PrepTest 114, Section 4, Question 24
Most land-dwelling vertebrates have rotating limbs terminating in(64 more words not shown)
The statements above, if true,(8 more words not shown)
- AMany anatomical characteristics(11 more words not shown)
- BNone of the(14 more words not shown)
- CAcanthostega originated as(13 more words not shown)
- DAll anatomical characteristics(17 more words not shown)
- ECertain anatomical characteristics(12 more words not shown)
Reading the argument
Fingers and toes can exist before an animal starts walking on land. That's the possibility this discovery forces the biologists to consider. The long name doesn't change the problem. Acanthostega had the rotating limbs and digits that are useful for land movement, yet the passage says it and its predecessors lived only in water. So a feature useful on land needn't have originated after animals moved there.
My first objection might be that an ancestor went onto land, developed the feature, and later returned to the water. That would change the story. But the last sentence rules out that escape: it says the predecessors were exclusively aquatic too. We have to use that fact, not replace it with an imagined history.
What we’re looking for
For the most strongly supported answer, I'd expect something challenging the biologists' proposed timing. The feature they thought developed after the move to land was already present in an animal that lived in water.
Working the answers
- A
Being useful on land doesn't establish that a characteristic is a disadvantage underwater. The passage doesn't say the limbs or digits harmed Acanthostega's ability to survive in water. A adds that negative effect and extends it to many characteristics.
- B
We haven't been given a survey of the characteristics shared by most aquatic animals. The example shows a land-useful characteristic in an aquatic animal, but it doesn't establish how widely that feature occurs among aquatic animals. B's sweeping claim that none of their common characteristics helps on land isn't supported.
- C
This is the land-then-water story we considered, but the passage says Acanthostega and its predecessors were exclusively aquatic. That directly undercuts C's claim of a land-dwelling origin. Imagining a different evolutionary history isn't evidence for it, particularly when it conflicts with a fact we're instructed to accept.
- D
The passage identifies a characteristic useful for land movement. It doesn't establish that all the other characteristics common to most land animals are advantageous underwater. A characteristic's lack of usefulness for one activity tells us nothing by itself about its value in a different environment.
- ECorrect
This gives us a narrower statement than my initial thought about the biologists' timing. Put the two relevant facts together: rotating limbs ending in digits are useful for land movement, and an exclusively aquatic animal had them. That supports the claim that some aquatic animals possess characteristics advantageous on land.
We don't need to claim that Acanthostega itself could walk on land. Its weak skeleton is a separate limitation. An organism can possess a useful feature without having every other feature needed to use it in that environment.
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