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Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”

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Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post Updated on: 13 Sep 2019, 03:37
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Two opposing scenarios, the “arboreal” hypothesis and the “cursorial” hypothesis, have traditionally been put forward concerning the origins of bird flight. The “arboreal” hypothesis holds that bird ancestors began to fly by climbing trees and gliding down from branches with the help of incipient feathers: the height of trees provides a good starting place for launching flight, especially through gliding. As feathers became larger over time, flapping flight evolved and birds finally became fully air-borne. This hypothesis makes intuitive sense, but certain aspects are troubling. Archaeopteryx (the earliest known bird) and its maniraptoran dinosaur cousins have no obviously arboreal adaptations, such as feet fully adapted for perching. Perhaps some of them could climb trees, but no convincing analysis has demonstrated how Archaeopteryx would have both climbed and flown with its forelimbs, and there were no plants taller than a few meters in the environments where Archaeopteryx fossils have been found. Even if the animals could climb trees, this ability is not synonymous with gliding ability. (Many small animals, and even some goats and kangaroos, are capable of climbing trees but are not gliders.) Besides, Archaeopteryx shows no obvious features of gliders, such as a broad membrane connecting forelimbs and hind limbs.

The “cursorial”(running) hypothesis holds that small dinosaurs ran along the ground and stretched out their arms for balance as they leaped into the air after insect prey or, perhaps, to avoid predators. Even rudimentary feathers on forelimbs could have expanded the arm’s surface area to enhance lift slightly. Larger feathers could have increased lift incrementally, until sustained flight was gradually achieved. Of course, a leap into the air does not provide the acceleration produced by dropping out of a tree; an animal would have to run quite fast to take off. Still, some small terrestrial animals can achieve high speeds. The cursorial hypothesis is strengthened by the fact that the immediate theropod dinosaur ancestors of birds were terrestrial, and they had the traits needed for high lift off speeds: they were small, agile, lightly built, long-legged, and good runners. And because they were bipedal, their arms were free to evolve flapping flight, which cannot be said for other reptiles of their time.


1. The primary purpose of the passage is to

(A) present counter evidence to two hypotheses concerning the origins of bird flight
(B) propose and alternative to two hypotheses concerning the origins of bird flight correct certain misconceptions about hypotheses concerning the origins of bird flight
(C) dismiss counter evidence to two hypotheses concerning the origins of bird flight
(D) refute a challenge to a hypothesis concerning the origins of bird flight
(E) evaluate competing hypotheses concerning the origins of bird flight



2. Which of the following is included in the discussion of the cursorial hypothesis but not in the discussion of the arboreal hypothesis?

(A) discussion of some of the features of Archaeopteryx
(B) description of the environment known to have been inhabited by bird ancestors
(C) possible reason why bird ancestors might have been engaging in activities that eventually evolved into flight
(D) description of the obvious features of animals with gliding ability
(E) An estimate of the amount of time it took for bird ancestors to evolve the kind of flapping flight that allowed them to become completely airborne



3. The passage presents which of the following facts as evidence that tends to undermine the arboreal hypothesis?

(A) Feathers tend to become larger over time
(B) Flapping flight is thought to have evolved gradually over time
(C) Many small animals are capable of climbing trees.
(D) Plants in Archaeopteryx’s known habitats were relatively small
(E) Leaping into the air does not provide as much acceleration as gliding out of a tree



4. The passage suggests which of the following regarding the climbing ability of Archaeopteryx?

(A) Its ability to climb trees was likely hindered by the presence of incipient feathers on its forelimbs.
(B) It was probably better at climbing trees than were its maniraptoran dinosaur cousins.
(C) It had certain physical adaptations that suggest it was skilled at climbing trees.
(D) Scientists have recently discovered fossil evidence suggesting it could not climb trees.
(E) Scientists are uncertain whether it was capable of climbing trees



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Originally posted by JarvisR on 15 Jun 2015, 20:12.
Last edited by SajjadAhmad on 13 Sep 2019, 03:37, edited 6 times in total.
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 16 Jun 2015, 11:23
I chose D for ques 2.....need explanation for its OA
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 30 Aug 2015, 05:01
campus1995 wrote:
I chose D for ques 2.....need explanation for its OA


we are asked to choose an option that refers CH but not AH.

