to quote the authors, quote, Here,
(33:32):
analysis of more than 20,000 vertebrate species with a model
of geoclimate and biological diversification shows that broad precipitation tolerance
and dispersal ability were key for exchange across the deep-time
precipitation gradient spanning the region. Sundanian, meaning Southeast Asian lineages,
evolved in a climate similar to the humid, quote, stepping
(33:55):
stones of Wallasea. facilitating colonization of the Sahulian, meaning Australian,
continental shelf. By contrast, Sahulian lineages predominantly evolved in drier conditions,
hampering establishment in Sunda and shaping faunal distinctiveness. So, in
other words, the islands in the transition zone here are
(34:19):
typically full of tropical rainforests, like you would find further
up on the Malay Peninsula and the Asian mainland. New Guinea,
on the same tectonic plate with Australia, is also dominated
with tropical rainforest. So animals adapted to Asian tropical rainforests
would find generally familiar and tolerably wet climates throughout most
(34:42):
of the islands that formed these stepping stones leading to
Australia and New Guinea. And once they reached Australia, there
would be different conditions. The continent would be much drier.
But they could have more evolutionary time to adapt by
colonizing the islands along the way or by landing in
the friendly climate of New Guinea. Meanwhile, animals adapted to
(35:06):
the dry climate of Australia would have a quite difficult
time finding hospitable conditions along the humid stepping stones. And
once you get to the Asian mainland, it's still just
humid and dominated by tropical rainforests. So because the transitional
islands in between were more easily tolerated by the Asian animals,
(35:27):
more Asian animals flowed into the land masses of the
Australian plate than the other way around. And I thought
that was really interesting because it, I don't know if
it often occurs to us to think when we think
about like animals dispersing from one region into another, we
think about the, the sort of end stage reach, like
(35:47):
they start here and then they end up here. But
the place that they're having to move through as they
migrate also plays a big role in whether that migration
can take place at all.
Speaker 1 (35:59):
Yeah, that's a great point.
Speaker 2 (36:01):
I was trying to think of a good analogy, and
this doesn't exactly work, but to grossly oversimplify. What if
there were a highway suddenly opened across the Atlantic Ocean
between Great Britain and the United States, and you could
cross it by car? Do you think you'd have more
people crossing one way or another? I mean, in reality,
(36:22):
there would be a lot of things influencing this. But
you could imagine that which side of the highway you
have to drive on on the highway going in between
might determine a lot about who decides to make the
journey more often. Like if you got to be if
you got to be on the left side of the road,
I can imagine a lot of Americans saying, I'm not
driving that far on the left side. I don't know
how to do that. So if we were driving on
(36:43):
the left side, maybe it would be a motorway and
not a highway. I'm not sure.
Speaker 1 (36:46):
Yeah, yeah. But I see what you're going for here.
Like the side of the car that you drive upon
is kind of like the environmental conditions of the passage. Yeah. Yeah.
And how conducive drivers on either side are going to
be to that passage. Yeah.
Speaker 2 (37:01):
It's not just how well adapted you already are to
the end point. It's like how well adapted you are
to each little step along the way.
Speaker 1 (37:08):
Yeah. Yeah.
Speaker 2 (37:10):
And in the case of Wallacea, it proved easier for
the humid adapted Asian mainland fauna to make the journey into,
especially into New Guinea.
Speaker 1 (37:22):
Now, as we begin to reach the end of this episode,
I want to bring it back a little bit to
my travel experience in Indonesia, where I actually visited these
stepping stones or some of these stepping stones we've been
talking about. So again, I was east of the Wallace
line in Raja Ampat. which situated us also in an
(37:42):
area that is often called the Coral Triangle. Joe, I've
included a map of the Coral Triangle for you here.
This one isn't heart-shaped, and it's also not really triangle-shaped.
It's located between the Pacific and Indian Oceans and encompasses
portions of two biogeographic regions, so the Indonesian Philippines region
(38:04):
and the far southwestern Pacific region. If it looks like anything,
it looks more like a rough sketch of a fish.
Speaker 2 (38:11):
Yeah, I can see that. It's a psilocybin triangle.
Speaker 1 (38:15):
Yeah, yeah. And so why do we call it the
coral triangle? Well, because it is super rich in coral.
According to the Coral Reef Alliance, this region contains over 76%
of the world's coral species and 37% of reef fish species.
So we're talking about somewhere around 500 of the 840
(38:38):
coral species that we know of. So just a rich
hotbed of marine biodiversity. There's a reason that snorkelers and
divers from around the world seek it out. There's just
breathtaking abundance there. I've never experienced snorkeling like this before.
Unless I go back there, I'm not sure I will again.
Speaker 2 (38:57):
Well, as I've said, I have no experience snorkeling myself,
but I'm still envious of what you got to see there.
But this does raise the question, Does the Wallace line
hold at all for underwater fauna? We know that it
holds to some extent for land mammals, but you might
(39:17):
think intuitively, well, surely there would be no way that
the Wallace line would have any impact on, say, fish
or other things that can swim through. They can just
swim right across the strait, right?
Speaker 1 (39:26):
Right. You would think, well, the fish just have a free-for-all,
and so they don't have to obey the rules of
the Wallace line, and neither do the birds. But this
is not the case. Despite the fact that we're under
the water here, we're still subject to some of the
boundary effects of deep ocean channels. So these deep trenches
are seem to act as persistent barriers that prevent the
(39:49):
widespread migration of many coral larvae and many other marine
invertebrates as well. Additionally, there's the Indonesian throughflow, which moves
massive amounts of water from the Pacific to the Indian Ocean.
And so it plays a crucial role here as well.
