Climate researchers examined sediments in a cave in Region Z, which now has a temperate climate, and they found arctic...

GMAT Critical Reasoning : (CR) Questions

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Critical Reasoning

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Climate researchers examined sediments in a cave in Region Z, which now has a temperate climate, and they found arctic lemming bones in a buried layer corresponding to about 14,000 B.C. Arctic lemmings are typical of arctic climates and are unlikely to have changed their adaptation for temperature over a period as short as 16,000 years. So Region Z probably had an arctic climate around 14,000 B.C.

Which of the following is an assumption that the argument requires?

Arctic lemmings were, around 14,000 B.C., well adapted to the climate of the region where the bones were found.

Adjacent layers did not contain bones of arctic lemmings.

Temperature shifts between arctic and temperate climates are common over the long term.

No bones of warm-weather animals from any period were found in nearby caves in Region Z.

The bones were from a species of lemming that was probably not well adapted for survival in the region where their bones were found.

Solution

Passage Analysis:

Text from Passage	Analysis
Climate researchers examined sediments in a cave in Region Z, which now has a temperate climate, and they found arctic lemming bones in a buried layer corresponding to about 14,000 B.C.	What it says: Researchers found arctic lemming bones from 14,000 B.C. in Region Z, which is temperate today What it does: Sets up the puzzle - why are arctic animal bones in a place that's not arctic now? What it is: Study finding/evidence
Arctic lemmings are typical of arctic climates and are unlikely to have changed their adaptation for temperature over a period as short as 16,000 years.	What it says: Arctic lemmings need arctic climates and haven't evolved much in 16,000 years What it does: Provides the key link - if the bones are there, the climate must have been different What it is: Biological fact/reasoning principle Visualization: Timeline: 16,000 years = "short" period for evolution (vs millions of years typically needed)
So Region Z probably had an arctic climate around 14,000 B.C.	What it says: Region Z was likely arctic 16,000 years ago What it does: Draws the conclusion by combining the bone evidence with lemming climate needs What it is: Author's conclusion

Argument Flow:

The argument starts with physical evidence (lemming bones from 14,000 B.C.), adds biological reasoning (lemmings need arctic climates and don't evolve quickly), then concludes the climate must have been different back then.

Main Conclusion:

Region Z probably had an arctic climate around 14,000 B.C.

Logical Structure:

If arctic lemming bones are found in Region Z from 14,000 B.C., and arctic lemmings only live in arctic climates and haven't evolved much since then, then Region Z must have been arctic back then. This is a causal reasoning argument linking animal presence to climate conditions.

Prethinking:

Question type:

Assumption - We need to identify what the argument must assume to be true for the conclusion to follow logically from the premises.

Precision of Claims

The argument makes specific claims about location (Region Z), time (14,000 B.C.), species behavior (arctic lemmings), and climate change over 16,000 years. We need assumptions that connect these elements.

Strategy

Look for gaps in the logical chain from 'arctic lemming bones found in cave' to 'Region Z had arctic climate in 14,000 B.C.' The argument assumes the bones indicate the lemmings lived there, that they lived when the sediment layer formed, and that their presence reliably indicates climate type.

Answer Choices Explained

Arctic lemmings were, around 14,000 B.C., well adapted to the climate of the region where the bones were found.

'Arctic lemmings were, around 14,000 B.C., well adapted to the climate of the region where the bones were found.' This is exactly what the argument must assume. The entire logic chain depends on the idea that finding arctic lemming bones tells us something meaningful about the climate. If these lemmings were poorly adapted to the climate in Region Z, then their presence wouldn't indicate anything about climate type. The argument assumes the lemmings were thriving there, which supports the conclusion that the climate was arctic. This assumption is necessary for the argument to work.

Adjacent layers did not contain bones of arctic lemmings.

'Adjacent layers did not contain bones of arctic lemmings.' The argument doesn't need to assume this. Even if other layers also contained arctic lemming bones, this wouldn't weaken the conclusion about the climate in 14,000 B.C. The argument is focused specifically on what the bones in that particular layer tell us about that specific time period. What's in other layers is irrelevant to this conclusion.

Temperature shifts between arctic and temperate climates are common over the long term.

'Temperature shifts between arctic and temperate climates are common over the long term.' The argument doesn't require this assumption. Whether such climate shifts are common or rare doesn't affect the logical connection between finding arctic animal bones and concluding that an arctic climate existed. The argument works regardless of how frequently such changes occur.

No bones of warm-weather animals from any period were found in nearby caves in Region Z.

'No bones of warm-weather animals from any period were found in nearby caves in Region Z.' This is not required for the argument. The absence of warm-weather animal bones in other caves doesn't affect the logical reasoning about what the arctic lemming bones in this specific cave tell us about the climate in 14,000 B.C. The argument stands on its own evidence.

The bones were from a species of lemming that was probably not well adapted for survival in the region where their bones were found.

'The bones were from a species of lemming that was probably not well adapted for survival in the region where their bones were found.' This directly contradicts what the argument needs to assume. If the lemmings were not well adapted to the region, then finding their bones there wouldn't tell us anything reliable about the climate type. This would actually undermine the argument rather than support it.

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