Correct Answer says - Reason for achieving the flight by the animals.

CH Says - The early animals choose to fly either to escape predators or to hunt.

But no such reason was mentioned in AH on why the animals choose to climb trees.

Hope its clear now.

Even i got it wrong when i practiced under timer.. but later understood.. :)
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 05 Aug 2016, 06:45
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Interesting passage . Took 3 mins to read and 8 mins and 40 seconds to answer :read

-The author talks about 2 hypotheses for the origin of flight in birds
-The author then evaluates both the hypotheses

1. E - evaluate competing hypotheses concerning the origins of bird flight

2. “holds that small dinosaurs ran along the ground and stretched out their arms for balance as they leaped into the air after insect prey or, perhaps, to avoid predators"
This reasoning as to why bird ancestors might have engaged in “flight” type activities is not provided in the discussion of arboreal hypothesis. Hence option (C ) is correct.

3. “and there were no plants taller than a few meters in the environments where Archaeopteryx fossils have been found"
the fact that small animals can climb does not in and of itself undermine the arboreal hypothesis. For C to be correct, it needs to present the entire undermining fact: that many small animals can climb but cannot fly.

also, D is clearly a piece of evidence used to undermine the hypothesis - the earliest known bird did not have trees high enough to glide from, so it probably developed flight in a different way. Once you have D, there really is no reason to go and talk yourself into choosing a trap answer choice such as C.
Option (D) is a clear word justification of the above, Hence correct.

4. “Even if the animals could climb trees, this ability is not synonymous with gliding ability"
The above suggests that scientists were unsure on whether the Archaeopteryx was capable of climbing trees. Option (E) is hence the right answer choice.
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 22 Aug 2018, 00:13
In Q4 why is option A incorrect??
It is stated that there is no convincing analysis to suggest that archaeopteryx could climb trees and fly with its forelimbs

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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 31 Aug 2018, 21:34
MithilaGauri wrote:
In Q4 why is option A incorrect??
It is stated that there is no convincing analysis to suggest that archaeopteryx could climb trees and fly with its forelimbs

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And that's what option (E) states: "Scientists are uncertain whether it was capable of climbing trees." Hence, it's the correct answer.

On the other hand, Option (A) is incorrect as there is no mention in the paragraph about presence of feathers on Archaeopteryx's limbs.
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 29 Oct 2018, 21:36
u1983 broall

Could you please the following question to the RC?

Which of the following best describes the organization of the passage?

A traditional hypothesis is presented, analyzed, and then rejected in light of newly discovered evidence.

A hypothesis is presented and critiqued, and then a second hypothesis, supported by more-compelling evidence, is presented.

Certain scientific facts are presented, and then two hypotheses attempting to account for those facts are discussed and evaluated.

Two opposing hypotheses are introduced and contrasted, and then evidence is presented to demonstrate that both hypotheses are likely flawed.

Two hypotheses are introduced, the evidence invoked to support them is evaluated, and then a recommendation is made for further research.
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 15 Jun 2019, 14:08
Hi Experts, Can you tell an efficient way to solve these long RC passages? I solved this with 75% accuracy but it took me a long time. For instance, Q3, in which it is asked which of the following was mentioned for discussion of A hypothesis vs B hypothesis? Now, since there are so many details here in both the hypothesis, it took me 4 minutes to figure out what was mentioned in paragraph 2 while not mentioned in paragraph 1. This is way too much time. How do I go about improving this?

I read the entire passage and answered the first question in about 4 minutes, but the third question took me 4 minutes to go through so much detail for both theories and similarly question 4th question took about 3+ minutes because I was focused on eliminating the answer choices.

GMATNinja, VeritasKarishma - can you please advise?

Please advise. thank you!
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 18 Jun 2019, 03:59
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sdlife wrote:
Hi Experts, Can you tell an efficient way to solve these long RC passages? I solved this with 75% accuracy but it took me a long time. For instance, Q3, in which it is asked which of the following was mentioned for discussion of A hypothesis vs B hypothesis? Now, since there are so many details here in both the hypothesis, it took me 4 minutes to figure out what was mentioned in paragraph 2 while not mentioned in paragraph 1. This is way too much time. How do I go about improving this?