It'll transport some larvae, but also acts as a barrier
(40:10):
to others. influencing the genetic flow between populations of coral
on either side of the line. So some coral species
have wide Indo-Pacific ranges and can be found on both sides,
but others follow distribution patterns that are more in line
with some of the terrestrial animals that are impacted by
the Wallace line. And we also see this play into
(40:31):
distinct species. For instance, there are two distinct subpopulations of
the Indo-Pacific leopard shark, and they are separated by the
Indonesian throughflow current, which again coincides roughly with the Wallace
line here. For instance, the Missoula Foundation, which runs the
Missoula Resort in Raja Ampat, they are part of a
(40:52):
Star Project ReShark program to help reestablish the eastern subpopulation
of the species in Raja Ampat. Meanwhile, there's another program,
Star Project Thailand, focused on the western variety. So one
variety of Indo-Pacific leopard shark on one side and then
another variety on the other side.
Speaker 2 (41:12):
Wow. I mean, even though we've established that the line
is not impermeable and sometimes animals do cross, uh, it,
I don't know. It's fascinating how, just like how much
of a barrier there could be. That's just totally invisible
to us. And, and even, even affects creatures. You would
think that it, that it couldn't like many underwater creatures.
Speaker 1 (41:33):
Yeah. That we can read it in the coral. We
can read it in the, uh, the leopard sharks and, Yeah,
it's fascinating. The powerful interplay of division and mixing between
rich ecosystems here. Another interesting thing about the leopard shark
program there is that they were having to get, you know,
(41:53):
you wanted to make sure that you were using the
right variety of Indo-Pacific leopard shark to repopulate the region
there in Raja Ampat. So they had to reach out
to various aquariums to get surplus eggs from Indo-Pacific leopard
sharks of the right variety. And one of the aquariums
is actually the Georgia Aquarium here in Atlanta. So it
(42:17):
was entirely, I don't know for certain, but it was
entirely possible that some of the baby leopard sharks that
I saw in the tanks that were then going to
be gradually reintroduced into the wild, perhaps were descendants of
eggs that were produced all the way back home in Atlanta.
Speaker 2 (42:34):
Circle of life.
Speaker 1 (42:35):
Yeah. circle of life with leopard shark eggs taking international
flights for days across the Earth, which in a way
is interesting because it brings it back into the vast
way that humans end up moving around. And of course,
we've talked before on the show about how the movements
of humans has, of course, been another huge factor in
(42:57):
the way other organisms are intentionally and unintentionally spread around
the world.
Speaker 2 (43:03):
Yeah, of course.
Speaker 1 (43:04):
But occasionally we get strategic about it. We attempt to
fix things that we had broken before because, you know,
there's a reason that sharks are missing, as we've discussed before,
or shark populations are greatly decreased in various areas. And
it's because we have hunted them. We fish for them.
We have feared them and so forth. And in doing so,
(43:25):
you know, we end up damaging the ecosystem that they
were a part of because they're Those apex predators are
essential to the overall structure of things. And also, as
we've discussed in the show, their place is fragile at
the top. Yeah. All right.
Speaker 2 (43:41):
Well, I have enjoyed this journey with Alfred Russell Wallace.
Speaker 1 (43:44):
Yeah. Like I say, a fascinating individual. If you want
to read more about him, that book Radical by Nature
by James T. Costa is quite good. And there are
a number of other books as well that have come
out in over the recent decades that about his work.
There's kind of been a resurgence of interest in Alfred
Russell Wallace.
Speaker 2 (44:03):
I enjoyed so much just reading Wallace's own writing in
the Malay Archipelago that I may want to come back
and plug that, plumb that maybe for some future topics.
Speaker 1 (44:11):
Yeah, yeah. Plus, yeah, he wrote many, many books on
a number of topics. So, yeah, he may pop up
again in a future.
Speaker 2 (44:18):
Episode of Stud to Play Your Mind.
Speaker 1 (44:20):
All right. We're going to go ahead and close it
up there. But we'd love to hear from everyone out
there if you have thoughts about the Wallace line and
about some of the organisms that we've discussed here. Just
a reminder, the Stuff to Blow Your Mind is primarily
a science and culture podcast with core episodes on Tuesdays
and Thursdays, short form episode on Wednesdays. And on Fridays,
we set aside most serious concerns to just talk about
a weird film on Weird House Cinema.
Speaker 2 (44:41):
Huge thanks, as always, to our excellent audio producer, J.J. Posway.
If you would like to get in touch with us
with feedback on this episode or any other, to suggest
a topic for the future, or just to say hello,
you can email us at contact at stufftoblowyourmind.com.
Speaker 1 (44:55):
Stuff to Blow Your Mind is a production of iHeartRadio.
Speaker 2 (45:05):
For more podcasts from iHeartRadio, visit the iHeartRadio app. Apple
Podcasts are wherever you listen to your favorite shows.
Speaker 1 (45:18):
Thank you.
Hey, welcome to Stuff to Blow Your Mind. My name
is Robert Lamb. It is a buffer week here on
Stuff to Blow Your Mind. We're gaining strength for our
October Halloween episodes ahead. So today is an episode for
you that originally aired 8-21-2025. It is The Wallace Line
Part 2. Please enjoy. The Wallace Line Part 2.
Speaker 2 (00:29):
Welcome to Stuff to Blow Your Mind, a production of iHeartRadio.
Speaker 1 (00:39):
Hey, welcome to Stuff to Blow Your Mind. My name
is Robert Lamb.
Speaker 2 (00:42):
And I am Joe McCormick. And we're back with part
two in our series on the 19th century British naturalist
Alfred Russell Wallace and on the Wallace Line, the faunal
boundary in the Malay archipelago that bears his name. If
you haven't heard part one yet, I would recommend you
go back and listen to that one first. But in
part one, we started off with a general character sketch
(01:05):
of Wallace. He's a man of many adventures and many opinions,
best known today for being the other guy who discovered
evolution by natural selection. He came up with a slightly
different version of the theory of natural selection around the
same time Darwin did, just some sort of differences of emphasis, basically.
(01:26):
Though Darwin's writings ultimately proved more influential in convincing his
peers on the reality of common descent and on articulating
the mechanisms by which species evolve. Wallace was also something
of a celebrity at his time. It's not like one
of those tragic cases of somebody else who also came
across a great idea but was just totally forgotten. Wallace
(01:47):
was kind of a celebrity, especially because of the quality
of his writings. And that would be in books like
The Malay Archipelago, published in 1869, in which he vividly
described his earlier travels in that region of the world,
including lots of very well-observed biological and cultural detail. And
we read a bunch of selections from that book in
(02:09):
the last episode. And it's somewhat magical to read. Yeah, absolutely.
