Q19 - Anthropologist: It was formerly believed

[Laura Damone]

What does the Question Stem tell us?
"assumption required" = Necessary Assumption. Keep an eye out for Defender Assumptions.

Break down the Stimulus:
This argument has a common form, beginning with an opposing point, redirecting to the main conclusion in the middle, and ending with the evidence. Our DNA is really different from Neanderthal DNA, therefore our prehistoric homo sapiens ancestors weren't interbreeding with Neanderthals.

Any prephrase?
There seems to be a pretty big gap between major differences in DNA and the conclusion that there was no interbreeding. Predict an answer that defends against the possibility that interbreeding could have happened and our DNA still ended up really different.

Correct answer:
C

Answer choice analysis:
A) If we were trying to conclude that there was interbreeding, we would need to assume (A). But we're trying to conclude there wasn't interbreeding, so we don't need to assume anything about the proximity of these prehistoric individuals.

B) Our argument hinges on DNA testing of Neanderthal remains, so this argument doesn't assume DNA testing is less reliable in this context than in others. Beware the irrelevant comparison!

C) Correct! This is a hard answer to pick on a first pass, but working wrong-to-right and using the negation technique can get us there. Consider for moment what it would mean if our homo sapiens ancestors had DNA more like Neanderthals' than we do. If there were genetic similarities between them and the Neanderthals, but those are decreasing over time, this could indicate that we once interbred but that our similarities are decreasing as we get further and further from the time of interbreeding.

D) This is the opposite of (A). However, just because we're concluding they didn't interbreed doesn't mean we have to assume that they had no opportunity to do so.

E) This is the opposite of what we need: we're trying to conclude that two species didn't interbreed because there are significant genetic differences. But that implies that there are also some genetic similarities. (E), then, would actually contradict our conclusion, because it would force us to conclude that interbreeding led to those similarities that do exist.

Takeaway/Pattern: On Necessary Assumption questions, particularly in the harder range of the section, be prepared for Defender Assumptions, and use the negation technique to rule out tricky answers and confirm the correctness of your pick.

#officialexplanation

[littlebibliophile]

Another explanation I wrote out. Hope it helps someone, and feel free to comment/criticize!

The argument says that DNA testing proves that homo sapiens did not interbreed with Neanderthals, the proof of this being that current human DNA is significantly different than that of the Neanderthal.

We need to find the assumption that is required to make this arg valid.

(A) shows that the two species lived in the same area. This is not a necessary assumption for the conclusion that the two species did not mate. In fact, it might make it more likely that the two groups might have, if we consider the close proximity.

(B) brings up a fact that questions the reliability of the DNA tests that were used _ definitely not a necessary assumption for an argument based on their results.

(D) might strengthen the argument that the two species did not interbreed, but it is not a necessary assumption for our conclusion. If the two species were in total isolation from one another, this might be sufficient for their not breeding, but it is not necessary.

(E) is extreme. It says that any similarity in DNA between two species must be the result of interbreeding. Nowhere do we see that the stimulus needs this fact, especially because the stimulus makes the conclusion based on the fact that the DNA is "significantly different", not completely different. This leaves room for there to be some similarities in the DNA, and still have the conclusion be valid.

(C) is correct, because if the only premise we have supporting the conclusion that there was no interbreeding is that the DNA of Neanderthals is significantly different from our DNA today, then it must not be true that the DNA of our confirmed ancestors is also significantly different.

[giladedelman]

Lovely explanation!

I think this question really forces us to work from wrong to right; the correct answer certainly didn't jump out at me. But you're right: if our only premise is that our DNA is significantly different from Neanderthals', and we're trying to conclude that this means our ancestors didn't interbreed, then we must be assuming that our DNA is not significantly different from our ancestors'. Otherwise, we would have to say that we're not related to them, either!

Thanks for the post!

[irini101]

Sorry but I am still confused by the expression of C. I understand the prephrase of the correct answer essentially conveys the meaning of "homo's DNA quite similar to contemporary human" and the other four answers are obviously wrong/irrelevant, but in my opinion what C says is: neither homo nor contemporary human's DNA resemble N's DNA.

It seems not quite close to the prephrase answer?

Is there anything I have missed or misunderstood? Could anyone please help and point out? Thanks a lot!

[zhanga]

Try negating (C) and it would say the DNA of prehistoric homo sapiens ancestors of contemporary humans was significantly more similar to that of Neanderthals than is the DNA of contemporary humans.

This would imply that the DNA of contemporary humans is also very different from our prehistoric ancestors. The only evidence for the conclusion that there was no interbreeding is the difference between contemporary human DNA and neanderthal DNA. If contemporary human DNA is also very different from our ancestors, whom we are clearly related to, then the argument falls apart because a difference in DNA wouldn't prove that we are not related to neanderthals.

[joseph.m.kirby]

The explanations in this thread already capture the gist and scope of this question. I'll add my own explanation so as to provide additional support for students tackling this problem.


Pre-HS are ancestors of Modern Humans
Modern Humans & N. DNA is significantly different
--------
C: Pre-HS and N. did not interbreed (N is not an ancestor of Modern Humans)

What's the assumption? If B's (N.'s) DNA is significantly different from A (Modern Humans), B cannot be an ancestor of A. What's the contrapositive of this assumption? If B can be an ancestor of A, B's DNA must not be significantly different from A.

