0:01 Imagine
0:03 a lungfish in a pond. It's the dry
0:05 season and the pond is shrinking. It
0:07 will soon disappear. But even though
0:09 this fish is about to be out of water,
0:11 it isn't totally out of luck. When the
0:13 water gets too shallow, the lungfish can
0:15 gulp air at the surface using its lungs.
0:17 And when the pond becomes too small, it
0:19 can use its sturdy fins to crawl across
0:20 the ground in search of a new one. Now,
0:22 these abilities seem almost tailorade
0:24 for surviving on land. You'd think this
0:26 fish was just one evolutionary step away
0:28 from becoming terrestrial. Okay, but
0:30 here's the thing. This lungfish has no
0:32 interest in land. Those terrestrial
0:34 adaptations exist to help it survive
0:36 long enough to get back to the water.
0:39 Over a century ago, one paleontologist
0:40 wondered if the lungfish strategy held
0:42 the key to understanding one of the
0:44 biggest evolutionary moments in history,
0:46 why our own ancestors left the water.
0:48 Maybe he thought the earliest tetropods
0:50 also had no interest in colonizing land.
0:52 And while parts of his hypothesis turned
0:54 out to not hold water, it looks like
0:56 lungfish have something to teach us
0:57 after all. because it's beginning to
0:59 look like our ancestors might have
1:01 originally evolved terrestrial traits in
1:06 order to stay fish.
1:07 Hundreds of millions of years ago, the
1:09 group of animals we belong to, for
1:11 limbmed vertebrates called tetropods,
1:13 conquered the land. They evolved from a
1:15 group of fish called sarcopterigians,
1:16 which include the ancestors of
1:18 tetropods, the tetropotamorphs. And for
1:20 over a century, scientists have been
1:22 trying to solve an important puzzle. How
1:23 and why did the earliest tetropods
1:25 evolve from these ancestors making the
1:28 monumental leap onto land? Through the
1:30 fossil record, we can track what
1:31 happened during this transition, more or
1:33 less. But why it happened has been much
1:35 harder to answer. During the early 20th
1:37 century, researchers contemplating what
1:39 drove our fish ancestors to leave the
1:41 water considered a bunch of ideas. Like
1:43 maybe the fish were escaping from
1:44 predators in the water or taking
1:46 advantage of food resources on land. But
1:49 in 1917, paleontologist Richard Lull
1:51 offered a different idea. He looked at
1:52 lungfish and wondered if scientists had
1:53 been thinking about this problem
1:56 backwards. LOL's idea, later championed
1:58 by paleontologist Alfred Sherwood Romer,
1:59 became known as the shrinking pond
2:02 hypothesis. LOL and Ror pointed to how
2:03 African and South American lungfish
2:05 living in aid seasonal environments use
2:07 a range of adaptations to cope with
2:08 changing amounts of water, like entering
2:10 a dormant state underground to escape
2:12 the dry season, as well as using those
2:14 skills of breathing air and sometimes
2:16 crawling around on land. and they
2:17 pointed out that while those traits
2:19 could be confused for having evolved for
2:21 life on land, their real function was
2:22 just to sustain the fish long enough to
2:24 get back to the water. LOL and ROR
2:26 viewed these fish as possible proxies
2:28 for our tetropod ancestors because
2:31 lungfish are also soprarigians closely
2:33 related to the ancestors of tetropods.
2:35 Maybe those ancestors also lived in
2:36 similar environments that changed
2:38 seasonally with bodies of water
2:39 shrinking and disappearing. And maybe
2:41 this led those tetropotammorphs to
2:42 evolve seemingly land friendly traits
2:44 for the same reason, to survive long
2:46 enough to find water again. But as new
2:48 fossils emerged over the next few
2:50 decades, the shrinking pond hypothesis
2:52 began to fall apart. For example, by the
2:54 1940s, the evidence suggested that lungs
2:56 actually evolved long before the fish
2:58 tetropod transition. In fact, lungs
3:00 likely appeared millions of years before
3:02 the arrival of tetropods, meaning they
3:03 must have developed to serve a different
3:05 purpose. Then the early 2000s brought a
3:07 ton of new discoveries of transitional
3:09 tetropottomorphs. giving us more insight
3:11 into the environments where they lived
3:13 like the famous tetropod ancestor
3:15 Tecttoik which fun fact was not named
3:16 after Tik Tok but lived during the
3:18 Deonian period in what's now northern
3:21 Canada around 375 million years ago.
