Charcoal from a hearth site in Colorado, 2,000 miles south of Alaska, is known to be 11,200 years old. Researchers...
GMAT Critical Reasoning : (CR) Questions
Charcoal from a hearth site in Colorado, 2,000 miles south of Alaska, is known to be 11,200 years old. Researchers reasoned that, since glaciers prevented human migration south from the Alaska-Siberia land bridge between 18,000 and 11,000 years ago, humans must have come to the Americas more than 18,000 years ago.
Which of the following pieces of new evidence would cast doubt on the conclusion drawn above?
Passage Analysis:
| Text from Passage | Analysis |
| Charcoal from a hearth site in Colorado, 2,000 miles south of Alaska, is known to be 11,200 years old. |
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| Researchers reasoned that, since glaciers prevented human migration south from the Alaska-Siberia land bridge between 18,000 and 11,000 years ago, humans must have come to the Americas more than 18,000 years ago. |
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Argument Flow:
The argument starts with physical evidence (11,200-year-old charcoal in Colorado), then adds information about when glacier barriers existed, and combines these facts to conclude when humans first arrived in the Americas.
Main Conclusion:
Humans must have come to the Americas more than 18,000 years ago.
Logical Structure:
If humans were in Colorado 11,200 years ago, and glaciers prevented migration from Alaska between 18,000-11,000 years ago, then humans must have arrived before the glacier barrier formed (before 18,000 years ago).
Prethinking:
Question type:
Weaken - We need to find new evidence that would make us doubt the conclusion that humans came to the Americas more than 18,000 years ago
Precision of Claims
The key claims involve specific timeframes (11,200 years old charcoal, 18,000-11,000 years ago glacier blockage), specific locations (Colorado, Alaska-Siberia land bridge), and a causal relationship between glacier timing and human migration patterns
Strategy
To weaken this conclusion, we need new evidence that provides an alternative explanation for how humans could have reached Colorado by 11,200 years ago WITHOUT needing to arrive before 18,000 years ago. The researchers assume the Alaska-Siberia land bridge was the only route, so we should look for alternative migration paths, exceptions to the glacier blockage, or different explanations for the Colorado evidence.
Using new radiocarbon dating techniques, it was determined that the charcoal from the Colorado site was at least 11,400 years old.
This choice actually strengthens the argument rather than weakening it. If the charcoal is even older (11,400 years instead of 11,200 years), this pushes back human presence in Colorado even further, making the researchers' conclusion that humans arrived before 18,000 years ago even more compelling. We need evidence that casts doubt on the conclusion, not supports it.
Another campsite was found in New Mexico with remains dated at 16,000 years old.
This finding of 16,000-year-old remains in New Mexico doesn't weaken the argument - it actually supports it. The remains are still older than the supposed glacier-free period (which ended 11,000 years ago), so this evidence would still require humans to have arrived before 18,000 years ago, just like the researchers concluded. This adds more support to their timeline.
A computer simulation of glacial activity showed that it would already have been impossible for humans to travel south overland from Alaska 18,500 years ago.
This choice actually strengthens the researchers' argument by extending the glacier blockage period. If humans couldn't travel south starting from 18,500 years ago (even earlier than the stated 18,000 years), this makes the case even stronger that humans must have arrived well before the glaciers formed the barrier. This supports rather than weakens the conclusion.
Using new radiocarbon dating techniques, it was proved that an ice-free corridor allowed passage south from the Alaska-Siberia land bridge at least 11,400 years ago.
This is the correct answer because it directly attacks the researchers' key assumption. The argument depends on the idea that glaciers completely blocked migration between 18,000-11,000 years ago. However, if an ice-free corridor existed at least 11,400 years ago, then humans could have migrated from Alaska after 18,000 years ago and still reached Colorado by 11,200 years ago. This eliminates the need for humans to have arrived before 18,000 years ago, directly weakening the conclusion.
Studies of various other hunting-gathering populations showed convincingly that, once the glaciers allowed passage, humans could have migrated from Alaska to Colorado in about 20 years.
This choice is irrelevant to weakening the argument. Knowing that humans could migrate quickly (20 years from Alaska to Colorado) once glaciers allowed passage doesn't change the fundamental timeline problem. The researchers' conclusion is based on when passage was possible, not how long the journey would take once it became possible. This doesn't address the glacier blockage issue at all.