WEBVTT - The Wallace Line, Part 2 0:00:03.040 --> 0:00:06.760 Welcome to Stuff to Blow Your Mind production of iHeartRadio. 0:00:12.920 --> 0:00:14.480 Hey A, you welcome to Stuff to Blow your Mind. 0:00:14.560 --> 0:00:15.880 My name is Robert Lamb. 0:00:16.079 --> 0:00:18.560 And I am Joe McCormick. And we're back with Part 0:00:18.600 --> 0:00:22.520 two in our series on the nineteenth century British naturalist 0:00:22.640 --> 0:00:27.080 Alfred Russell Wallace and on the Wallace Line, the faunal 0:00:27.160 --> 0:00:31.440 boundary in the Malay Archipelago that bears his name. If 0:00:31.480 --> 0:00:33.400 you haven't heard part one yet, I would recommend you 0:00:33.440 --> 0:00:35.320 go back and listen to that one first. But in 0:00:35.760 --> 0:00:38.960 part one we started off with a general character sketch 0:00:39.000 --> 0:00:42.840 of Wallace. He's a man of many adventurers and many opinions, 0:00:43.600 --> 0:00:46.599 best known today for being the other guy who discovered 0:00:46.640 --> 0:00:50.080 evolution by natural selection. He came up with a slightly 0:00:50.080 --> 0:00:53.519 different version of the theory of natural selection around the 0:00:53.600 --> 0:00:58.920 same time Darwin did, just some sort of differences of emphasis. Basically, 0:01:00.040 --> 0:01:04.320 Zarwin's writings ultimately proved more influential in convincing his peers 0:01:04.319 --> 0:01:07.720 on the reality of common descent and on articulating the 0:01:07.760 --> 0:01:11.480 mechanisms by which species evolved. Wallace was also something of 0:01:11.520 --> 0:01:13.760 a celebrity at his time. It's not like one of 0:01:13.760 --> 0:01:17.119 those tragic cases of somebody, you know, somebody else who 0:01:17.319 --> 0:01:19.560 also came across a great idea but was just like 0:01:19.680 --> 0:01:23.440 totally forgotten. Wallace was kind of a celebrity, especially because 0:01:23.440 --> 0:01:26.399 of the quality of his writing, and that would be 0:01:26.440 --> 0:01:30.479 in books like The Malay Archipelago, published in eighteen sixty nine, 0:01:30.920 --> 0:01:34.520 in which he vividly described his earlier travels in that 0:01:34.600 --> 0:01:38.240 region of the world, including lots of very well observed 0:01:38.280 --> 0:01:41.160 biological and cultural detail. And we read a bunch of 0:01:41.240 --> 0:01:44.360 selections from that book in the last episode, and they 0:01:44.360 --> 0:01:46.440 are kind of it's somewhat magical to read. 0:01:47.400 --> 0:01:50.400 Yeah. Absolutely, again, you can see why this book was 0:01:50.400 --> 0:01:53.280 such a success, and it was read by other naturalists, 0:01:53.280 --> 0:01:56.520 but also just members of the general public who are 0:01:56.560 --> 0:01:59.520 interested in the topic, interested in far away lands and 0:01:59.600 --> 0:01:59.880 in the net. 0:02:00.760 --> 0:02:03.520 Yeah. So we read some selections from the chapters where 0:02:03.520 --> 0:02:06.640 he goes exploring, initially against his will, by the way, 0:02:07.440 --> 0:02:10.960 on the islands of Bali and Lombach. And this included 0:02:11.000 --> 0:02:14.720 everything from these evocative descriptions of the land itself, talking 0:02:14.760 --> 0:02:17.240 about the terrorist agriculture and the way the fields are 0:02:17.240 --> 0:02:22.040 irrigated and all that, two discussions about the geographic distribution 0:02:22.160 --> 0:02:25.560 of ghosts to complaints about how hard it is to 0:02:25.639 --> 0:02:29.720 do science when everything smells putrid and discovered in ants. 0:02:29.639 --> 0:02:32.200 Right now, To be clear, it was putrid smelling because 0:02:32.360 --> 0:02:35.480 of all the birds skinning gsty This is not a 0:02:35.560 --> 0:02:39.920 general comment on the region or the people's there. This 0:02:40.120 --> 0:02:40.720 was his nest. 0:02:40.919 --> 0:02:43.240 No, no, it was not like Lombox smells bad. It 0:02:43.280 --> 0:02:48.920 was my room smells bad. Yea yeah. Also we talked 0:02:48.919 --> 0:02:52.720 about the concept of biogeography, the study of what lives, 0:02:52.800 --> 0:02:57.160 where and why. Wallace is considered a very important founding 0:02:57.200 --> 0:03:00.840 