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

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!
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Conclusion: humans must have come to the Americas more than 18,000 years ago.
Reasoning: A blockade prevented humans traveling south between 18,000 to 11,000 years ago. If some information showed that the blockade was passable, then the new evidence would cast doubt on the conclusion. Correct answer weakens the conclusion; incorrect answers are neutral to the conclusion or strengthen the conclusion.

A.Using new radiocarbon dating techniques, it was determined that the charcoal from the Colorado site was at least 11,400 years old. Neutral answer. If the charcoal was exactly 11,400 years old, then the conclusion is weakened. Whereas, if the charcoal was greater than 18,000 years old, then this would strengthen the conclusion. It can go both ways.

B.Another campsite was found in New Mexico with remains dated at 16,000 years old. Neutral. The correct answer must prove that people crossed in between 18,000 and 11,000 years ago. This proves that someone was there 16,000 years ago; the humans could have crossed 2,000 years prior, around 18,000, or sometime shortly after 18,000 years ago.

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. Strengthens. If the it were impossible to travel south overland from 18,500 years ago, then the conclusion would be strengthened.

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. Correct. If it were possible to cross at least 11,400 years ago, it provides doubt that the glacier prevented all human migration between 18,000 and 11,000 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. Out of scope/Neutral. This does nothing the conclusion.

IMO D

Agree with D. Humans could have arrived as early as 11400 years ago, not 18000.

D for me too.
Answer coming from the man who stays in Colorado.

Awesome Dr Watson,

This is a classical problem. It is of a type "Alternative Explanation" -- kaplan 800

These type of CR's pose a implicit problem where they state that there is only 1 way of doing things. The best answer to weaken such assumptions is to provide another way of doing the same thing.

Like in this case,
It tries to pose the problem that Alaska-Siberia bridge was the only way to travel to that place and since the bridge was blocked there was no way a human could reach there between the period
Solution,
find an argument that shows another way of travelling till there

Hence the correct answer is the one that confirms my argument.

by
Sheerluck Jones
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PREMISE 1. Charcoal from a hearth site in Colorado, 2,000 miles south of Alaska, is known to be 11,200 years old.
PREMISE 2. Researchers reason that, glaciers prevented human migration south from the Alaska-Siberia land bridge between 18,000 and 11,000 years ago.
CONCLUSION- Humans must have come to the Americas more than 18,000 years ago.

ASSUMPTION- CHARCOAL FOUND AT COLORADO COULD NOT HAVE BEEN THAT OF HUMAN WHO MIGRATED BETWEEN 18-11000 YEARS AGO, FROM ALASKA....BUT OF HUMANS THAT WOULD HAVE MIGRATED BEFORE THIS BLOCK PERIOD...

WEAKENER---- COULD BE.....1. SOME EVIDENCE THAT HUMANS DID COME SOUTH DURING THIS BLOCK PERIOD ALSO....
2. NATIVE HUMANS( NOT THE MIGRATED ONES) KNOWN TO HAVE RESIDED ALL THROUGH THIS PERIOD IN COLORADO..
3. NOT POSSIBLE FOR HUMANS TI COME SOUTH MORE THAN 18000 YEARS BACK...

A.Using new radiocarbon dating techniques, it was determined that the charcoal from the Colorado site was at least 11,400 years old. STILL FALLS UNDER THE BLOCK PERIOD.....DOES NOT WEAKEN THE ASSUMPTION, ABOVE, IN ANY CASE

B.Another campsite was found in New Mexico with remains dated at 16,000 years old. STILL FALLS UNDER THE BLOCK PERIOD.....DOES NOT WEAKEN THE ASSUMPTION, ABOVE, IN ANY CASE

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. THATS OK BUT THEY COULD HAVE MIGRATED IN THE PERIOD-- 18,500 - 18,000

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....CORRECT... AS PER OUR ASSUMPTION ABOVE.....

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....TIME TAKEN TO MIGRATE IS IRRELEVANT

Can someone help me with option B. Doesn't this mean that people/human were there in 16000 and hence weakens the conclusion.
I chose D becaus it mentions that there was a way to cross the bridge but need a reason to understand why B is wrong.

This option is a far fetched information. Even if there was another campsite dated at 16000 years , the conclusion is about people who crossed the land bridge and not about people who could have already been there !! The conclusion talks about the migration south from the Alaska Siberia bridge at least 18000 years ago!
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I use timeline in Math to solve this question,
check the picture, author is talking about charcoal and human migration on different timeline.
So, if we can figure out the what happened at what time
the argument will be much easier for us.
kudos, if you like my way.

