Charcoal from a hearth site in Colorado, 2,000 miles south of Alaska,

(Option F) Based on recently discovered historical records, human migration occured from Brazil and the Argentinas into the Americas because of trade, 13000 years ago.

Normally I would NOT have considered this a legitimate weakeness because this weakness doesnt touch on the 'link' between the premise and the conclusion in the argument.

But I think, in this case this weakness is passable because of the question stem

If the question was asking specifically about North America, then your answer F would be a perfectly good weakener, because it would point out a possibility the argument hadn't even considered. There might be other ways to get into North America besides through the Alaska-Siberia land bridge, and perhaps people first came by those other routes between 11,000 and 18,000 years ago. But as the question is written, your answer is not a weakener; instead it's a lot like answer B. Brazil and Argentina are part of "the Americas" (the Americas includes South America), so your answer F just establishes that people were already in the Americas more than 13,000 years ago. Maybe they first got there more than 18,000 years ago, and maybe they didn't.

(Option F) Based on recently discovered historical records, human migration occured from Brazil and the Argentinas Asia into the Americas because of trade, 13000 years ago.

Premise : migration CANNOT take place 18,000 years ago to 11,000 years ago
OA : D -- actually migration DID take place 11,400 years ago and before.

This is a strange OA because we are not allowed to question the premise (in red)

I have seen many answer choices marked INCORRECT in other CR's because the answer choice (a) 'challenged' the premise or (b) the answer choice stated something different to what the premise states

How come in this CR - the D and the premise, both cannot be true at the same time ?

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.

Let's take a fresh look at the prompt:

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).

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.

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.

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.

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).

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.

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!

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?

(A) Using new radiocarbon dating techniques, it was determined that the charcoal from the Colorado site was at least 11,400 years old.

(B) Another campsite was found in New Mexico with remains dated at 16,000 years old.

(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.

(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.

(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.

Option D -at least 11,400 years ago
means that year >=11,400.
it can be 11,500, 11,600 years ago too right?

Yes, option (D) says that the passage was open at least 11,400 years ago. It could have been open 11,500 years ago or 12,000 years ago etc too.

This is a strange OA because we are not usaully allowed to question the premise (marked in red)

I have seen many answer choices marked INCORRECT in other CR's because the answer choice (a) 'challenged' the premise or (b) the answer choice stated something different to what the premise states

Normally I would NOT have considered this a legitimate weakeness because this weakness doesnt touch on the 'link' between the premise and the conclusion in the argument.

Which of the following pieces of new evidence would cast doubt on the conclusion drawn above?

Let's take a fresh look at the prompt:

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).

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.

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.

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.

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).

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.

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!

Hi GMATNinja

Is following reasoning wrong on the option C?

Option C has No Impact, analogically, this option is x<6 but my argument’s conclusion says x<2 so this option doesn’t help me to change my confidence in the conclusion as x can be 3/4/5.