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!