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Difficulty: Very hardRuling Out Alternate Explanations (Defender)

A comprehensive analysis of fossilized hominid dental remains from the Pleistocene epoch reveals a distinct pattern: populations residing in high-altitude environments exhibited significantly greater enamel thickness compared to their low-altitude contemporaries. Because thicker enamel provides increased resistance to the rapid abrasive wear caused by masticating tough, fibrous vegetation, anthropologists conclude that the harsh conditions of high-altitude environments must have forced these hominids to rely heavily on such vegetation for their caloric needs.

Which one of the following is an assumption on which the anthropologists' argument depends?

  1. The greater enamel thickness observed in the high-altitude hominids was not a physiological byproduct of biological adaptations necessary for surviving in lower-oxygen environments.Answer
  2. B
    Hominid populations do not develop thicker tooth enamel unless their environment forces them to rely heavily on tough, fibrous vegetation.
  3. C
    The tough, fibrous vegetation available in high-altitude environments was not significantly more abrasive than the vegetation found in low-altitude environments.
  4. D
    Low-altitude Pleistocene hominids did not consume any tough, fibrous vegetation as part of their regular diet.
  5. E
    The rate at which hominid tooth enamel wears down is determined exclusively by the abrasiveness of the individual's diet.

Answer

The argument assumes that the greater enamel thickness observed in the high-altitude hominids was not a physiological byproduct of biological adaptations necessary for surviving in lower-oxygen environments.
The argument concludes that a dietary need (relying on fibrous vegetation) caused an evolutionary trait (thicker enamel) simply because the trait is useful for that diet. This is a causal claim based on an observed physical characteristic. For this conclusion to be logically sound, the author must assume that there is no other, non-dietary cause responsible for the thicker enamel in these specific high-altitude hominids. The correct answer acts as a 'defender' by ruling out an alternate physiological cause (low-oxygen adaptation). If we negate this statement—meaning the thicker enamel *was* merely a biological byproduct of low-oxygen adaptation—the dietary conclusion falls apart completely.

Step-by-Step Solution

1
Analyze the stimulus structure.
Premise: High-altitude hominids had thicker enamel, which resists wear from fibrous vegetation. Conclusion: High-altitude environments forced reliance on fibrous vegetation.
Deconstructing the argument is necessary to identify the logical gap between the evidence and the conclusion.
2
Determine the author's causal claim.
The author claims that a dietary requirement (reliance on fibrous plants) caused the evolutionary adaptation (thicker enamel).
Understanding the direction of causation highlights potential vulnerabilities, such as alternate explanations for the physical adaptation.
3
Apply the Negation Test to find the necessary defender assumption.
If the thicker enamel WAS a physiological byproduct of adapting to lower-oxygen environments, then the cause of the adaptation was respiratory/metabolic, not dietary. This completely invalidates the anthropologists' conclusion.
A necessary assumption is one that, if proven false, causes the argument's logic to collapse entirely.

Key Concept

Ruling Out Alternate Explanations (Defender Assumption)
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Ruling Out Alternate Explanations (Defender)

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