Even the earliest known species of land animals, known from fossils

Question

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. Since neither aquatic nor amphibious animals exhibit these adaptations, early species of land animals must have evolved very rapidly after leaving an aquatic environment.

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

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

(A) Known fossils of early land animals include fossils of animals that lived relatively soon after the first emergence of land animals

(B) Fossils from the late Silurian period represent only a small number of the animal species that were alive at that time

(C) No plants were established on land before the late Silurian period

(D) No present-day species of aquatic animal is descended from a species of animal that once lived on land

(E) All animals alive in the late Silurian period lived either exclusively on land or exclusively in the water

AshutoshB · Answer

Conclusion;  early species of land animals must have evolved very rapidly after leaving an aquatic environment.

Let's consider this way, based on the earliest known fossil to us we are concluding these land animals must have adapted very quickly 
there are many holes in the argument  what if these earliest known fossils are not the earliest one,  ( we have not found the earliest one yet)
so in order to overcome this, we have to assume  " Known fossils of early land animals include fossils of animals that lived relatively soon after the first emergence of land animals "

MikeScarn · Answer

Identify the argument:    "early species of land animals must have evolved very rapidly after leaving an aquatic environment."

Let's negate this answer choice.

(A) Known fossils of early land animals  do not  include fossils of animals that lived relatively soon after the first emergence of land animals.

Wait, we don't have fossils of early land animals? If we don't even have fossils of early land animals, how can we make the argument about early land animals showing highly evolved adaptations to life on land?

This wrecks our argument. Hold.

This wrecks our argument. Hold.

This assumption would weaken the argument. The argument does not  depend  on this assumption.

"Plants" are way out of scope. We don't  need to assume  anything about "plants" for this argument.

First of all, this is some strong language, which we should be cautionary of in Assumption questions. Let's negate this answer choice and see if it wrecks the argument. If it does, this is our answer.

(D)  No   Some  present-day species of aquatic animals are descended from a species of animal that once lived on land.

Okay so if some animals evolved by going from land to water. Interesting... But that doesn't have anything to do with our argument. Our argument is about evolution of land animals going from water to land. So this choice is irrelevant.

Once again, we have some strong language; we'll proceed with caution.

Animals that live on land or in the water are amphibians. This assumption says that we didn't have any amphibians in the Silurian period. This doesn't have anything to do with the argument. Further, the last sentence of the stem says that amphibians didn't exhibit the adaptations. So whether we had amphibians or not, they did not show highly evolved adaptations to life on land.

VerbalBot · Answer

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Even the earliest known species of land animals, known from fossils

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Even the earliest known species of land animals, known from fossils

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