Speaker 1 (02:14):
Again, you can see why this book was such a success.
And it was read by other naturalists, but also just biologists.
just members of the general public who are interested in
the topic, interested in faraway lands and in the natural world.
Speaker 2 (02:27):
Yeah, so we read some selections from the chapters where
he goes exploring, initially against his will, by the way,
on the islands of Bali and Lombok. And this included
everything from these evocative descriptions of the land itself, talking
about the terrorist agriculture and the way the fields are
irrigated and all that, to discussions about the geographic distribution
(02:48):
of ghosts to complaints about how hard it is to
do science when everything smells putrid and is covered in ants. Right.
Speaker 1 (02:56):
Now, to be clear, it was putrid smelling because of
all the bird skin. This is not a general comment
on on the region or the peoples there. This was
this was his mess. No, no.
Speaker 2 (03:07):
It's not like Lombok smells bad. It was my room
smells bad.
Speaker 1 (03:11):
Yeah. Yeah.
Speaker 2 (03:14):
Also, we talked about the concept of biogeography, the study
of what lives where and why. Wallace is considered a
very important founding figure in biogeography, with one of his
most enduring observations being what is now called, in his honor,
the Wallace Line, an invisible boundary passing in the ocean
between the islands of originally Borneo and Sulawesi, and even
(03:38):
more astonishingly, in the tiny, narrow strait between Bali and Lombok.
which marks the westernmost boundary of a lot of characteristic
Australasian fauna, marsupials, cockatoos, and things like that. So we
are back today to talk more about Wallace and the
Wallace line. Now, one thing I think we've got to
do before we move on any further is just a
(04:01):
bit of cleanup, because as I mentioned in the last episode,
our model of the Wallace line has undergone some some
major revisions since Wallace first proposed it in the mid-19th century.
I'm not going to do a whole detailed play-by-play of
the shifting history of the line, but for a brief
summary of developments, I was just looking at the abstract
(04:24):
of a paper called Wallace's Line, Wallacea, and Associated Divides
and Areas, History of a Tortuous Tangle of Ideas and Labels.
This is by Ali and Heaney. published in the journal
Biological Reviews in 2021. So the authors go through the concept,
which we discussed in the last episode, the idea of
(04:44):
a faunal boundary being basically a place where you find
animals mainly of one sort on one side of the
line and animals mainly of a different sort on the
other side of the line. And this raises questions, how
did it end up like that? The authors write that
the earliest dividing lines in this region were considered quite sharp,
especially the line Wallace drew in 1863. And this was
(05:08):
based on looking at the distribution of land mammals and
some birds. This is the line we've been talking about
so far, passing between Bali and Lombok, with Asian animals
found west from the western coast of Bali and Australasian
animals running east from Lombok. This particular theoretical boundary proved
(05:30):
very influential and it got, you know, reproduced in a
lot of texts. It was baked into maps and plates
and stuff all over. So like it had a big
cultural footprint and within the discipline of biogeography, it came
to be seen as something of a fixed marker in nature,
maybe like the tree line on a mountain. But over time,
(05:51):
it also became obvious to experts in the field that Yeah.
Speaker 1 (05:54):
Yeah.
Speaker 2 (06:20):
By later in the 19th century, many biogeographers had started
to think about zones instead of lines. For example, there
is now a biogeographic region known as Wallasea. I've also
heard it pronounced Wallasia. So Wallasia or Wallasea, named after, again,
Alfred Russell Wallace. This region was sort of proposed in
(06:40):
the 1920s, which is a faunal transition zone where you
essentially have Australasian fauna. to the east of the zone,
Asian fauna to the west of the zone, and more
of a mix within. Though the western boundary of Wallacea
is still basically Wallace's line, passing between Bali and Lombok,
and in between Borneo and Sulawesi. So we're actually here
(07:03):
creeping a bit back toward that gradual transition idea of
animal ranges that we talked about in part one, though
some of the divisions you see, especially between Bali and Lombok,
are still actually quite striking. So, Rob, I included for
you to look at in our outline here an illustration
from one of the papers I was looking at. The
region highlighted in red and all the islands within that
(07:25):
is what is generally considered Wallasea today.
Speaker 1 (07:27):
Yeah. And on this map here, it looks like a
big old heart, like a big old Valentine. They even
colored it red.
Speaker 2 (07:32):
Happy Valentine's Day. You get some marsupials.
Speaker 1 (07:36):
Yeah. And I was in the eastern part of this
very region.
Speaker 2 (07:39):
Okay. So Ali and Heaney write that in the last
decade before their paper, even more new regions and boundary
modifications have been proposed, and the authors paint a truly
headache-inducing picture of the historical understanding of all this. As
they say in their title, it is a tortuous tangle
of ideas and labels. For example, I'm just going to
(08:01):
read one section of their abstract to give you an idea. Quote,
Wallace's 1863 line is not the one he finally settled
upon in 1910. Its path around Sulawesi was transferred from
the west to the east of the island. Ideally, Huxley's Divide,
and that's referring to Thomas Henry Huxley, who proposed a
(08:22):
modification to where the line goes, Huxley's Divide, 1868, should
carry his name rather than Wallace's. The latter never accepted
the proposition. Leidecker's line of 1896 ought to be labeled
the Heilprin-Leidecker line in recognition of Angelo Heilprin's 1887 contribution.
(08:43):
Concerning transition zones, ideally Wallasea should correspond to its original
1924 description, which incorporated the Philippine islands bar the Palawan group. Notably, though,
a smaller form introduced by Darlington in 1957, used frequently
from 1998 onwards, in which all of the Philippine islands
are excluded, is entrenched within the recent literature, but this
(09:06):
is often without evident justification. It should also be recognized
that the reduced, meaning southern, Wallacea area was effectively defined
by Heilprin in 1887, but was then labeled the Austro-Malaysian
Transition Zone. So it's a mess. This is one reason
why I really enjoyed researching for these episodes, but when
(09:27):
I was reading all this stuff for the last episode,
I was just like, Oh, I can't.