figure in biogeography, with one of his most enduring observations 0:03:01.040 --> 0:03:04.840 being what is now called in his honor, the Wallas Line, 0:03:04.919 --> 0:03:08.560 an invisible boundary passing in the ocean between the islands 0:03:08.560 --> 0:03:13.200 of originally Borneo and Sulawesi, and even more astonishingly in 0:03:13.280 --> 0:03:17.919 the tiny narrow strait between Bali and Lombach, which marks 0:03:17.960 --> 0:03:23.840 the westernmost boundary of a lot of characteristic Australasian fauna marsupials, 0:03:23.960 --> 0:03:27.079 cockatoos and things like that. So we are back today 0:03:27.120 --> 0:03:30.679 to talk more about Wallace and the Wallas Line. Now, 0:03:30.760 --> 0:03:32.880 one thing I think we've got to do before we 0:03:32.919 --> 0:03:36.280 move on any further is just a bit of clean up, because, 0:03:36.400 --> 0:03:40.320 as I mentioned in the last episode, our model of 0:03:40.360 --> 0:03:45.320 the Wallas line has undergone some major revisions since Wallace 0:03:45.400 --> 0:03:48.840 first proposed it in the mid nineteenth century. I'm not 0:03:48.880 --> 0:03:51.080 going to do a whole detailed play by play of 0:03:51.120 --> 0:03:53.960 the shifting history of the line, but for a brief 0:03:53.960 --> 0:03:57.560 summary of developments, I was just looking at the abstract 0:03:57.560 --> 0:04:02.520 of a paper called Wallace's Line WALLACEI and Associated Divides 0:04:02.600 --> 0:04:07.480 and Areas History of a tortuous tangle of ideas and labels. 0:04:07.800 --> 0:04:10.840 This is by Ali and Heene, published in the journal 0:04:10.880 --> 0:04:14.360 Biological Reviews in twenty twenty one. So the authors go 0:04:14.480 --> 0:04:17.280 through the concept which we discussed in the last episode, 0:04:17.279 --> 0:04:20.160 the idea of a faunnel boundary being basically a place 0:04:20.160 --> 0:04:24.000 where you find animals mainly of one sort on one 0:04:24.040 --> 0:04:26.280 side of the line and animals mainly of a different 0:04:26.360 --> 0:04:28.800 sort on the other side of the line. And this 0:04:28.920 --> 0:04:32.400 raises questions how did it end up like that? The 0:04:32.440 --> 0:04:35.840 authors write that the earliest dividing lines in this region 0:04:35.880 --> 0:04:39.839 were considered quite sharp, especially the line Wallace drew in 0:04:39.960 --> 0:04:43.320 eighteen sixty three, and this was based on looking at 0:04:43.320 --> 0:04:47.159 the distribution of land mammals and some birds. This is 0:04:47.279 --> 0:04:49.839 the line we've been talking about so far, passing between 0:04:49.880 --> 0:04:53.800 Bali and Lombach, with Asian animals found west from the 0:04:53.800 --> 0:04:57.160 west coast or from the eastern coast of Bali and 0:04:57.360 --> 0:05:03.320 Australasian animals running east from Lombak. This particular theoretical boundary 0:05:03.400 --> 0:05:06.599 proved very influential and it got you know, reproduced in 0:05:06.640 --> 0:05:08.960 a lot of texts. It was baked into maps and 0:05:09.000 --> 0:05:11.440 plates and stuff all over so like it had a 0:05:11.440 --> 0:05:16.280 big cultural footprint, and within the discipline of biogeography it 0:05:16.360 --> 0:05:20.279 came to be seen as something of a fixed marker 0:05:20.360 --> 0:05:22.840 in nature, maybe like the tree line on a mountain. 0:05:23.560 --> 0:05:27.200 But over time it also became obvious to experts in 0:05:27.240 --> 0:05:30.000 the field that there were a lot of exceptions to 0:05:30.040 --> 0:05:33.440 the Wallace line, especially a lot of Asian fauna found 0:05:33.560 --> 0:05:37.000 east of the so called line. And this would not 0:05:37.080 --> 0:05:40.000 have been very surprising to Wallace himself, I think, who 0:05:40.080 --> 0:05:44.160 argued that biogeographic boundaries were all to some extent permeable, 0:05:44.880 --> 0:05:46.880 But it might have been more surprising to people who 0:05:47.000 --> 0:05:49.320 got a you know, a map printed with a line 0:05:49.360 --> 0:05:53.320 on it that says animals do not cross this line. Yeah. 0:05:53.600 --> 0:05:57.520 By later in the nineteenth century, many biogeographers had started 0:05:57.560 --> 