Logical Reasoning

PT158 · S3 · Q20 The study of primates is interesting

A free, expert breakdown of this official LSAT Logical Reasoning question.

Save & drill this skill — build targeted practice sets from questions like this one
Video walkthroughs — watch every question solved step by step
81 official LSATs — as questions, timed sections & full-length tests

Start practicing free

The study of primates is interesting for many reasons, including the fact that only primates have opposable thumbs.

Information

conditionals (only / the only / all)

have opposable thumbs → primate

primate indigenous → lemurs to Madagascar

lower primates and → certain lemurs diurnal and living

higher primate → evolved from one single diurnal lower primate

Evaluate

We’re trying to figure out whether we can combine any of these claims. We got multiple conditionals, but it doesn’t seem like any of them chain together. What are the Overlapping Concepts? Lots. Primates, lower primates, lemurs, and diurnal all show up multiple times.

Given that we were told that all higher primates evolved from a single diurnal species of lower primates, and given that some species of lemur are the only living lower primates that are diurnal, we have some support for the inference that all higher primates evolved from a lemur species, but this is Must Be True, not Most Supported. We don’t know that for sure.

Goal

Look for a provable answer. It’s hard to predict what it will be since this assortment of facts didn’t lend itself to any obvious combining / synthesizing.

20.

Which one of the following can be properly inferred from the information above?

The chimpanzee, a higher primate,
Supported But Not Certain
The passage states that all higher primates evolved from a single diurnal lower primate species, but it never identifies that species as a lemur. The passage says some lemurs are diurnal lower primates, and higher primates evolved from a diurnal lower primate — but these could be different species of diurnal lower primates. The passage does not establish that the ancestral species was a lemur, let alone a chimpanzee's specific ancestor. Furthermore, "evolved from" describes a distant ancestral relationship, not a direct lineage claim linking chimpanzees specifically to lemurs. This inference requires assumptions beyond what the passage states.
17% picked this
Correct
No primates indigenous to Madagascar
Why this is right
The passage states that lemurs are the only primates indigenous to Madagascar (F3) and that lemurs are lower primates (F2). From these two facts, it follows necessarily that all primates indigenous to Madagascar are lower primates — and therefore no primates indigenous to Madagascar are higher primates. Since no higher primates are indigenous to Madagascar, no diurnal higher primates are indigenous to Madagascar either. This answer captures exactly this inference: "no primates indigenous to Madagascar are diurnal higher primates." The reasoning is airtight — if the entire set (higher primates) is excluded from Madagascar, then any subset of that set (diurnal higher primates) is also excluded. No additional assumptions are needed.
Skill tested: Must be True · how this choice captures the argument's function is the move to repeat next time.
62% picked this
No higher primate is nocturnal.
Too Strong
The passage says all higher primates are thought to have evolved from a single diurnal species of lower primates. This tells us the ancestral species was diurnal, but it does not tell us that all current higher primates are diurnal. Evolution from a diurnal ancestor does not guarantee that all descendants remain diurnal. Many higher primate species could have evolved nocturnal habits over millions of years. In fact, some higher primates (like certain prosimians reclassified or owl monkeys) are nocturnal. The passage provides no information about the current activity patterns of higher primates as a group — only about their evolutionary origin.
13% picked this
There are some lemurs without
No Support
The passage states that only primates have opposable thumbs — meaning ALL primates have them, and nothing else does. This means lemurs, as primates, have opposable thumbs. The passage does not say or imply that any lemurs lack opposable thumbs. The "only primates" formulation establishes that opposable thumbs are exclusive to primates, but it does not address variation within the primate order. Since lemurs are primates, the passage actually implies they DO have opposable thumbs, making this answer contradict the passage rather than follow from it.
6% picked this
There are no nocturnal lemurs.
Too Strong
The passage says some species of lemurs are the only living lower primates that are diurnal. This means some lemur species are diurnal, but it does not say all lemurs are diurnal. The word "some" leaves open the possibility that other lemur species are nocturnal. In fact, many lemur species in the real world are nocturnal, and the passage's careful use of "some species of lemurs" rather than "all lemurs" signals that not every lemur is diurnal. This answer claims there are no nocturnal lemurs, which goes beyond — and likely contradicts — the passage's careful "some" qualifier.
3% picked this

Continue the review in LSAT Lab

Save this question, watch the video walkthrough, and drill similar questions in your LSAT Lab account.

Save and practice this skill

LSAT Lab

Turn this review into a targeted study plan.

Save this question, drill more like it, watch the video walkthrough, and track your progress in your LSAT Lab account.

Start practicing free