Speaker 1 (09:32):
Yeah. I mean, it's again, it's it's not like if
you take a kangaroo and have it walk over the
Wallace line, it will explode. So there's no it becomes
very difficult to actually test out some of these things.
It's based on a number of observations and vectors.
Speaker 2 (09:48):
Well, these modifications absolutely are based on empirical observations. And
it's not just people kind of jousting around about absolutely
nothing There are reasons based on the different kinds of
fauna that have been observed and the underlying theory about
the geologic causes and so forth. But unfortunately, the theoretical
(10:10):
definition of the Wallace line and Wallacea are kind of
a mess. There have been a few details that have persisted,
but there has not been clear or consistent agreement across
time on where to place the boundaries, whether it's a
line or a zone, what to call those lines and zones,
and what exactly they mean. So you might be wondering
(10:31):
based on that, like, well, if we can't agree on
what we're talking about, is the idea of a faunal
boundary here just useless or is it nonsense? I think
the answer is no. I mean, for one thing, Some
of this confusion is historical, like there have been general
trends in how the idea is getting refined over time.
So there's nothing wrong with that. I mean, obviously ideas
(10:51):
get updated, but some of the confusion is also because
we're trying to figure out what exactly is being proposed here.
And so like you have gotten away from the idea
of a simple boundary line on a map that, you know,
where you have one thing on one side and one
on another and more into say the idea of a
transition zone. And it is absolutely still safe to say
(11:12):
that there is something very interesting with reference to biogeography
that's happening in the islands between Borneo and Bali to
the west and Papua New Guinea to the east, let's say.
And this boundary line or transition zone can tell us
a lot about the history of life on the surrounding
continents and the history of the Earth itself.
Speaker 1 (11:32):
Yeah, and this is what they were trying to figure out, obviously,
in the late 19th century. But as we'll be discussing here,
they didn't have all the information they needed in order
to really understand and to make what we would think
of as a modern theory as to why it was
like this.
Speaker 2 (11:52):
That's right. And so that brings us to the question
of what causes the boundary. We raised this in the
last episode, but we didn't really have time to answer it.
What causes the faunal boundary between the islands of, say,
Bali and Lombok? Why do you get mostly one set
of animals on Lombok and a fairly different set on Bali,
(12:12):
even though the straight between these two islands is just
a few dozen kilometers wide. It's like it's barely any
ocean at all. And also the environment on the two
islands is very similar. So it seems like you would
expect that the animals you find on one to be
the same as the ones you'd find on the other.
Speaker 1 (12:30):
Right, right.
Speaker 2 (12:31):
So to summarize sort of the original mainstream explanation, and
there will be some additions to this as we move
on in this episode, Wallace understood the cause of the
apparent faunal boundary between Bali and Lombok to be a
result of geological history. This is a core insight of biogeography.
(12:52):
Studying where animals live can tell us not only about
the animals, but also about the land. And there are
actually really awesome examples of this. If you get into
the way paleontology has helped interact with the geohistory of Earth,
like fossil organisms from hundreds of millions of years ago
(13:17):
that appeared to live only in Africa and South America.
Speaker 1 (13:21):
Huh?
Speaker 2 (13:23):
Well, one example of this, there's this early Permian reptile
called Mesosaurus that sort of shaped like a little crocodile.
It lived exclusively in freshwater lakes and rivers almost 300
million years ago, and its remains have only been found
in southern South America and southern Africa. Wait a minute.
(13:44):
If it only lived in freshwater, like lakes and rivers,
how did it get from Africa to South America and,
as far as we can tell, nowhere in between? And
then you've got other examples like this, like there's a
Triassic land-based therapsid called Sinonathus, also found in the fossil
record of South America and Africa, among a few other
(14:07):
equally baffling locations like Antarctica. These examples showed, along with
lots of other evidence that we have now, that these
two land masses used to be one, that the range
was one continuous range hundreds of millions of years ago.
and these animals died, their remains were deposited, they were
quickly buried, they became fossilized, and then the land split apart,
(14:31):
and the Atlantic Ocean in between was formed. So that's
one example of how biogeography can tell us not just
about the animals, but also about the land. Coming back
to Wallace, at the time Wallace lived plate tectonics was
(14:55):
not yet an accepted theory. In fact, it wasn't even
really proposed in a form that would be recognizable to
us until I think like the 1910s around then. And
it wouldn't be widely accepted by geologists until the 1960s.
So Wallace and his contemporaries did not know that the
continents and land masses of the earth moved around, splitting
(15:18):
and rejoining on the timescale and of tens or hundreds
of millions of years. But Wallace did know that the
surface of the Earth could change drastically due to other factors,
and one of those was climate. Climate patterns determine Earth's
average temperature and weather, and in instances long ago when
(15:40):
Earth's surface was colder and more of Earth's water was
locked up in polar ice caps, sea levels were lower,
much lower. As a result, many areas that are now
ocean were once dry land, and many parts of the
world that are now islands surrounded by shallow seas were
once continuous land masses, allowing land animals to cross freely
(16:04):
between them, which is how you end up with large
land mammals like tigers and elephants living on what appear
to be remote islands. In most cases, they didn't swim there,
certainly not in stable breeding populations. They crossed on land
at a time when there was a land bridge. And
for the most part, Wallace correctly identified that many islands
(16:27):
east of the line, of the Wallace line, were once
joined as a single land mass with Australia. This land
mass now known as the Paleocontinent Sahul, with islands west
of the line being once being part of a more
continuous landmass with Asia now known as Sunda. However, just
(16:48):
because islands are close to each other does not mean
a land bridge always forms between them when sea levels drop.