0:06:01.760 to think about zones instead of life. For example, there 0:06:01.839 --> 0:06:05.760 is now a biogeographic region known as Wallace Sea. I've 0:06:05.760 --> 0:06:09.760 also heard it pronounced to Wallasia, so Wallasia or Wallacea 0:06:09.880 --> 0:06:13.520 named after again Alfred Russell Wallace. This region was sort 0:06:13.560 --> 0:06:17.039 of proposed in the nineteen twenties, which is a faunal 0:06:17.240 --> 0:06:21.719 transition zone where you essentially have Australasian fauna to the 0:06:21.760 --> 0:06:24.400 east of the zone, Asian fauna to the west of 0:06:24.440 --> 0:06:27.760 the zone, and more of a mix within. Though the 0:06:27.800 --> 0:06:32.280 western boundary of Wallacea is still basically Wallace's line, passing 0:06:32.279 --> 0:06:35.760 between Bali and Lombach and in between Borneo and Sulawesi. 0:06:36.080 --> 0:06:39.080 So we're actually here creeping a bit back toward that 0:06:39.240 --> 0:06:42.320 gradual transition idea of animal ranges that we talked about 0:06:42.360 --> 0:06:45.480 in part one. Though some of the divisions you see, 0:06:45.560 --> 0:06:49.359 especially between Bali and Lombach, are still actually quite striking, 0:06:49.680 --> 0:06:52.040 So Rabbi included for you to look at in our outline. 0:06:52.080 --> 0:06:54.480 Here an illustration from one of the papers I was 0:06:54.520 --> 0:06:57.599 looking at, the region highlighted in red, and all the 0:06:57.640 --> 0:07:01.120 islands within that is what is generally considered Wallacea today. 0:07:01.960 --> 0:07:03.440 Yeah, and on this map here it looks like a 0:07:03.440 --> 0:07:05.320 big old heart, like a big old Valentine. They even 0:07:05.320 --> 0:07:05.760 colored it. 0:07:05.760 --> 0:07:08.360 Red Happy Valentine's Day. You get some marsupials. 0:07:09.520 --> 0:07:11.880 Yeah, and I was in the eastern part of this 0:07:12.080 --> 0:07:12.640 very region. 0:07:12.880 --> 0:07:17.840 Okay. So Ali and Heeney write that in the last 0:07:17.880 --> 0:07:21.480 decade before their paper, even more new regions and boundary 0:07:21.480 --> 0:07:24.960 modifications have been proposed, and the authors paint a truly 0:07:25.080 --> 0:07:28.880 headache inducing picture of the historical understanding of all this. 0:07:28.960 --> 0:07:31.240 As they say in their title, it is a tortuous 0:07:31.320 --> 0:07:35.160 tangle of ideas and labels. For example, I'm just going 0:07:35.240 --> 0:07:37.240 to read one section of their abstract to give you 0:07:37.320 --> 0:07:42.200 an idea quote. Wallace's eighteen sixty three line is not 0:07:42.440 --> 0:07:46.160 the one he finally settled upon in nineteen ten. Its 0:07:46.200 --> 0:07:49.680 path around Sulawesi was transferred from the west to the 0:07:49.720 --> 0:07:53.600 east of the island, ideally Huxley's divide, And that's referring 0:07:53.640 --> 0:07:57.240 to Thomas Henry Huxley, who proposed a modification to this 0:07:57.720 --> 0:08:01.400 to the where the line goes. Hucks divide eighteen sixty 0:08:01.440 --> 0:08:05.080 eight should carry his name rather than Wallace's. The latter 0:08:05.240 --> 0:08:09.360 never accepted the proposition. Lydacer's line of eighteen ninety six 0:08:09.480 --> 0:08:12.480 ought to be labeled the hil Prin Lydacer line in 0:08:12.560 --> 0:08:18.920 recognition of Angelo Hilprin's eighteen eighty seven contribution concerning transition zones. Ideally, 0:08:18.960 --> 0:08:23.000 Wallacea should correspond to its original nineteen twenty four description, 0:08:23.360 --> 0:08:28.280 which incorporated the Philippine Islands bar the Palawan group. Notably, 0:08:28.320 --> 0:08:31.760 though a smaller form introduced by Darlington in nineteen fifty 0:08:31.800 --> 0:08:34.840 seven used frequently from nineteen ninety eight onwards, in which 0:08:34.880 --> 0:08:37.960 all of the Philippine Islands are excluded, is entrenched within 0:08:38.040 --> 0:08:41.800 the recent literature, but this is often without evident justification. 