Logical reasoning PrepTest 158 · Section 3 · Question 20

As discussed in the Answer Anticipation section, we cannot conclude that any higher primates evolved from the lemur. So we cannot conclude that the chimpanzee, or any other higher primate, evolved from the lemur.

Argument or Facts:
Facts

Question Type:
Must Be True

Stimulus Summary:
Primates! Fun facts about primates include: they're the only animals with opposable thumbs. Actually, enough about primates in general, let's talk lemurs. They're lower primates. And they're the only primate indigenous to Madagascar. And they're the only living lower primate that is diurnal. Oh, and back to primates in general — we think all higher primates evolved from a diurnal lower primate species.

Strategy Overview:

1. Argument or facts in passage?
   1. If argument, identify premises and conclusions
2. Sufficient/necessary conditions in the passage?
   1. If so, diagram statements and draw any deductions
3. Use anticipation to find the answer choice that is absolutely 100% true

Answer Anticipation:
This set of facts features many conditional keywords ("only," "the only," "all"), which is a strong signal that diagramming may help us figure out what must be true. Recall that "only" refers to necessary conditions, while "the only" and "all" point to sufficient conditions. We can diagram these statements as:

1. Opposable Thumbs → Primate
2. Lemur → Lower Primate
3. Primate Indigenous to Madagascar → Lemur
4. Living Diurnal Lower Primate → Lemur
5. Higher Primate → Evolved from Diurnal Lower Primate

As expected from the disjointed nature of this passage, we cannot combine many of these conditional statements. We can, however, combine the second and third premises to form the following conclusion:

Conclusion: Primate Indigenous to Madagascar → Lower Primate

Other than that, however, we cannot make any conclusions. (Well, technically, we could combine the second and fourth as well, but that would lead to a redundant conditional statement.) In fact, we should be cautious with these facts. A heedless test-taker may conclude that all higher primates evolved from the lemur, but that conclusion is unjustified. We think all higher primates evolved from a diurnal lower primate species, but that doesn't mean all higher primates actually evolved from such a species. Even if we knew all higher primates evolved from a diurnal lower primate species, we still couldn't conclude they evolved from the lemur. Lemurs are the only living diurnal lower primate, but higher primates may have evolved from an extinct diurnal lower primate. So instead of trying to force our anticipation, let’s try to assess each answer choice, using the conclusion we drew and the premises we diagrammed.

Answer Choice Explanation:
This answer choice must be true. In fact, it is consistent with the one conclusion we drew in the Answer Anticipation section. As discussed in that section, the only primate indigenous to Madagascar is the lemur, and we know lemurs are lower primates. Therefore, if a primate is indigenous to Madagascar, it is a lower primate. So it is true that "No primates indigenous to Madagascar are ... higher primates." They're all lower primates.

This answer choice also mentions that these primates indigenous to Madagascar are not diurnal higher primates. That's also true. Really, they could have also put nocturnal, insomniac, narcoleptic, or any other sleep pattern in the place of diurnal. We know all primates indigenous to Madagascar are lower primates, which means they're not any kind of higher primates, no matter the sleep pattern.

Key Takeaway:
It's OK to occasionally "over-diagram" a question with conditional statements. We only needed two conditional statements to draw a valid conclusion and find the correct answer. However, diagramming the remaining conditional statements can still help us organize the information, which is useful in eliminating wrong answer choices.

This is not supported. Higher primates may have evolved from a diurnal lower primate (or so we think), but that doesn't mean that they're all diurnal as well. Beyond that, the passage doesn't provide any information about the sleep patterns of higher primates.

Although this answer choice has the weak, wobbly language ("some") we usually see on correct answers to Must Be True questions, this answer choice is incorrect. We have no idea if some lemurs lack opposable thumbs. The passage states that if a species has an opposable thumb, it must be a primate. That could imply that all primates have opposable thumbs or that some primates lack opposable thumbs. We can't say for sure. And even if we knew that some primates lacked opposable thumbs, we can't conclude that lemurs are among the species that lack opposable thumbs.

This answer choice is too strong. The passage tells us that "some" species of lemur are diurnal. But that doesn't imply that all species of lemur are diurnal. Some, or even most, lemurs could be nocturnal.