What Wallace did not know for sure, but what later
proved perfectly consistent with his observation of the faunal boundary,
is that between Bali and Lombok, there is a deep
(17:10):
ocean trench. The Strait of Lombok is not wide. Again,
it's just a few kilometers wide at the narrowest point,
but it is very deep, meaning that even when sea
levels were at their lowest, a land bridge between the
two islands never formed, and thus there was much less
opportunity for land-based animals to colonize one island from the other.
(17:33):
The reason for the existence of this deep ocean trench,
by the way, brings us back once again to plate tectonics.
This trench is the meeting point between two plates where
one is undergoing subduction, meaning it's being driven down underneath
the other plate at the meeting point. Subduction zones tend
to create deep ocean trenches along with other geologic activity
(17:54):
like earthquakes and volcanic eruptions. So plate tectonics also, it
explains that, the deep ocean trench that keeps the islands apart,
never forming a land bridge. But plate tectonics also helps
explain why there were such different collections of animals on
Sunda and Sahul to begin with. They were once separated
(18:17):
by much more ocean than they are now. Sahul, and again,
this is modern-day Australia and New Guinea and some other
associated islands, this was once part of the same landmass
as what is now Antarctica. And over tens of Millions
of years in isolation from the Afro-Eurasian and American landmasses,
(18:38):
this ancient continent experienced its own distinct branch of evolution,
giving us, for example, marsupials instead of the placental mammals
that you would find on the rest of the globe.
It was only a little more than 30 million years
ago that Sahul actually broke away from the landmass that
would become Antarctica. And so who will, after that slowly
(18:58):
drifted north and then collided with the continental plates of
Southeast Asia, giving us the arrangement of islands that we
see today in Indonesia and the Malay archipelago. And some
of the islands we see, uh, in that archipelago were, uh,
were created by the collision. They were raised up. Um,
but again, at the place where the plates collide, we
(19:19):
get a subduction zone and a deep ocean trench, which
means even though the islands are now close. Land bridges
never form between Bali and Lombok, so even though they
have been close now for millions of years, there is
relatively little interchange of large land animals back and forth. However,
there is some interchange, and as we've discussed already, all
(19:42):
faunal boundaries are to some extent permeable, and studying those
crossover examples can tell us even more about how this
boundary works. I'm going to come back to that in
a bit. But Rob, I think you also had some
good stuff about how this interacts with plate tectonics.
Speaker 1 (19:56):
Yeah, and just getting a little bit into a little
more into just sort of the history of Wallace and
Darwin in trying to figure out why the Wallace line
seemed to be a thing. How this boundary worked and
what were the factors behind it. Again, as you mentioned,
we didn't have continental drift in a sort of crystallized
form until 1912, and we didn't have plate tectonics until
(20:18):
the 1960s, again, in its more crystallized accepted form. At
the time, a leading approach to understanding all this was
a theory of continental extensionism, which Radical by Nature author
James T. Costa describes as land bridges on steroids. So,
you know, we've been talking here about the concept of
(20:40):
land bridges and how they work. And again, they're certainly real.
The Isthmus of Panama is a current land bridge. Lost
land bridges once connected India and Sri Lanka. And a
landmass known as the Bassian Plain once connected Tasmania to
mainland Australia. And of course, we've also talked about the
now submerged North Sea landmass of Doggerland.
Speaker 2 (21:01):
Yeah, we did a couple of episodes on that. Go
back and look them up. They're from sometime last year.
Speaker 1 (21:06):
So land bridges as an explanation for certain regional slash
local examples of geological distribution, that's all good. The problem
with continental extensionism as it was used at the time
was that it leaned heavily on this as the prime
or even the only way that many forms of fauna
had spread, employing actual and likely examples of land bridges,
(21:28):
you know, sunken land masses and shallow seas and but
then adding in other necessary lost land masses and land
bridges as required to fill in the gaps, often with
little or no actual evidence, and even dipping into lost
continent myth-making and pseudoscience in order to get the job done.
Speaker 2 (21:47):
Gotta love a lost continent.
Speaker 1 (21:49):
I mean, we all do. It's a wonderful concept in
myth and in fiction and science fiction. But yeah, this
was especially the case when trying to connect the dots
across abyssal seas. So you'd have respected naturalists of the
day suddenly leaning into the idea of Lemuria, but even Atlantis.
Speaker 2 (22:09):
We may have also talked about them in a series
of episodes we did, I think, starting with the El
Tannin antenna, where we were talking about how people take
anomalous underwater imagery that kind of looks interesting and then
kind of run wild with it, deciding that, oh, this
thing underwater has got to be an alien antenna. And actually,
(22:29):
it's probably a sponge. And this other thing has got
to be evidence of Atlantis when actually it's probably a rock.
Speaker 1 (22:36):
That's right. And we definitely talked about Lemuria. This one's
a little different compared to Atlantis. It was proposed in
1864 by zoologist Philip Sclater to explain the presence of
lemur fossils, thus Lemuria, on Madagascar and in India, but
not in Africa or the Middle East. So you can
see how Lemuria is very much about filling in that gap.
(22:59):
saying these creatures had to get from one side of
the world as we know it to another. How did
they do it? There had to be a land bridge,
but not just any land bridge. There had to be
a lost continent right there in the middle.
Speaker 2 (23:11):
That's funny because it would be trying to explain actually
what is evidence for continental drift or plate tectonics.
Speaker 1 (23:18):
Right, right. So initially it's just, it was an idea to,
Lemuria was just a way to hypothetically fill in the
blanks here using continental extensionism. to explain everything. And other
individuals like biologist Ernst Haeckel considered the idea that human
origins might tie to this lost continent as well. But
(23:38):
then in the late 19th century, occultists began to appropriate
the concept. And Lemuria, of course, dies as a scientific
hypothesis in the 1960s, but refuses to die in occultism,
conspiracy theory, and of course, fiction. You can look to
various examples, such as the fiction of Robert E. Howard,
the fiction of Lynn Carter. You'll find Lemuria showing up
(24:01):
as a location for various fantastic magical adventures.
Speaker 2 (24:05):
It's a great place to have a leather diaper barbarian
face some psychic monsters.