0:08:42.440 --> 0:08:46.160 It should also be recognized that the reduced meaning southern 0:08:46.280 --> 0:08:50.839 Wallacea area was effectively defined by hilprint in eighteen eighty seven, 0:08:51.040 --> 0:08:55.800 but was then labeled the Austro Malaysian transition zone. So 0:08:55.960 --> 0:08:58.920 it's a mess. This is one reason why I've really 0:08:59.000 --> 0:09:01.400 enjoyed researching for the episodes. But like when I was 0:09:01.440 --> 0:09:03.280 reading all this stuff for the last episode, I was 0:09:03.360 --> 0:09:05.439 just like, ah, I can't. 0:09:05.920 --> 0:09:08.600 Yeah, I mean it's again, It's it's not like if 0:09:08.640 --> 0:09:10.720 you take a kangaroo and have it walk over the 0:09:10.720 --> 0:09:14.680 Wallas line, it will explode. Like, so, there's no it 0:09:14.720 --> 0:09:19.080 becomes very difficult to actually test out some of these things. 0:09:18.840 --> 0:09:21.600 It's based on a number of observations and factors. 0:09:22.080 --> 0:09:25.960 Well, these modifications absolutely are based on empirical observations. I mean, 0:09:26.040 --> 0:09:29.960 it's not just people kind of jousting around about absolutely nothing. 0:09:30.080 --> 0:09:34.640 Like there are reasons based on the different kinds of 0:09:34.760 --> 0:09:37.440 fauna that have been observed and the underlying theory about 0:09:37.520 --> 0:09:41.640 like the geologic causes and so forth. But unfortunately, the 0:09:42.320 --> 0:09:47.400 theoretical definition of the Wallace line and Wallace are kind 0:09:47.480 --> 0:09:50.120 of a mess. There have been a few details that 0:09:50.160 --> 0:09:53.960 have persisted, but there has not been clear or consistent 0:09:54.080 --> 0:09:58.240 agreement across time on where to place the boundaries, whether 0:09:58.280 --> 0:10:01.160 it's a line or a zone, to call those lines 0:10:01.160 --> 0:10:04.360 and zones, and what exactly they mean. So you might 0:10:04.360 --> 0:10:06.680 be wondering based on that, like, well, if you know, 0:10:06.760 --> 0:10:09.040 if we can't agree on what we're talking about, is 0:10:09.080 --> 0:10:13.000 the idea of a faunal boundary here just useless or 0:10:13.040 --> 0:10:15.520 is it nonsense? I think the answer is no. I mean, 0:10:15.559 --> 0:10:19.360 for one thing, some of this confusion is historical, Like 0:10:19.400 --> 0:10:21.680 there have been general trends in how the idea is 0:10:21.679 --> 0:10:24.240 getting refined over time. So there's nothing wrong with that. 0:10:24.320 --> 0:10:26.920 I mean, obviously ideas get updated. But some of the 0:10:26.960 --> 0:10:30.000 confusion is also because we're trying to figure out what 0:10:30.200 --> 0:10:33.000 exactly is being proposed here, and so like you have 0:10:33.080 --> 0:10:36.840 gotten away from the idea of a simple boundary line 0:10:36.840 --> 0:10:38.520 on a map that you know, where you have one 0:10:38.520 --> 0:10:40.880 thing on one side and one on another and more 0:10:40.920 --> 0:10:43.680 into say the idea of a transition zone. And it 0:10:43.760 --> 0:10:46.440 is absolutely still safe to say that there is something 0:10:46.760 --> 0:10:50.839 very interesting with reference to biogeography that's happening in the 0:10:50.840 --> 0:10:54.920 islands between Borneo and Bali to the west and Papa 0:10:54.960 --> 0:10:57.720 New Guinea to the east, let's say, and this, this 0:10:57.800 --> 0:11:00.520 boundary line or transition zone can tell us a lot 0:11:00.640 --> 0:11:03.960 about the history of life on the surrounding continents in 0:11:04.000 --> 0:11:05.400 the history of the Earth itself. 0:11:06.160 --> 0:11:07.880 Yeah, and this is what they were trying to figure 0:11:07.880 --> 0:11:12.319 out obviously in the late nineteenth century. But so we'll 0:11:12.320 --> 0:11:16.800 be discussing here they didn't have all the information they 0:11:16.840 --> 0:11:21.120 needed in order to really understand and to make what 0:11:21.120 --> 0:11:23.640 we would think of as a modern theory as to 0:11:23.760 --> 0:11:25.079 why it was like this. 0:11:25.480 --> 0:11:27.600 That's right, and so that brings us to the question 0:11:27.679 --> 0:11:31.640 of what causes the boundary. We raised this in the 0:11:31.720 --> 0:11:34.000 last episode, but didn't really have time to answer it. 0:11:34.360 --> 0:11:37.960 What causes the faunal boundary between the islands