Hi!

Thank you for this wonderful explanation. Now i have just one doubt remaining and that is, the purpose of 1st statement. I don't understand from where the charcoal came into the whole scene. Should i have ignored this statement and read rest of the argument?
Thanks

"Charcoal from a hearth site in Colorado, 2,000 miles south of Alaska, is known to be 11,200 years old." You meant this sentence?

I think the author mentions this point to avoid the possibility that humans might come to Colorado after 11,000 years ago. Just assume that humans must have come to Colorado before any charcoal existed. Therefore, author states the first sentence to eliminate a weakener, thus strengthening his conclusion.

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.

EMPOWERgmatRichC, egmat, GMATNinja, VeritasKarishma

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.

Conclusion: Humans must have come to the Americas more than 18,000 years ago.
Reasoning: A blockade prevented humans traveling south between 18,000 to 11,000 years ago. If some information showed that the blockade was passable, then the new evidence would cast doubt on the conclusion. Correct answer weakens the conclusion; incorrect answers are neutral to the conclusion or strengthen the conclusion.
A. Neutral answer. If the charcoal was exactly 11,400 years old, then the conclusion is weakened. If the charcoal was more than 18,000 years old, then this would have strengthened the conclusion. It can go both ways.
B. Neutral. The correct answer must prove that people crossed in between 18,000 and 11,000 years ago. This proves that someone was there 16,000 years ago; the humans could have crossed 2,000 years prior, around 18,000, or sometime shortly after 18,000 years ago.
C. Strengthens. If it was impossible to travel south overland from 18,500 years ago, then the conclusion would be strengthened.
D. Correct. If it was possible to cross at least 11,400 years ago, it provides doubt that the glacier prevented all human migration between 18,000 and 11,000 years ago. Hence, D is the answer.
E. Out of scope/Neutral. This does nothing the conclusion.

Although the question is from official GMAT source I highly doubt the quality of this question because this question expects the reader to have some geographical knowledge. I did not at all understood this question being non-native to that area because it expects me to know that Alaska=Americas (Which really took me a while to understand), also where Mexico lies relative to alaska (Mentioned in one of the answer choices)? and some sort of geographical knowledge of siberia as well.
Any thoughts on this??

If we are able to challenge the author's statement that "Humans migrated to the Americas more than 18,000 years ago" with counter-evidence. That would weaken the argument's conclusion

(A) Using new radiocarbon dating techniques, it was determined that the charcoal from the Colorado site was at least 11,400 years old.
It doesn't talk about human migration

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

(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 supports the argument conclusion by saying that human migration was not possible 18500 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.
So there is a way for humans t migrate during the 11,200- 18,000 year period. So humans need not have settled prior to 18,000

(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.
Irrelevant

Great, thanks this is really helpful.

But my question is, does choice (D) not attack the premise "..glaciers prevented human migration south from the Alaska-Siberia land bridge between 18,000 and 11,000 years ago" - to say that actually it didn't seems to conflict with the fact. Or are the ice-free corridor and the land-bridge completely separate entities?

I was also thrown off by the new word 'hearth' - if you have any tips?

avigutman -

I thought option D when combined with the premise -- strengthened the conclusion

Given the word 'ago' -- we are talking about items left of zero on the number line.

Analogy -

I owe at-least 10,000 dollars

That means i owe minimum 10,000 dollars and i may owe MORE (i may owe 11,000 dollars or i may owe 12,000 dollars or i may owe a billion dollars ) -- i am going MORE TO THE LEFT on the number line given we are talking about negatives.

I came at least 10 days before the wedding

That means i may have come 20 days before the wedding ) -- i am going MORE TO THE LEFT on the number line given we are talking about negatives.

Simirlarly, in Option D, this must mean the ice-free corridor allowed for passage at-least 11,400 years ago

This impies, the ice-free corridor allowed for passage perhaps 11,500 years ago or 12,000 years ago or 15,000 years ago or 18,000 years ago, all the way to negative infinity

If i draw the timeline -- option D is saying -- passage of humans was allowed during the time zone highlighted in green (all the way to negative infinity)

Per the premise, no one travelled during the red period

So if combine option D + what it is the premise (premises cant be false) - what remains is the gray zone (all the way to negative infinitiy)

So option D is pointing to the fact that humans probably could travel in the grey zone (i.e. prior to 18,000 years ago)

This strengthens the conclusion, not weakens it

Clearly something wrong in my logic