Speaker 1 (24:10):
Yes. So continental extensionism, this was a concept that Wallace
himself was rather taken with early on. Again, it seemed
a possible way to explain geological distribution of fauna and
explain some of what he was seeing. Though obviously, as
we've also discussed, he clearly wasn't opposed to viewpoints outside
(24:31):
of the scientific mainstream. Again, he was very interested in
spiritualism and defended spiritualism and so forth. So it's not
a stretch to imagine that he wouldn't have strong objections
to some of these concepts, which again, at the time,
were not like firmly wrapped up in a lot of
occultist dreaming and was more firmly within the realm of
(24:54):
possible scientific explanations for the world.
Speaker 2 (24:57):
That's right. And again, to clarify, we talked about this
last time, but spiritualism is different than what people mean
when they say, like, I'm spiritual today. Spiritualism then meant
the belief that you could make contact with and communicate
with the dead or with spirits other than living humans.
And that kind of contact was exciting to a lot
of people who were in the sciences. They were like, hey, look,
(25:18):
here's something we can study empirically. Let's study it. Let's
take notes.
Speaker 1 (25:23):
And this is another place where we kind of get
into the butting heads of Darwin and Wallace. So again,
these two were not enemies. They wrote to each other.
They seemed friendly with each other. And Wallace looked up
to Darwin. But Darwin really hated all of this. He
hated that naturalists would even flirt with the idea of
Atlantis and And Darwin believed that over the course of
(25:46):
life's history on Earth, wind, wing, flotation, these were sufficient
to explain everything that we saw. In fact, Darwin favored
the permanence of ocean basins and continents in his calculations.
So not to say he thought everything was set in stone.
You know, he recognized that there would be regional uplift
and subsidence. For example, this was a key part of
(26:08):
his understanding of how reefs and atolls formed. We've talked
about that on the show before. But, you know, he
was he was not keen on the idea that there
were lost continents that would have connected one landmass to
another and didn't think they were necessary. More to the point.
(26:29):
And this is all interesting because everyone here had some
really good ideas going on. But those ideas had to
ultimately be overclocked to make up for what we had
yet to settle on regarding continental drift and plate tectonics.
So again, land bridges are a local slash regional reality,
but you can't apply them globally. It's not a solution
(26:52):
to every problem in the distribution of fauna. Right.
Speaker 2 (26:56):
Land bridges explain a lot, but they don't explain how
life forms got to Easter Island or to Hawaii, for
that matter.
Speaker 1 (27:02):
Right, right. And wing wind and flotation can get the
job done over the fullness of time, sometimes in remarkable ways,
but there are limitations to that as well. But there
are also some pretty amazing examples. I'm reminded of my
recent interview with Tom Lathan about his book, Lost Wonders,
regarding a number of species that wind up in rare,
far-flung places, including both the Galapagos tortoise, which that's, as
(27:27):
we've discussed in the show before, that's an example of
flotation in action. But then you have the now extinct St.
Helena olive of St. Helena Island in the South Atlantic.
The olive, as listeners might remember, possibly became established there
by dropping off of an albatross. So it's one of
these things that was extremely unlikely to occur, but unlikely
(27:50):
rare events do occur over the course of geological time.
So Wallace did eventually come around to a different understanding
and wrote the 1880 book, Island Life, in which he
sets aside really any thought of lost continents in favor
of kind of a combo of both land bridges and
long-distance dispersal via wind, wing, and flotation. And so in this,
(28:14):
it brought his line of thinking closer to Darwin's own,
but ultimately leaning on a combination of factors.
Speaker 2 (28:21):
And again, he was aware of the idea of past
sea level changes due to changes in how much of
Earth's water is locked up in ice.
Speaker 1 (28:29):
Absolutely. And again, those deep ocean trenches, though, would have
just been too deep to have been impacted by these
sea level drops, you know, I mean, sea level can
only drop so much and you're still going to have
to contend with trenches at that point.
Speaker 2 (28:51):
So I was looking for some more recent papers on
the Wallace line, and I came across one that I
thought was pretty interesting. This was a 2023 study published
in the journal Science by Skeels et al. called Paleoenvironments
Shaped by Exchange of Terrestrial Vertebrates Across Wallace's Line. The
authors in this paper start off by establishing a concept
(29:15):
called biotic interchange. This is what happens when the flora
and fauna of two previously separate regions are suddenly allowed
to move into each other. They're allowed to freely colonize
one another's regions. Examples of this usually occur when a
barrier to travel between two places is suddenly removed, and
(29:37):
suddenly here would be a relative modification. suddenly on geologic time.
One example of this is the American Interchange, where land
and freshwater animals from North and South America were suddenly
able to migrate across into each other's territory and After, Rob,
you mentioned this earlier, the idea of the Panama Isthmus. Oh, man,
(29:58):
that word is so hard to say. Isthmus for me.
It's difficult. After the Panama Isthmus was raised by volcanic activity.
So that came up from the seafloor. And previously there
had been a saltwater barrier between North and South America
for some time. But you have this volcanic activity. Now
there's a land bridge in Central America that happens between
(30:19):
two and three million years ago. And now there's all
this exchange of life forms across the barrier, or across
the former barrier. Another one of these great biotic mingling
events is the Indo-Australian Interchange, which arose when the Australian
and Eurasian tectonic plates smashed together, forming some islands in
(30:41):
between within the region we were talking about earlier, known
as Wallasea. Now, we've already talked about how there are
some barriers to exchange along this connection process. The extent
of that barrier to cross colonization was the original observation
of the Wallace line. It's like what Wallace observed is, hey,
there's not a lot of animal species going across here.
(31:02):
Unlike the American interchange, there there is not and was
never a full land bridge between Sunda and Sahul. But
as Wallace also observed, all boundaries are permeable. And we
now know a good bit of exchange does happen across
this this line. And there are some more faunally mixed
(31:23):
transition zones in between in the islands of Wallacea. But
the authors of this paper point out something interesting. Biotic
interchanges are rarely symmetrical. They're rarely totally even. Usually you
have more life flowing in one direction across the new
corridor than flowing in the other direction. Why would that be?