3:22 Tectalic and its relatives including
3:25 panderices and alpisti are sometimes
3:27 called fishopods because they look sort
3:29 of halfway between a fish and a tetropod
3:32 and among their more tetropodike traits
3:34 were limblike fins that even had the
3:36 precursors to toes. Tectalic was also
3:38 one of the first to evolve a neck,
3:40 giving it a distinct head and torso,
3:41 unlike the classic fish body plan with
3:43 no neck. This allowed for greater
3:45 mobility of its head, and it's a key
3:46 trait that distinguishes the tetropod
3:48 body plan from that of a fish. But as
3:50 more fishopod fossils began to surface
3:51 and their habitats were studied,
3:53 scientists realized they lived in
3:55 shallow water environments like tidal
3:57 channels, braided streams, and swamps.
3:59 These weren't the seasonal droughtprone
4:01 landscapes that LOL and Ror had
4:03 envisioned. So that's that then, right?
4:05 The shrinking pot hypothesis is wrong.
4:07 Well, maybe not entirely. Lol and rumor
4:08 might have been on to something even if
4:11 they had the wrong habitat. See, amid
4:12 all the new tetropottomorph discoveries
4:15 of the early 2000s, another researcher
4:17 suggested that the tetropod body plan
4:19 might have evolved mostly or entirely in
4:21 the water, not on land. One animal she
4:23 pointed to was a canthastega from the
4:25 swamps of what's now Greenland during
4:27 the late Deonian period about 365
4:29 million years ago. While it wasn't a
4:32 true tetropod, it was getting close with
4:34 a salamander-like body and four limbs
4:36 instead of fins. This was also one of
4:37 the first tetropodomorphs known to
4:39 evolve a rearpowered mode of locomotion.
4:41 That is, its movement was powered
4:42 primarily by its back limbs instead of
4:44 its front limbs. This form of locomotion
4:46 would one day become a feature common to
4:48 tetropods. But while a Kenthga looked
4:50 like a tetropod, its arms and weakly
4:52 built rib cage seem unlikely to have
4:54 been able to support its weight on land,
4:56 suggesting it was entirely aquatic.
4:58 Those seemingly land-friendly features
5:00 like its hands were instead probably
5:01 being used for navigating the waters of
5:03 its swampy habitat and holding on to
5:05 plants in the water. And when tectonic
5:06 was described around this time, its
5:08 discoverers suggested that its
5:10 tetropodlike traits might have evolved
5:12 in order to walk underwater as well. Or
5:13 actually be more like this cuz it has
5:15 four limbs. While they thought it might
5:16 have come ashore occasionally, most of
5:18 its time was probably spent navigating
5:20 stream beds and shallow channels
5:22 fluttered with plants and debris. It was
5:23 looking like some of the key pieces of
5:26 the tetropod body plan represented an
5:28 adaptation, a trait that initially
5:29 evolved as an adaptation for one thing,
5:31 but then later became used for something
5:33 else. In this case, our fish ancestors
5:35 evolved tetropod traits while adapting
5:37 for life in the shallows. And only later
5:39 was that body plan co-opted for life on
5:41 land. It was the old lungfish paradox
5:43 all over again. Traits that looked like
5:45 adaptations for land were actually just
5:47 better ways to be fish. And more
5:49 evidence supporting this aquatic picture
5:51 continued to emerge. For example, a 2012
5:52 study reexamined the limbs of
5:55 Ichthyostga, another tetropottomorph
5:57 that shared Aanthastega's environment.
5:58 Scientists originally thought its limbs
6:00 could have supported its body on land,
6:02 but these researchers found that the
6:03 range of motion would have limited
6:05 Echostga's terrestrial movement to
6:07 dragging its body like a seal. Not
6:09 exactly the most effective evolutionary
6:11 strategy. Ichthyastega probably did
6:13 spend some time on land, but not a lot.
