Narrator: Listen to part of a lecture in a marine biology class.
Professor: So we’ve already talked about the evolution of aquatic mammals… say, how whales and dolphins evolved from land-dwelling mammals, making their way into the water. But our next question takes us back a step—how did a whale’s ancestors… our ancestors… end up on land in the first place? Remember that all life initially started in the sea.
Mammals are actually a form of tetrapod… this comes from the Greek tetra, meaning “four,” and pous, meaning “foot.” So the tetrapods are any animals with four “feet,” or limbs. This includes humans and other mammals, but also includes birds, reptiles, and amphibians. Early tetrapods appeared in the fossil record about 400 million years ago, but were all aquatic. It took another 25 million years before they made their way onto land. In fact, there is still a lot of debate on exactly how this happened.
But let’s start with where these tetrapods came from… this is actually pretty interesting. It turns out that fish and tetrapods both evolved from early vertebrates, or animals with spines. These early animals had bony fins and gills like a fish, but they also had air sacs that were connected to the back of their throats, which meant they could breathe air, not just pull oxygen out of the water. They could also use this air sac to control their ability to float at different water depths—this ability is called buoyancy.
Now over time, fish and our tetrapod ancestors took two different evolutionary paths. Some fish kept their gills, but the air sacs lost connection to the throat and became exclusively for buoyancy control. Others developed a more advanced lung that started to look more and more like what you and I have today. One example is a lungfish… this is, uh… a modern freshwater fish with lungs that allow it to breathe air at the water’s surface. Eventually, many of these fish would completely lose the gills and buoyancy control. Of course, there are modern tetrapods, like amphibians, that still have gills… or at least start off with gills that they shed as they move into adulthood.
Another notable difference occurred in the fins. The fish that you’re probably familiar with are considered ray-finned, in that there are a lot of little bones that create a web or ray shape in the fin. Their early aquatic ancestors had the same fin shape. But our tetrapod ancestors began to develop lobe fins… the lungfish is a lobe-finned fish. These lobe fins are different from ray fins in that there is a single bone extending from the fish’s body that then connects to almost finger-like bones. As you can see, the early needs for living on land were starting to appear… feet to walk around on and lungs in order to breathe air.
Um…so most lobe-finned fish are actually extinct. Scientists thought the lungfish was the last remaining one. But in 1938 a fisherman caught a different kind of fish—a coelacanth… bet you didn’t think that was how to pronounce that word! Scientists had already been studying fossils of the coelacanth but it was thought to have gone extinct 66 million years ago! So, of course, the fisherman had no idea that he’d caught something so rare, but a museum curator happened to be at a fish market in South Africa, where it was caught, and noticed it. I bet you can imagine her surprise!
So the puzzle of how our early lobe-finned ancestors made their way to land is still unsolved. There are only two living species of lobe-finned fish… the coelacanth, and the lungfish. The question stands—how are we all related? Recent studies of DNA, which is the carrier of all genetic information in a living organism, focus on the three groups—coelacanths, lungfish, and tetrapods. How are we all related, and are some of us more related than others? For example, are coelacanths and lungfish closely tied, while the tetrapods represent a different evolutionary lineage? Or maybe we are evolved from one of the other ancient lines of fish? It is an exciting question to try to answer, but unfortunately, the DNA evidence of several studies hasn’t yet been able to provide support for any of these hypotheses!