of say 0:11:37.960 --> 0:11:42.000 Bali and Lombok. Why do you get mostly one set 0:11:42.040 --> 0:11:45.800 of animals on Lombach and a fairly different set on Bali. 0:11:45.920 --> 0:11:49.080 Even though the strait between these two islands is just 0:11:49.120 --> 0:11:52.440 a few dozen kilometers wide, it's like it's barely any 0:11:52.480 --> 0:11:55.720 ocean at all. And also the environment on the two 0:11:56.080 --> 0:11:58.680 islands is very similar, so it seems like you would 0:11:58.720 --> 0:12:01.400 expect that the animals find on one to be the 0:12:01.440 --> 0:12:04.520 same as the ones you'd find on the other, right, right, 0:12:04.880 --> 0:12:08.680 So to summarize sort of the original mainstream explanation, and 0:12:08.720 --> 0:12:11.200 there will be some additions to this as we move on. 0:12:11.280 --> 0:12:16.000 In this episode, Wallace understood the cause of the apparent 0:12:16.080 --> 0:12:19.880 faunnel boundary between Bali and Lombach to be a result 0:12:20.040 --> 0:12:25.760 of geological history. This is a core insight of biogeography. 0:12:26.000 --> 0:12:30.880 Studying where animals live can tell us not only about 0:12:30.960 --> 0:12:35.320 the animals, but also about the land, and there are 0:12:35.360 --> 0:12:38.960 actually really awesome examples of this. If you get into 0:12:38.960 --> 0:12:44.960 the way paleontology has helped interact with the geohistory of 0:12:45.000 --> 0:12:50.000 Earth like fossil organisms from hundreds of millions of years 0:12:50.040 --> 0:12:56.560 ago that appeared to live only in Africa and South America. Huh. 0:12:56.880 --> 0:13:02.160 One example of this, there's this early Permian reptile called Mesosaurus, 0:13:02.200 --> 0:13:05.200 the sort of shaped like a little crocodile. It lived 0:13:05.280 --> 0:13:09.360 exclusively in freshwater lakes and rivers almost three hundred million 0:13:09.440 --> 0:13:12.679 years ago, and its remains have only been found in 0:13:13.000 --> 0:13:18.480 southern South America and southern Africa. Wait a minute, If 0:13:18.480 --> 0:13:21.679 it only lived in fresh water like lakes and rivers, 0:13:22.200 --> 0:13:25.640 how did it give from Africa to South America and 0:13:25.840 --> 0:13:28.959 as far as we can tell nowhere in between. And 0:13:29.000 --> 0:13:31.320 then you've got other examples like this, like there's a 0:13:31.440 --> 0:13:36.960 Triassic land based therapsid called Syno Nathas, also found in 0:13:37.000 --> 0:13:40.480 the fossil record of South America and Africa, among a 0:13:40.480 --> 0:13:46.199 few other equally baffling locations like Antarctica. These examples showed, 0:13:46.360 --> 0:13:48.839 along with lots of other evidence that we have now, 0:13:49.160 --> 0:13:52.360 that these two land masses used to be one. That 0:13:52.480 --> 0:13:55.760 you know, the range was one continuous range hundreds of 0:13:55.760 --> 0:13:59.160 millions of years ago, and these animals died, their remains 0:13:59.200 --> 0:14:02.880 were posited, they were quickly buried, they became fossilized, and 0:14:02.920 --> 0:14:05.920 then the land split apart and the Atlantic Ocean in 0:14:06.000 --> 0:14:09.439 between was formed. So that's one example of how biogeography 0:14:09.480 --> 0:14:12.400 can tell us not just about the animals, but also 0:14:12.440 --> 0:14:25.760 about the land. Coming back to Wallace. At the time 0:14:25.880 --> 0:14:30.320 Wallace lived, plate tectonics was not yet an accepted theory. 0:14:30.680 --> 0:14:33.840 In fact, it wasn't even really proposed in a form 0:14:33.920 --> 0:14:36.280 that would be recognizable to us until I think, like 0:14:36.320 --> 0:14:39.480 the nineteen tens around them, and it wouldn't be widely 0:14:39.520 --> 0:14:44.520 accepted by geologists until the nineteen sixties. So Wallace in 0:14:44.560 --> 0:14:48.760 his contemporaries did not know that the continents and land 0:14:48.880 --> 0:14:53.320 masses of the Earth moved around, splitting and rejoining on 0:14:53.360 --> 0:14:56.960 the timescale of tens or hundreds of millions of years. 