We know that Pre-HS is an ancestor of Modern Humans; therefore, Pre-HS's DNA must not be significantly different from that of Modern Humans, which is what (C) puts forth. If Pre-HS's DNA is significantly more similar (51%) to N, then it cannot also be significantly more similar to Modern Humans. The negated version of (C) would undermine this argument.

Overall, as others pointed out, I think this answer requires eliminating bad answers (ABD). Besides the correct answer, I felt (E) was the most tempting; (E), however, is too strong with "any" (as pointed out earlier in this thread). On a more important, and logical note, (E) puts forward "similarity in DNA --> interbred." As you'll note in the stimulus, we have as evidence "~similarity in DNA," and we have as a conclusion "~interbed." Thus, logically, this answer is reverse of what we would want.

[FarOutsidetheBox]

...Otherwise, we would have to say that we're not related to them, either!

I agree with you that this is an ugly one! I see your argument and I'm still digesting whether or not I agree; it seems like a little bit too much of an inference to go into the logical inconsistencies of assuming our DNA is different from our ancestors.

Instead I thought about it backwards, and I found it very helpful.

Obviously we are descended from our ancestors. We are claiming that these ancestors were pure-blooded homo sapiens, unadulterated by Neanderthals. How do we know? Because our DNA is different from Neanderthals.

But wait a minute, what if our ancestors did interbreed and only in the course of millenia did our DNA diverge? I think that this is what (C) is getting at. If our DNA and Neanderthal DNA is different because our DNA has changed, that then it doesn't prove that we aren't interbred.

In other words, I don't think C is addressing the problem of that not(C) would lead to an absurdity, I think (C) is simply eliminating an alternative explanation for why our DNA is different from the Neanderthals'.

[lsatfreak2013]

I got halfway through typing this and then realized the Anthropologists argument + assumption question, which made it apparent why A was, in fact, not incorrect. Decided to post because it made some of the other explanations on this feed make more sense... Don't know if anyone else will find it useful.

Why A actually weakens the argument:

If not at least some neanderthals lived at the same time and place as the ancestors of the contemporary humans DNA samples are being compared to, then how could they mate/interbred and pass along their DNA? The two species lacked opportunity if this were true.

[ganbayou]

I think I'm confused because when I searched it says Homo sapiens=contemporary humans.
But actually they are ancestors?? Or
prehistoric Homo sapiens=ancestors and Homo sapiens=contemp. humans?

[maria487]

I think what is helpful for this question is seeing it as premise-sub conclusion-main conclusion.

Our conclusion is: Homo sapien ancestors (from this point "HS") didn't interbreed with Neanderthals (from this point on "N").
Why? Because:
DNA testing of N's remains tells us that they didn't.
Why? Because:
Human DNA today is significantly different from N's DNA.

Core: N's DNA different from contemporary human's --> N did not interbreed with HS

We are assuming that our DNA from today being different from N's DNA is enough to tell us that N did not interbreed with HS in prehistoric times. So our DNA today is giving us information about what happened in prehistoric times. (C) gets at this by affirming that HS (our ancestor's) DNA was not more similar to N's DNA than our DNA is now.


I got this question right timed through process of elimination; the 4 incorrect answer choices are very easily eliminated.

[CharlesS800]

I had some trouble with this question, particularly in choosing between C and E for my answer.

When I diagrammed the argument, I put down:

Humans DNA significantly different than neanderthals ---> Homo sapiens' ancestors did not interbreed with neanderthals

There seems like a pretty big gap here, in particular assuming that the Homo sapiens ancestors also did not have DNA that was significantly different than current day humans.

I eliminated A, B, and D because they all seemed out of scope.

When I negated answer choice C, it seemed to give me something somewhat in line with my prephrase, so I kept it around.

I had a tough time negating E in a way that was easy to understand, so I just focused on the language. As others have noted, the "Any" in this choice makes it somewhat problematic and, as Laura has noted, it's the opposite of what we're looking for. I originally eliminated this answer because of the absolute language but upon further review, I think Laura's method of analysis was easier and, ultimately, a more secure way to arrive at the conclusion. This question is just another reminder of the importance of not simply negating answer choices to determine if they break the argument and eliminating those that are out of scope, but also consider what answer choices MEAN in the context of an argument.

[andreperez7]

Premise: The DNA of modern humans is different from the DNA of neanderthals.

Conclusion: Ancient humans and neanderthals did not interbred.

Assumption: If ancient humans and neanderthals interbred modern human DNA would be similar to neanderthals. In other words, different DNA means no interbreeding. As the correct AC shows, this hinges on the assumption that neanderthals and ancient humans had really different DNA -- but we don’t know that. Another possible objection would be that over 100,000 years or so other factors didn’t change modern human DNA enough so that we can’t fairly use them to make a comparison about what happened so long ago; remember we’re using modern human DNA as a proxy for ancient human DNA.

Wrong ACs:

A) There’s no reason to think that neanderthals and ancient humans needed to live in the same place and time. They could have just gone into the woods somewhere far away from home and interbed.
B) Testing samples is not the issue here. This seems to attack the premises, a no-no.
C) Bingo -- See above.
D) “Completely isolated” is too strong for a necessary assumption question. This is more of a sufficient assumption answer choice.
E) Illegal negation.