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What is this lecture mainly about? |
Gist-content. The lecture is primarily concerned with the question of how tetrapods ended up becoming land-based. |
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| ✗ | A Recent research connecting whales to ray-finned fish |
Whales are mentioned at the start of the lecture, but no recent research about them is discussed. |
| ✗ | B Differences between mammals and other tetrapods |
There is no contrast made between mammals and other tetrapods. |
| ✓ | C Discussing the evolution of tetrapods |
Correct. The question of how mammals and many other tetrapods evolved from ocean-dwelling animals into land-dwelling animals is the central idea of the lecture. |
| ✗ | D The importance of DNA evidence in evolutionary research |
DNA research is only mentioned near the end of the lecture. |
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According to the professor, what are two evolutionary changes that occurred in the ancestors of tetrapods? Choose 2 answers. |
Detail. The professor cites the development of lungs and the development of lobe fins as two evolutionary changes among tetrapod ancestors. |
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| ✗ | a Their swimming ability improved dramatically. |
Swimming ability is not discussed in the lecture. |
| ✓ | b They developed lobe fins. |
Correct. Tetrapod ancestors developed lobe fins. These lobe fins are implied to be the predecessors of hands and feet. |
| ✗ | c They became more like ray-finned fish. |
The opposite is true. While the ancestors of modern fish were ray-finned, tetrapod ancestors developed lobe fins. |
| ✓ | d Their air sacs developed into a more advanced lung. |
Correct. The air sacs in tetrapod ancestors became better-developed to support breathing air, while the air sacs in ray-finned fish evolved to primarily support buoyancy. |
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According to the professor, what was surprising about the discovery of a living coelacanth? |
Detail. The surprise derives from the fact that scientists had previously believed that coelacanths became extinct millions of years ago. |
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| ✗ | A Its DNA did not match that found in earlier fossils. |
The DNA of living and fossil specimens of coelacanths is not discussed in the lecture. |
| ✓ | B Coelacanths were thought to have become extinct 66 million years ago. |
Correct. The professor indicates that this was the cause of the surprise. |
| ✗ | C The fisherman who caught the coelacanth was testing out a new method of net fishing. |
Nothing is mentioned about the methods used by the fisherman to catch the living coelacanth. |
| ✗ | D Researchers had not previously thought to search fish markets for coelacanth specimens. |
Prior searches for coelacanth specimens are not discussed in the lecture. |
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What evidence suggests that modern, land-dwelling mammals are descended from lobe-finned fish? |
Detail. The professor notes that the structure of the lobe fin is “a single bone extending from the fish’s body that then connects to almost finger-like bones.” This is implied to be comparable to the structure of a hand or foot. |
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| ✗ | A Lobe-finned fish are able to walk on land, even if only awkwardly. |
This idea is not presented in the lecture. |
| ✗ | B The coelacanth moves by “walking” on the ocean floor. |
This idea is not presented in the lecture. |
| ✓ | C Lobe fins include a bone extending from the body with smaller, finger-like bones attached. |
Correct. The professor implies that this structure of the lobe fin is similar to that of a hand or foot. |
| ✗ | D No lobe-finned fish use lungs to breathe air. |
On the contrary, the lungfish is given as an example of a lobe-finned fish with lungs that allow it to breathe air. |
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What is the professor’s opinion about the studies investigating the DNA of lungfish, coelacanths, and tetrapods? |
Speaker’s Attitude. The professor mentions these studies in the context of determining the degree of ancestral relationship among lungfish, coelacanths, and tetrapods. She states: “It is an exciting question to try to answer.” |
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| ✗ | A She is disappointed that DNA research is relatively new. |
She does not express disappointment about how new the DNA research is. |
| ✓ | B She is excited to discover what conclusions may arise from such studies. |
Correct. She calls the issue an exciting one, but indicates that no conclusive evidence has yet been found. |
| ✗ | C She hopes that future researchers will be more accurate in their reporting. |
Accuracy of the researchers involved in completed studies is never called into question in the lecture. |
| ✗ | D She does not agree with the findings of most studies. |
The professor does not indicate her disagreement with the results of any studies. |
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What does DNA evidence indicate about the ancestral relationship among lungfish, coelacanths, and tetrapods? |
Detail. The professor notes that no definitive conclusions have yet been reached about these relationships. |
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| ✓ | A It is not clear which of the three are most closely related. |
Correct. The professor states that “the DNA evidence of several studies hasn’t yet been able to provide support for any” of the hypotheses about the relationships among the three groups. |
| ✗ | B Earlier beliefs about the relationships among the three were proven incorrect. |
The professor never mentions earlier beliefs about these relationships. Furthermore, she notes that any hypotheses about these relationships are not yet proven or disproven. |
| ✗ | C Lungfish and tetrapods have the highest degree of similarity among the three. |
The studies have not yet been able to conclude which relationships are the strongest. |
| ✗ | D The DNA of tetrapods is too varied to study effectively. |
On the contrary, tetrapod DNA has been researched in these studies, but no definitive conclusions have yet been drawn from them. |