(31:44):
Why the asymmetry? Some answers lie in characteristics of the organisms.
Maybe some organisms are just more adaptable. They're better at
dispersing into new environments. Maybe they're better at evolving more
quickly and changing what they're adapted to. But some answers,
on the other hand, might lie in characteristics of the
land and environment on either side of the new corridor
(32:08):
or within the corridor itself. What if something about the
geography favors dispersal one way but not the other? And
to be clear here, the authors do not argue that
Wallace was wrong about the deep ocean trench in the
Lombok Strait being a barrier to dispersal.
Speaker 1 (32:26):
It is.
Speaker 2 (32:26):
It does stop a lot of exchange. And they note
that the only mammals which seem to have successfully crossed
that boundary naturally are bats and rodents. Big mammals seemingly
do not cross. But of the animals that have crossed,
one thing that is clear is which way the asymmetry goes.
Over the last 20 million years, way more animals from
(32:50):
Asia have successfully crossed the barrier and made their way
into New Guinea and onto the Australian continent. Relatively little
dispersal has gone the other direction, from Australia and New
Guinea to the west into Asia. Why would that be? Well,
the authors argue, based on analysis of a huge dataset
(33:11):
and a sophisticated computer model, that the answer has to
do with the climate of the islands within and on
either side of Wallacea, especially how wet or dry those
islands are and the precipitation tolerance of the animals that
evolved on either side. So to quote the authors, quote, Here,
(33:32):
analysis of more than 20,000 vertebrate species with a model
of geoclimate and biological diversification shows that broad precipitation tolerance
and dispersal ability were key for exchange across the deep-time
precipitation gradient spanning the region. Sundanian, meaning Southeast Asian lineages,
evolved in a climate similar to the humid, quote, stepping
(33:55):
stones of Wallasea. facilitating colonization of the Sahulian, meaning Australian,
continental shelf. By contrast, Sahulian lineages predominantly evolved in drier conditions,
hampering establishment in Sunda and shaping faunal distinctiveness. So, in
other words, the islands in the transition zone here are
(34:19):
typically full of tropical rainforests, like you would find further
up on the Malay Peninsula and the Asian mainland. New Guinea,
on the same tectonic plate with Australia, is also dominated
with tropical rainforest. So animals adapted to Asian tropical rainforests
would find generally familiar and tolerably wet climates throughout most
(34:42):
of the islands that formed these stepping stones leading to
Australia and New Guinea. And once they reached Australia, there
would be different conditions. The continent would be much drier.
But they could have more evolutionary time to adapt by
colonizing the islands along the way or by landing in
the friendly climate of New Guinea. Meanwhile, animals adapted to
(35:06):
the dry climate of Australia would have a quite difficult
time finding hospitable conditions along the humid stepping stones. And
once you get to the Asian mainland, it's still just
humid and dominated by tropical rainforests. So because the transitional
islands in between were more easily tolerated by the Asian animals,
(35:27):
more Asian animals flowed into the land masses of the
Australian plate than the other way around. And I thought
that was really interesting because it, I don't know if
it often occurs to us to think when we think
about like animals dispersing from one region into another, we
think about the, the sort of end stage reach, like
(35:47):
they start here and then they end up here. But
the place that they're having to move through as they
migrate also plays a big role in whether that migration
can take place at all.
Speaker 1 (35:59):
Yeah, that's a great point.
Speaker 2 (36:01):
I was trying to think of a good analogy, and
this doesn't exactly work, but to grossly oversimplify. What if
there were a highway suddenly opened across the Atlantic Ocean
between Great Britain and the United States, and you could
cross it by car? Do you think you'd have more
people crossing one way or another? I mean, in reality,
(36:22):
there would be a lot of things influencing this. But
you could imagine that which side of the highway you
have to drive on on the highway going in between
might determine a lot about who decides to make the
journey more often. Like if you got to be if
you got to be on the left side of the road,
I can imagine a lot of Americans saying, I'm not
driving that far on the left side. I don't know
how to do that. So if we were driving on
(36:43):
the left side, maybe it would be a motorway and
not a highway. I'm not sure.
Speaker 1 (36:46):
Yeah, yeah. But I see what you're going for here.
Like the side of the car that you drive upon
is kind of like the environmental conditions of the passage. Yeah. Yeah.
And how conducive drivers on either side are going to
be to that passage. Yeah.
Speaker 2 (37:01):
It's not just how well adapted you already are to
the end point. It's like how well adapted you are
to each little step along the way.
Speaker 1 (37:08):
Yeah. Yeah.
Speaker 2 (37:10):
And in the case of Wallacea, it proved easier for
the humid adapted Asian mainland fauna to make the journey into,
especially into New Guinea.
Speaker 1 (37:22):
Now, as we begin to reach the end of this episode,
I want to bring it back a little bit to
my travel experience in Indonesia, where I actually visited these
stepping stones or some of these stepping stones we've been
talking about. So again, I was east of the Wallace
line in Raja Ampat. which situated us also in an
(37:42):
area that is often called the Coral Triangle. Joe, I've
included a map of the Coral Triangle for you here.
This one isn't heart-shaped, and it's also not really triangle-shaped.
It's located between the Pacific and Indian Oceans and encompasses
portions of two biogeographic regions, so the Indonesian Philippines region
(38:04):
and the far southwestern Pacific region. If it looks like anything,
it looks more like a rough sketch of a fish.
Speaker 2 (38:11):
Yeah, I can see that. It's a psilocybin triangle.
Speaker 1 (38:15):
Yeah, yeah. And so why do we call it the
coral triangle? Well, because it is super rich in coral.
According to the Coral Reef Alliance, this region contains over 76%
of the world's coral species and 37% of reef fish species.
So we're talking about somewhere around 500 of the 840
(38:38):
coral species that we know of. So just a rich
hotbed of marine biodiversity. There's a reason that snorkelers and
divers from around the world seek it out. There's just
breathtaking abundance there. I've never experienced snorkeling like this before.
