saurabh9gupta
Hi everyone,
Request one of the experts to take a stab at this.
I am still not able to understand why D is the answer.
Swat40
Dear Experts,
Please clear my queries:
1) I feel both option C and D weakens the conclusion. However, I am unable to understand why option D is preferred.\
option C: Weakens the author's conclusion by eliminating the possibility of human arrival before 18500 years. However, it is not full proof since leaves open the 18000-18500 duration.
option D: Casts doubt for the duration between 18000 & 11400 years ago. However, not full proof since leaves open the possibility of more than 18k years ago.
Moreover, i am not comfortable with the "charcoal" info with reference to the overall argument. The charcoal is if dated 11200 years old, how the argument is coorelating it with humans coming more than 18k years ago.
Let's take a fresh look at the prompt:
Quote:
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.
The researchers’ conclusion is that
humans must have come to the Americas more than 18,000 years ago. Here's how they reach this conclusion:
- Charcoal from a hearth site in Colorado is 11,200 years old. A hearth is a human-made fireplace, so this implies that the charcoal was the result of human activity. (Presumably, the charcoal was laced with THC. Hey, I'm a Coloradan, so I'm allowed to make that joke!)
- In other words, humans were in Colorado at least 11,200 years ago.
- Colorado is 2,000 miles south of Alaska.
- Between 18,000 and 11,000 years ago, glaciers prevented human migration south from the Alaska-Siberia land bridge (which connects Alaska and Siberia).
- Therefore, humans must have come to the Americas more than 18,000 years ago.
The charcoal is evidence that humans were in Colorado 11,200 years ago or prior. But humans could not have migrated into the Americas between 18,000 and 11,000 years ago, because they would have been blocked by glaciers during this period. Therefore, humans could not have come to the Americas between 18,000 and 11,200 years ago. So researchers conclude that humans must have come to the Americas prior to 18,000 years ago (implying that the descendants of those initial migrants lit a fire that left behind some charcoal 11,200 years ago).
Quote:
Which of the following pieces of new evidence would cast doubt on the conclusion drawn above?
If there were any reason to doubt the evidence, then we’d have a harder time accepting the conclusion. The conclusion focuses on when humans came to the Americas, so any sign that humans could have migrated between 18,000 and 11,200 years ago —
despite the glaciers — would certainly weaken the conclusion. New information about the charcoal’s age might help us as well, but knowing the age of the charcoal is kind of far from what we really care about:
when humans could have migrated into the Americas.Quote:
(A) Using new radiocarbon dating techniques, it was determined that the charcoal from the Colorado site was at least 11,400 years old.
So what? All we know from choice (A) is that humans
made a fire at least 200 years earlier than we had originally thought. This doesn't shed any light on when humans came to the Americas, and doesn’t challenge the role of glaciers in blocking human migration. Even if (A) is true, we must still accept the fact that humans couldn’t migrate into the Americas between 18,000 and 11,000 years ago. (A) does nothing to weaken the conclusion, so let's eliminate it.
Quote:
(B) Another campsite was found in New Mexico with remains dated at 16,000 years old.
This has no bearing on the argument. Knowing that another human-made site in the Americas existed 16,000 years ago doesn't tell us anything about whether humans could have migrated between 18,000 and 16,000 years ago (a time period when we know that they could not). Consequently, (B) does not weaken the argument, and we'll eliminate it.
Quote:
(C) 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 slightly
strengthens the argument by providing a second source of evidence supporting what we already know about glaciers. If it had been
already impossible for humans to travel south 18,500 years ago, then it certainly was impossible for humans to migrate into the Americas between 18,000 and 11,200 years ago.
And since the age of the charcoal remains 11,200 years old, (C) reinforces the author's conclusion that humans must have arrived prior to 18,000 years ago. To be precise, (C) would lead us to believe that humans arrived prior to 18,500 years ago (which by definition is more than 18,000 years ago).
That's why we eliminate (C).
Quote:
(D) 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 right on the money! Choice (D) cuts straight through the argument by telling us that the glaciers, in fact,
did not prevent all human migration between 18,000 and 11,200 years ago. This new evidence suggests that human migration could have taken place between 11,400 and 11,200 years ago.
Remember, the conclusion states that humans must have come to the Americas more than 18,000 years ago. If (D) is true, then humans could have migrated after 18,000 years ago. This definitely casts doubt on the conclusion, so let’s keep (D) around.
Quote:
(E) 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.
Who cares? Choice (E) only tells us about what could have happened
once the glaciers allowed passage — i.e., 11,000 years ago. We don’t care about what happened 11,000 years ago. We want to know whether humans could have migrated between 18,000 and 11,200 yearse ago. Eliminate (E).
(D) is the only choice that weakens the conclusion, so it’s our winner.
I hope this helps!
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