Logical reasoning PrepTest 133 · Section 1 · Question 19

Incorrect. Since the conclusion is that Homo sapiens and Neanderthals didn't interbreed, there's no need for them to be in the same place at the same time.

Incorrect. The argument relies on testing Neanderthal DNA—to be able to compare it to modern humans—as being at least somewhat reliable. This answer is also comparative, which isn't required for the argument to work. The argument requires testing Neanderthal DNA to be reliable to some uncertain degree, not nearly as reliable as testing living humans.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Former belief - Ancient humans and Neanderthals interbred
New evidence - Modern human DNA is significantly different from Neanderthal DNA
Conclusion - Ancient humans and Neanderthals didn't interbreed

Answer Anticipation:
This stimulus is all about comparisons! The relevant premise compares DNA from modern humans to that of Neanderthals and determines that they're "significantly" different. From this, it concludes that ancient humans and Neanderthals didn't interbreed.

But note that shift—from a premise about modern human DNA to a conclusion about ancient humans. For this argument to work, what we learned about modern humans by analyzing their DNA and comparing it to Neanderthals must hold true for ancient humans, as well. If the ancient human DNA is much more similar to Neanderthal DNA than ours is, the argument falls apart.

Since this argument jumps from modern humans to ancient ones, it assumes that our DNA is relevantly similar to our ancestors' DNA, so let's find an answer establishing that.

Answer Explanation:
The stimulus compares modern human DNA to Neanderthal DNA to conclude something about ancient human ancestors. That requires some assumptions about the similarity between modern humans and ancient ones! If ancient human DNA was more similar to Neanderthal DNA than modern human DNA is, the argument falls apart because it's possible that Neanderthals and ancient humans did interbreed, and then evolution over the intervening millennia changed human DNA to what it is today, without similarity to Neanderthal DNA. This answer is necessary for the argument.

Key Takeaway:
When an argument uses a premise about one group to justify a conclusion about another, it's assuming that those groups are relevantly similar. As always, noting the groups being discussed is key to finding such gaps.

Incorrect. The argument concludes that ancient humans and Neanderthals didn't interbreed, but that doesn't require complete geographic isolation. They could have lived in the same place and still avoided interbreeding.

Incorrect. First, this answer is too extreme in stating that "any" similarity must be the result of interbreeding. Second, this answer is the negation of the assumption made by the argument—that a lack of similarity shows a lack of interbreeding.