Unless I go back there, I'm not sure I will again.
Speaker 2 (38:57):
Well, as I've said, I have no experience snorkeling myself,
but I'm still envious of what you got to see there.
But this does raise the question, Does the Wallace line
hold at all for underwater fauna? We know that it
holds to some extent for land mammals, but you might
(39:17):
think intuitively, well, surely there would be no way that
the Wallace line would have any impact on, say, fish
or other things that can swim through. They can just
swim right across the strait, right?
Speaker 1 (39:26):
Right. You would think, well, the fish just have a free-for-all,
and so they don't have to obey the rules of
the Wallace line, and neither do the birds. But this
is not the case. Despite the fact that we're under
the water here, we're still subject to some of the
boundary effects of deep ocean channels. So these deep trenches
are seem to act as persistent barriers that prevent the
(39:49):
widespread migration of many coral larvae and many other marine
invertebrates as well. Additionally, there's the Indonesian throughflow, which moves
massive amounts of water from the Pacific to the Indian Ocean.
And so it plays a crucial role here as well.
It'll transport some larvae, but also acts as a barrier
(40:10):
to others. influencing the genetic flow between populations of coral
on either side of the line. So some coral species
have wide Indo-Pacific ranges and can be found on both sides,
but others follow distribution patterns that are more in line
with some of the terrestrial animals that are impacted by
the Wallace line. And we also see this play into
(40:31):
distinct species. For instance, there are two distinct subpopulations of
the Indo-Pacific leopard shark, and they are separated by the
Indonesian throughflow current, which again coincides roughly with the Wallace
line here. For instance, the Missoula Foundation, which runs the
Missoula Resort in Raja Ampat, they are part of a
(40:52):
Star Project ReShark program to help reestablish the eastern subpopulation
of the species in Raja Ampat. Meanwhile, there's another program,
Star Project Thailand, focused on the western variety. So one
variety of Indo-Pacific leopard shark on one side and then
another variety on the other side.
Speaker 2 (41:12):
Wow. I mean, even though we've established that the line
is not impermeable and sometimes animals do cross, uh, it,
I don't know. It's fascinating how, just like how much
of a barrier there could be. That's just totally invisible
to us. And, and even, even affects creatures. You would
think that it, that it couldn't like many underwater creatures.
Speaker 1 (41:33):
Yeah. That we can read it in the coral. We
can read it in the, uh, the leopard sharks and, Yeah,
it's fascinating. The powerful interplay of division and mixing between
rich ecosystems here. Another interesting thing about the leopard shark
program there is that they were having to get, you know,
(41:53):
you wanted to make sure that you were using the
right variety of Indo-Pacific leopard shark to repopulate the region
there in Raja Ampat. So they had to reach out
to various aquariums to get surplus eggs from Indo-Pacific leopard
sharks of the right variety. And one of the aquariums
is actually the Georgia Aquarium here in Atlanta. So it
(42:17):
was entirely, I don't know for certain, but it was
entirely possible that some of the baby leopard sharks that
I saw in the tanks that were then going to
be gradually reintroduced into the wild, perhaps were descendants of
eggs that were produced all the way back home in Atlanta.
Speaker 2 (42:34):
Circle of life.
Speaker 1 (42:35):
Yeah. circle of life with leopard shark eggs taking international
flights for days across the Earth, which in a way
is interesting because it brings it back into the vast
way that humans end up moving around. And of course,
we've talked before on the show about how the movements
of humans has, of course, been another huge factor in
(42:57):
the way other organisms are intentionally and unintentionally spread around
the world.
Speaker 2 (43:03):
Yeah, of course.
Speaker 1 (43:04):
But occasionally we get strategic about it. We attempt to
fix things that we had broken before because, you know,
there's a reason that sharks are missing, as we've discussed before,
or shark populations are greatly decreased in various areas. And
it's because we have hunted them. We fish for them.
We have feared them and so forth. And in doing so,
(43:25):
you know, we end up damaging the ecosystem that they
were a part of because they're Those apex predators are
essential to the overall structure of things. And also, as
we've discussed in the show, their place is fragile at
the top. Yeah. All right.
Speaker 2 (43:41):
Well, I have enjoyed this journey with Alfred Russell Wallace.
Speaker 1 (43:44):
Yeah. Like I say, a fascinating individual. If you want
to read more about him, that book Radical by Nature
by James T. Costa is quite good. And there are
a number of other books as well that have come
out in over the recent decades that about his work.
There's kind of been a resurgence of interest in Alfred
Russell Wallace.
Speaker 2 (44:03):
I enjoyed so much just reading Wallace's own writing in
the Malay Archipelago that I may want to come back
and plug that, plumb that maybe for some future topics.
Speaker 1 (44:11):
Yeah, yeah. Plus, yeah, he wrote many, many books on
a number of topics. So, yeah, he may pop up
again in a future.
Speaker 2 (44:18):
Episode of Stud to Play Your Mind.
Speaker 1 (44:20):
All right. We're going to go ahead and close it
up there. But we'd love to hear from everyone out
there if you have thoughts about the Wallace line and
about some of the organisms that we've discussed here. Just
a reminder, the Stuff to Blow Your Mind is primarily
a science and culture podcast with core episodes on Tuesdays
and Thursdays, short form episode on Wednesdays. And on Fridays,
we set aside most serious concerns to just talk about
a weird film on Weird House Cinema.
Speaker 2 (44:41):
Huge thanks, as always, to our excellent audio producer, J.J. Posway.
If you would like to get in touch with us
with feedback on this episode or any other, to suggest
a topic for the future, or just to say hello,
you can email us at contact at stufftoblowyourmind.com.
Speaker 1 (44:55):
Stuff to Blow Your Mind is a production of iHeartRadio.
Speaker 2 (45:05):
For more podcasts from iHeartRadio, visit the iHeartRadio app. Apple
Podcasts are wherever you listen to your favorite shows.
Speaker 1 (45:18):
Thank you.
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