Logical Reasoning

PT17 · S3 · Q21 Even the earliest known species

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Even the earliest known species of land animals, known from fossils dating from the late Silurian period, 400 million years ago, show highly evolved adaptations to life on land.

Conclusion (must)

Early species of land animals must have evolved very rapidly after leaving an aquatic environment.

Evidence (since)

The earliest known species of land animals show highly evolved adaptations to life on land, which neither aquatic nor amphibious animals exhibited.

Evaluate

Because this argument seems to be real world / no conditional logic / no repeating terms, we're probably going to see an answer play a Defender role, rather than a Linking one.

Given that ... the earliest species of land animals had highly evolved adaptations that neither aquatic nor amphibious animals had.

How could we still argue that ... the early species of land animals did not evolve very rapidly after leaving an aquatic environment?

Hmm. I'm pretty stumped. It would have been natural to try to object that what seem like very quick adaptations to land are really just adaptations that were developing while the ancestor species were still transitioning from aquatic life to life on land. But the premise kind of ruled out that option.

Goal

Look for an answer that provides the Linking idea that gets us from Evidence to Conclusion:

highly evolved earliest species adaptations in → evolved very rapidly earliest known after leaving land species aquatic environment

Or look for an answer that, if negated, would weaken the argument / would allow us to argue that the early land animals did not evolve very rapidly to land.

21.

Which one of the following is an assumption on which the argument depends?

  1. Correct

    Known fossils of early land

    Why this is right

    This answer is playing off a distinction LSAT has tested numerous times: earliest known fossils ? earliest fossils If we negate this answer, it's saying, "The fossils we have of early land animals do not include any fossils of animals that lived relatively soon after the first emergence of land animals". That weakens a ton! The author's evidence is, "look at these fossils! these animals had highly evolved adaptations. They must have evolved them very rapidly." But if we respond with this negation, we're saying, "Those fossils don't belong to the early crop of land animals. You've only got fossils of animals that lived long after the first emergence of land animals. So there's no reason we need to think they evolved these adaptations very rapidly. Now, if you could show me a fossil of an animal that lived relatively soon after the first emergence of land animals and it had these highly evolved adaptations, then, sure, I'd agree they evolved very rapidly."

    Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.

    61% picked this

  2. Fossils from the late Silurian

    Too Strong: only a small number

    There's no reason the author needs to assume anything about what proportion of all animals these fossils represented. Whether they represent only a small number vs. a pretty moderate number would make no difference to this argument.

    2% picked this

  3. No plants were established on

    Out of Scope

    Out of Scope: plants Too Strong: no This argument has nothing to do with plants. The author definitely hasn't committed herself to the extreme position that "there were zero plants established on land prior to 400 million years ago". (I'm pretty sure trees pre-date all animal life)

    1% picked this

  4. No present-day species of aquatic

    Out of Scope

    Out of Scope: present-day Too Strong: no Nothing in this argument has anything to do with present-day species, so the author doesn't need to assume a single thing about them, especially not an extreme claim like "not a single present day aquatic species descended from an animal that once lived on land."

    13% picked this

  5. All animals alive in the

    Too Strong: all

    The author hasn't committed to an incredibly unlikely and extreme position that "every single animal alive 400 million years ago lived exclusively on land or exclusively in water". In fact, by mentioning amphibious animals, the author seemed to even imply a contradiction to this.

    23% picked this

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