TOEFL Sample Essays

Were Dinosaurs Endotherms?

Reading Passage: 

Endotherms are animals such as modern birds and mammals that keep their body temperatures constant. For instance, humans are endotherms and maintain an internal temperature of 37°C, no matter whether the environment is warm or cold. Because dinosaurs were reptiles, and modern reptiles are not endotherms, it was long assumed that dinosaurs were not endotherms. However, dinosaurs differ in many ways from modern reptiles, and there is now considerable evidence that dinosaurs were, in fact, endotherms. Polar dinosaurs One reason for believing that dinosaurs were endotherms is that dinosaur fossils have been discovered in polar regions. Only animals that can maintain a temperature well above that of the surrounding environment could be active in such cold climates. Leg position and movement There is a connection between endothermy and the position and movement of the legs. The physiology of endothermy allows sustained physical activity, such as running. But running is efficient only if an animal’s legs are positioned underneath its body, not at the body’s side, as they are for crocodiles and many lizards. The legs of all modern endotherms are underneath the body, and so were the legs of dinosaurs. This strongly suggests that dinosaurs were endotherms. Haversian canals There is also a connection between endothermy and bone structure. The bones of endotherms usually include structures called Haversian canals. These canals house nerves and blood vessels that allow the living animal to grow quickly, and rapid body growth is a characteristic of endothermy. The presence of Haversian canals in bone is a strong indicator that the animal is an endotherm, and fossilized bones of dinosaurs are usually dense with Haversian canals. 

Listening:

[Narrator] Now listen to part of a lecture on the topic you just read about. 

[Professor] Many scientists have problems with the arguments you read in the passage. They don’t think those arguments prove that dinosaurs were endotherms. Take the polar dinosaur argument. When dinosaurs lived, even the polar regions where dinosaur fossils have been found were much warmer than today—warm enough during part of the year for animals that were not endotherms to live. And during the months when the polar regions were cold, the so-called polar dinosaurs could have migrated to warmer areas or hibernated as many modern reptiles do. So the presence of dinosaur fossils in polar regions doesn’t prove the dinosaurs were endotherms. Well, what about the fact that dinosaurs had their legs placed under their bodies, not out to the side, like a crocodile’s? That doesn’t necessarily mean dinosaurs were high-energy endotherms built for running. There’s another explanation for having legs under the body: this body structure supports more weight. So with legs under their bodies, dinosaurs could grow to a very large size. Being largely had advantages for dinosaurs, so we don’t need the idea of endothermy and running to explain why dinosaurs evolved to have their legs under their bodies. OK, so how about bone structure? Many dinosaur bones do have Haversian canals, that’s true, but dinosaur bones also have growth rings. Growth rings are a thickening of the bone that indicates periods of time when the dinosaurs weren’t rapidly growing. These growth rings are evidence that dinosaurs stopped growing or grew more slowly during cooler periods. This pattern of periodic growth—you know, rapid growth followed by no growth or slow growth and then rapid growth again—is characteristic of animals that are not endotherms. Animals that maintain a constant body temperature year-round, as true endotherms do, grow rapidly even when the environment becomes cool. 

Prompt: Summarize the points made in the lecture, being sure to explain how they challenge the specific points made in the reading passage. 

Life on Mars 

Reading Passage: 

For years, scientists have been trying to discover whether life exists on Mars. Most have concluded that the Martian environment today is too harsh to support living organisms. However, three recent discoveries lend support to the idea that there may very well be life on Mars after all. First, large amounts of methane have been detected in the Martian atmosphere. This suggests the presence of living methanogens, tiny organisms that release methane into the atmosphere as a by-product of their life processes. Methanogens get their energy from hydrogen in rocks and do not need oxygen or sunlight to live, so they can survive in harsh environments like Mars. Since methane can remain in Mars’ atmosphere only for a few centuries after it’s created, these methanogens must be present and producing methane today.             

Second, the existence of life on Mars is also strongly supported by a meteorite discovered in Antarctica in 1996. The particular chemical composition of this meteorite indicates that it is a rock from the surface of Mars. The meteorite also contains chemically pure and flawless crystals of the compound magnetite. Such magnetite crystals are produced in nature only by bacteria. Third, observations of hydrogen on Mars made in 2001 by NASA’s Mars Odyssey spacecraft suggest the presence of liquid water there that could support organisms. The spacecraft detected the hydrogen in the top layer of Martian soil. This hydrogen is just what one would expect if there were liquid water under the surface. A subterranean reservoir of liquid water is an extremely suitable environment for microscopic life. 

Listening: 

[Narrator] Now listen to part of a lecture on the topic you just read about. 

[Professor] Mars is a very unfriendly place for life. The evidence that has been put forward as supporting there being life on Mars is actually very ambiguous. The facts as we know them can easily be accounted for without supposing life on Mars. First, methanogens are not necessarily the source of methane. Methane can be produced by non-biological processes like volcanic eruptions. The same experiments that detected the atmospheric methane also indicated recent volcano activity. Moreover, unlike methanogens, volcanoes have actually been found on Mars. So since no methanogens have been found yet, it’s more likely that volcanoes are the source of Mars’s methane. Second, while the magnetite crystals found in the meteorite from Mars were probably made by bacteria, there is no indication that the bacteria themselves originated on Mars. The meteorite arrived on Earth about 13,000 years ago, which means it has had thousands of years to become exposed to bacteria on Earth. So it could have been bacteria on Earth that formed the crystals we now observe. Indeed, recent research performed on the meteorite found that, in fact, it has become contaminated by Earth bacteria. Since the sample has been contaminated, the magnetite crystals on the meteorite cannot be used as evidence of life on Mars. Third, what about the supposed existence of liquid water near enough to the surface of Mars to support life? The recent investigations of Mars detected hydrogen; they didn’t detect liquid water per se. It’s actually much more likely that if there’s any water close to the Martian surface, it’s frozen; in other words, it’s ice, not liquid water. And that’s not a great environment for life. 

Prompt: Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.