0:14:57.760 --> 0:15:00.600 But Wallace did know that the the surface of the 0:15:00.640 --> 0:15:05.360 Earth could change drastically due to other factors, and one 0:15:05.360 --> 0:15:10.880 of those was climate. Climate patterns determine Earth's average temperature 0:15:10.920 --> 0:15:14.880 and weather, and in instances long ago, when Earth's surface 0:15:15.080 --> 0:15:17.640 was colder and more of Earth's water was locked up 0:15:17.680 --> 0:15:22.040 in polar ice caps, sea levels were lower much lower. 0:15:22.440 --> 0:15:25.800 As a result, many areas that are now ocean were 0:15:25.960 --> 0:15:29.080 once dry land, and many parts of the world that 0:15:29.200 --> 0:15:33.960 are now islands surrounded by shallow seas were once continuous 0:15:34.080 --> 0:15:38.760 land masses, allowing land animals to cross freely between them, 0:15:38.800 --> 0:15:42.040 which is how you end up with large land mammals 0:15:42.080 --> 0:15:45.000 like tigers and elephants living on what appeared to be 0:15:45.160 --> 0:15:49.120 remote islands. In most cases, they didn't swim there, certainly 0:15:49.120 --> 0:15:53.160 not in stable breeding populations. They crossed on land at 0:15:53.160 --> 0:15:56.880 a time when there was a land bridge, and for 0:15:56.960 --> 0:16:01.640 the most part, Wallace correctly identified that many islands east 0:16:01.800 --> 0:16:05.280 of the line of the Wallas line were once joined 0:16:05.560 --> 0:16:09.920 as a single land mass with Australia, this land mass 0:16:09.960 --> 0:16:13.800 now known as the paleocontinent Sahul, with islands west of 0:16:13.840 --> 0:16:16.800 the line once being part of a more continuous land 0:16:16.880 --> 0:16:22.040 mass with Asia, now known as Sunda. However, just because 0:16:22.120 --> 0:16:25.320 islands are close to each other does not mean a 0:16:25.400 --> 0:16:29.120 land bridge always forms between them when sea levels drop. 0:16:30.040 --> 0:16:33.880 What Wallace did not know for sure, but what later 0:16:33.960 --> 0:16:38.160 proved perfectly consistent with his observation of the faunal boundary, 0:16:38.960 --> 0:16:43.440 is that between Bali and Lombach there is a deep 0:16:43.600 --> 0:16:48.000 ocean trench. The Strait of Lombach is not wide again, 0:16:48.040 --> 0:16:50.360 it's just a few kilometers wide at the narrowest point, 0:16:50.600 --> 0:16:53.680 but it is very deep, meaning that even when sea 0:16:53.760 --> 0:16:57.160 levels were at their lowest, a land bridge between the 0:16:57.160 --> 0:17:00.840 two islands never formed, and thus there was much less 0:17:00.880 --> 0:17:05.200 opportunity for land based animals to colonize one island from 0:17:05.280 --> 0:17:09.320 the other. The reason for the existence of this deep 0:17:09.320 --> 0:17:11.960 ocean trench, by the way, brings us back once again 0:17:12.000 --> 0:17:16.000 to plate tectonics. This trench is the meeting point between 0:17:16.080 --> 0:17:19.800 two plates where one is undergoing subduction, meaning it's being 0:17:19.960 --> 0:17:23.040 driven down underneath the other plate at the meeting point. 0:17:23.400 --> 0:17:26.960 Subduction zones tend to create deep ocean trenches along with 0:17:27.000 --> 0:17:31.560 other geologic activity like earthquakes and volcanic eruptions, so plate 0:17:31.560 --> 0:17:35.800 tectonics also it explains that the deep ocean trench that 0:17:35.920 --> 0:17:39.600 keeps the islands apart and never forming a land bridge, 0:17:39.600 --> 0:17:44.400 but it Plate tectonics also helps explain why there were 0:17:44.480 --> 0:17:47.920 such different collections of animals on Sunda and so Whul. 0:17:48.080 --> 0:17:51.639 To begin with, they were once separated by much more 0:17:51.680 --> 0:17:54.959 ocean than they are now Sahul And again, this is 0:17:55.160 --> 0:17:58.800 modern day Australia and New Guinea and some other associated islands. 0:17:59.160 --> 0:18:02.520 This was one part of the same land mass as 0:18:02.560 --> 0:18:06.760 what is now Antarctica, and over tens of millions of 0:18:06.840 --> 0:18:11.160 years in isolation from the Afro Eurasian and American land masses, 0:18:11.560 --> 0:18:16.000 this ancient continent experienced its own distinct branch of evolution, 0:18:16.560 --> 0:18:20.640 giving us, for example, marsupials instead of the placental mammals 0:18:20.640 --> 0:18:22.240 that you would find on the rest of the globe. 