I read the entire passage and answered the first question in about 4 minutes, but the third question took me 4 minutes to go through so much detail for both theories and similarly question 4th question took about 3+ minutes because I was focused on eliminating the answer choices.

GMATNinja, VeritasKarishma - can you please advise?

Please advise. thank you!



sdlife - Actually, I would say the passage is not complicated. It is structured in a really neat fashion and I don't have to waste time looking for an answer at the wrong place.
I give a quick read to the passage and this is what I note:
Passage 1 - Talks about "arboreal". In second half, talks about "issues with arboreal" taking example of Archaeopteryx.
Passage 2 - Talks about "cursorial". Doesn't talk much about issues of cursorial. Seems more convinced with this one.

Answer 1 is straight forward then.
Answer 2, I agree is a bit more complicated to locate though my strategy was to focus on the second paragraph. I looked for things that cursorial did discuss and then cross checked to see which one was missing from first paragraph.
Answer 3 was very easy to locate. I focused on the second half on first paragraph. First point - not known whether could climb trees. Didn't find it in the options. Second point - No trees tall enough - found it in options. Answer
Answer 4 was then obviously (E) from the analysis of answer 3 above.
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”  [#permalink]

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New post 27 Jun 2019, 14:45
1
sdlife wrote:
Hi Experts, Can you tell an efficient way to solve these long RC passages? I solved this with 75% accuracy but it took me a long time. For instance, Q3, in which it is asked which of the following was mentioned for discussion of A hypothesis vs B hypothesis? Now, since there are so many details here in both the hypothesis, it took me 4 minutes to figure out what was mentioned in paragraph 2 while not mentioned in paragraph 1. This is way too much time. How do I go about improving this?

I read the entire passage and answered the first question in about 4 minutes, but the third question took me 4 minutes to go through so much detail for both theories and similarly question 4th question took about 3+ minutes because I was focused on eliminating the answer choices.

GMATNinja, VeritasKarishma - can you please advise?

Please advise. thank you!

Great analysis, VeritasKarishma!

sdlife, also remember that you do not need to get ALL of the questions right to get a great verbal score! If you are struggling that much with a question, make an educated guess and move on! Otherwise, you'll find yourself rushing and making mistakes on questions that you SHOULD get right.

If you're looking for a general framework for how to get a bit more efficient on RC, you might enjoy this thread, or potentially this video. I'm not sure whether they'll resonate with you, but they might be worth a quick peek.

Manukaran wrote:
Can someone explain why not option C for Q3.

Question #3 asks which answer choice undermines the "arboreal hypothesis." Let's take a look at answer choice (C):
Quote:
Many small animals are capable of climbing trees.

This one is tempting because the author does mention it while listing potential flaws in the arboreal hypothesis. Remember, the arboreal hypothesis holds that the ancestors of birds started by climbing trees and gradually began to glide to the ground, and then to fly. So if many small animals are capable of climbing trees, then it is possible that some of those animals eventually began to glide and then to fly. Thus, by itself, the fact that "many small animals are capable of climbing trees" is totally consistent with the arboreal hypothesis!

More context is given in the full sentence from the passage: "Many small animals, and even some goats and kangaroos, are capable of climbing trees but are not gliders." The last bit of the sentence -- that animals that are capable of climbing trees are not necessarily gliders -- is the part that really undermines potential evidence for the arboreal hypothesis. If certain animals can climb but cannot glide, then any arboreal adaptations in Archaeopteryx cannot be used as evidence that it began gliding down from trees.

Because (C) does not include the key information that potentially undermines the arboreal hypothesis, it is not the correct answer.

Quote:
(D) Plants in Archaeopteryx’s known habitats were relatively small

Again, the arboreal hypothesis "holds that bird ancestors began to fly by climbing trees and gliding down from branches with the help of incipient feathers: the height of trees provides a good starting place for launching flight, especially through gliding."

This hypothesis relies on the height of trees to provide a good starting place for launching flight. Sure, we don't know exactly how tall the trees need to be, but the fact that "there were no plants taller than a few meters in the environments where Archaeopteryx fossils have been found" certainly undermines the arboreal hypothesis.

(D) is the best choice.

I hope that helps!
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Re: Two opposing scenarios, the “arboreal” hypothesis and the “cursorial”   [#permalink] 27 Jun 2019, 14:45
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