6:16 Then a 2013 study found that jaws of
6:18 earlier fishopods like tict wouldn't
6:20 have allowed them to swallow prey on
6:22 land. And that's a big deal because it
6:23 means that even if they did occasionally
6:25 venture onto land, the fishopods would
6:26 have been mostly incapable of taking
6:28 advantage of prey there. This suggests
6:30 that at this point in their evolution,
6:32 tetraotamorphs were still very much
6:34 shallow water dwellers and the abundant
6:36 resources on land probably played a
6:37 minimal role in their evolution. And
6:39 most recently, there was a study from
6:41 2021 that analyzed bone micro structure
6:43 of tetropods from Nova Scotia that lived
6:45 in the early carbonifpherous period,
6:47 several million years after athosta and
6:49 ichthyostga. It found that the stress
6:51 patterns in their bones were consistent
6:53 with aquatic habitats and inconsistent
6:55 with movement on land. So, it looks like
6:57 not only were most tetropot fishes
6:59 mostly or fully aquatic, but that even
7:01 the later first true tetropods still
7:03 weren't living on land. Okay, but now we
7:05 should be careful about oversimplifying
7:07 the water to land transition overall.
7:09 Not all tetropottomorphs form a
7:10 perfectly neat progression in their
7:12 traits or through time, and their
7:14 habitats weren't all identical. There
7:16 was probably tons of variation, but many
7:17 scientists are beginning to think that
7:19 most transitional tetropottomorphs were
7:21 primarily shaped by shallow water
7:23 habitats choked with plants and other
7:25 debris. So, L and Ror were right about
7:27 one crucial thing. The colonization of
7:29 land was probably an evolutionary
7:31 accident. So, these animals crawl onto
7:34 land by accident. And now I have to pay
7:36 taxes. Those limbs, necks, and other
7:37 adaptations that later help
7:40 tetropottomorphs conquer continents
7:41 initially evolved to navigate the
7:43 shallows, not escape them. And
7:45 possessing these traits beforehand
7:46 probably made the transition to life on
7:48 land much easier for our ancestors when
7:50 they eventually did come ashore and
7:52 stayed there. So, in the end, or rather
7:55 in the beginning, our ancestors didn't
7:57 set out to revolutionize life on Earth.
7:58 Our success on land and that of all
8:01 tetropods from frogs to dogs to
8:03 dinosaurs was just a lucky side effect
8:11 Before you go, I need to talk about PBS.
8:13 Congress recently eliminated funding to
8:15 the Corporation for Public Broadcasting,
8:17 which funds public television and radio
8:19 stations across the United States. Many
8:21 of these stations are PBS members, and
8:23 while PBS itself is not entirely
8:25 dependent on federal funds, these cuts
8:27 will significantly impact the system.
8:30 Local stations need direct support now
8:31 more than ever, especially smaller
8:33 stations who are facing the very real
8:35 threat of closure. If you want to
8:37 support PBS's mission, follow the link
8:39 in the description or scan the QR code
8:41 on screen to donate to your local
8:44 station. Any amount goes a long way.
8:55 [Music]
8:56 Okay, what do horseshoe crabs, ginko
8:58 trees, and lungfish have in common?
9:00 They've all been referred to as living
9:01 fossils. And while these organisms
9:03 appear to have persisted for millions of
9:05 years unchanged, they also show us that
9:07 evolution is always at work. Learn more
9:09 in our episode, living fossils aren't
9:10 really a thing. And thanks to this
9:12 month's eontologists who give us a big
9:15 step up. Addie, Annie and Eric Higgins,
9:18 Carl Wolfful, Jackie Scott Rston, Jake
9:21 Hart, John Davidson, Ing, Juan M.
9:23 Melanie Lamb, Carnival, Nico Robin,
9:27 Rafael Hassa, Tony Dye, and Steve.
9:30 Become an eonite at patreon.com/eons.
9:32 And you can get fun perks like access to
9:34 a monthly digital puzzle of paleo art
9:36 commissioned just for eons. And as
9:38 always, thanks for joining me in the
9:39 Annie and Eric Higgins studio. Subscribe
9:41 at youtube.com/eons
9:44 for more fabulous fossils. [Music]
9:50 [Music]
9:51 I just want everyone to know I'm doing
9:53 resort casual for this shoot. This is
9:55 vacation dad. [Music]