0:18:22.840 --> 0:18:25.400 It was only a little more than thirty million years 0:18:25.440 --> 0:18:28.040 ago that sa who will actually broke away from the 0:18:28.119 --> 0:18:31.480 land mass that would become Antarctica, and so who will? 0:18:31.520 --> 0:18:34.960 After that, slowly drifted north and then collided with the 0:18:35.040 --> 0:18:38.879 continental plates of Southeast Asia, giving us the arrangement of 0:18:38.920 --> 0:18:42.320 islands that we see today in Indonesia and the Malay Archipelago. 0:18:42.359 --> 0:18:45.320 And some of the islands we see in that archipelago 0:18:45.359 --> 0:18:49.919 were created by the collision. They were raised up. But 0:18:50.119 --> 0:18:53.160 again at the place where the plates collide we get 0:18:53.200 --> 0:18:56.160 a subduction zone and a deep ocean trench, which means 0:18:56.200 --> 0:18:59.240 even though the islands are now close, land bridge is 0:18:59.280 --> 0:19:02.800 never formed between Bali and Lombach, So even though they 0:19:02.920 --> 0:19:05.800 have been close, now for millions of years. There is 0:19:05.920 --> 0:19:11.360 relatively little interchange of large land animals back and forth. However, 0:19:11.560 --> 0:19:15.640 there is some interchange, and as we've discussed already, all 0:19:15.720 --> 0:19:19.480 faunnel boundaries are to some extent permeable, and studying those 0:19:19.520 --> 0:19:22.479 crossover examples can tell us even more about how this 0:19:22.600 --> 0:19:24.800 boundary works. I'm going to come back to that in 0:19:24.840 --> 0:19:26.480 a bit, but Rob, I think you also had some 0:19:26.520 --> 0:19:29.520 good stuff about how this interacts with plate tectonics. 0:19:29.960 --> 0:19:31.800 Yeah, and just getting a little bit into a little 0:19:31.840 --> 0:19:34.399 more into just sort of the history of Wallace and 0:19:34.520 --> 0:19:39.080 Darwin in trying to figure out why the Wallace line 0:19:39.119 --> 0:19:41.720 seemed to be a thing, how this boundary worked, and 0:19:41.720 --> 0:19:44.320 what were the factors behind it. Again, as you mentioned, 0:19:44.400 --> 0:19:47.160 you know, we didn't have continental drift in a sort 0:19:47.160 --> 0:19:50.720 of crystallized form until nineteen twelve, and we didn't have 0:19:50.760 --> 0:19:53.600 plate tectonics until the nineteen sixties, again in its more 0:19:53.640 --> 0:19:57.480 crystallized accepted form. At the time, a leading approach to 0:19:57.600 --> 0:20:02.760 understanding all this was a theory of continental extensionism, which 0:20:03.119 --> 0:20:07.280 Radical by Nature author James T. Costa describes as land 0:20:07.320 --> 0:20:11.120 bridges on steroids. So you know, we've been talking here 0:20:11.200 --> 0:20:14.680 about the concept of land bridges and how they work, 0:20:14.720 --> 0:20:18.960 and again they're certainly real. The Isthmus of Panama is 0:20:19.119 --> 0:20:22.960 a current land bridge, lost land bridges once connected India 0:20:23.000 --> 0:20:25.760 and Sri Lanka, and a land mass known as the 0:20:25.880 --> 0:20:29.800 Basian Plaine once connected Tasmania to mainland Australia. And of 0:20:29.800 --> 0:20:32.600 course we've also talked about the now submerged north Sea 0:20:32.760 --> 0:20:34.440 land mass of Doggerland. 0:20:34.680 --> 0:20:36.560 Ah. Yeah, we did a couple of episodes on that. 0:20:36.640 --> 0:20:39.120 Go back and look them up. Yeah, sometime last year. 0:20:39.560 --> 0:20:43.360 So land bridges as an explanation for certain regional slash 0:20:43.400 --> 0:20:47.800 local examples of geological distribution, that's all good. The problem 0:20:47.920 --> 0:20:51.320 with continental extensionism as it was used at the time 0:20:51.720 --> 0:20:54.520 was it leaned heavily on this as the prime, or 0:20:54.560 --> 0:20:58.080 even the only way that many forms of fauna had spread, 0:20:58.640 --> 0:21:02.240 employing actual and likely examples of land bridges, you know, 0:21:02.320 --> 0:21:06.520 sunken land masses and shallow seas, but then adding in 0:21:06.600 --> 0:21:10.800 other necessary lost land masses and land bridges as required 0:21:10.840 --> 0:21:14.240 to fill in the gaps, often with little or no 0:21:14.480 --> 0:21:18.639 actual evidence, and even dipping into lost continent myth making 0:21:18.760 --> 0:21:21.080 and pseudoscience in order to get the job done. 0:21:21.280 --> 0:21:22.680 God love a lost continent. 0:21:23.320 --> 0:21:25.440 I mean, we all do. It's a it's a wonderful 0:21:25.480 --> 0:21:30.640 concept in myth and in fiction and science fiction. But yeah, 0:21:30.680 --> 0:21:32.840 this was especially the case when trying to connect the 0:21:32.880 --> 0:21:37.240 dots across like Abystle Seas, so you'd have respected naturalists 0:21:37.240 --> 0:21:40.240 of the days suddenly leaning into well, you know, the 0:21:40.240 --> 0:21:42.520 idea of Lemuria, but even Atlantis. 0:21:43.080 --> 0:21:45.040 We may have also talked about them in a series 0:21:45.080 --> 0:21:49.800 of episodes we did, I think starting with the Eltannan Antenna, 0:21:49.840 --> 0:21:54.200 where we were talking about how people take anomalous underwater 0:21:54.320 --> 0:21:56.800 imagery that kind of looks interesting and then kind of 0:21:56.880 --> 0:21:59.920 run wild with it, deciding that oh, this thing under 0:22:00.119 --> 0:22:03.240 water has got to be an alien antenna, and actually 0:22:03.320 --> 0:22:06.480 it's probably a sponge, and this other thing has got 0:22:06.520 --> 0:22:09.920 to be evidence of Atlantis, when actually it's probably a rock. 0:22:10.160 --> 0:22:13.639 That's right. And we definitely talked about Lemurria. This was 0:22:14.480 --> 0:22:17.639 This one's a little different compared to Atlantis. It was 0:22:17.640 --> 0:22:21.040 proposed in eighteen sixty four by zoologists Phillip Sklater to 0:22:21.160 --> 0:22:25.440 explain the presence of lemur fossils, thus Lmuria on Madagascar 0:22:25.560 --> 0:22:28.760 and in India, but not in Africa or the Middle East. 0:22:28.800 --> 0:22:31.000 So you can you can see how Lumurria is very 0:22:31.080 --> 0:22:34.359 much about filling in that gap, saying, these creatures had 0:22:34.400 --> 0:22:36.480 to get from one side of the world as we 0:22:36.560 --> 0:22:39.080 know it to another. How did they do it? There 0:22:39.119 --> 0:22:41.120 had to be a land bridge, but not just any 0:22:41.160 --> 0:22:44.040 land bridge. There had to be a lost continent right 0:22:44.080 --> 0:22:44.800 there in the middle. 0:22:45.000 --> 0:22:47.360 That's funny because it would be trying to explain, actually, 0:22:47.400 --> 0:22:51.879 what is evidence for continental drift or plate tectonics. 0:22:51.600 --> 0:22:54.919 Right right, So initially it's just it was an idea 0:22:55.000 --> 0:22:58.760 to Lemuria was just a way to hypothetically fill in 0:22:58.760 --> 0:23:04.720 the blanks here continental extensionism to explain everything into other 0:23:05.000 --> 0:23:09.080 Individuals like biologist Ernst Haekel, considered the idea that human 0:23:09.119 --> 0:23:12.119 origins might tie to this lost continent as well. But 0:23:12.200 --> 0:23:15.760 then in the late nineteenth century, occultists began to appropriate 0:23:15.800 --> 0:23:19.360 the concept, and Lumiria of course dies as a scientific 0:23:19.440 --> 0:23:23.800 hypothesis in the nineteen sixties, but refuses to die in occultism, 0:23:24.000 --> 0:23:27.520 conspiracy theory, and of course fiction. You can look to 0:23:27.600 --> 0:23:30.919 various examples such as the fiction of Roberty Howard the 0:23:30.920 --> 0:23:35.080 fiction of Lynn Carter. You'll find Lumiria showing up as 0:23:35.119 --> 0:23:38.560 a location for various fantastic magical adventures. 0:23:38.760 --> 0:23:42.000 It's a great place to have a leather diaper, barbarian face, 0:23:42.080 --> 0:23:43.280 some psychic monsters. 0:23:43.720 --> 0:23:50.359 Yes, so continental extensionism. This was a concept that Wallace 0:23:50.440 --> 0:23:53.800 himself was rather taken with early on. Again, it seemed 0:23:53.840 --> 0:23:58.119 a possible way to explain geological distribution of fauna and 0:23:58.200 --> 0:24:01.160 explain some of what he was seeing. Though obviously, as 0:24:01.160 --> 0:24:04.840