A fossil recently discovered in Marlandia, a chain of islands, proves

A fossil recently discovered in Marlandia, a chain of islands, proves that a present-day reptile indigenous to Marlandia is descended from an ancient reptile species that lived on the islands millions of years ago. - A fossil was recently discovered. The fossil links the lineage of a current reptile with an ancient reptile that lived in the same area many years ago.

The finding is surprising since the ancestral species was thought to have become extinct when Marlandia was submerged in a global sea-level rise twenty-five million years ago. - The author is surprised by this finding since the ancient species was presumed to have become extinct when the area was submerged a few million years ago (The surprising part is that if the ancestral species had become extinct, how do we have its living descendants!)

Based on the new discovery, many scientists have concluded that the sea-level rise in question left at least part of Marlandia unsubmerged. - Given this surprising finding, many scientists have concluded that the entire area must not have submerged (and thus the ancestral species may not have become extinct at that time.)

Gist:

The passage talks about certain reptiles in a particular chain of islands.

Recent development: A discovered fossil proves that a current species is descended from an ancient one.

Previous understanding: People used to believe that the islands had completely submerged in the past, making the ancient species extinct.

The coexistence of these two scenarios is surprising.(support), Thus, many scientists conclude that the islands must not have entirely submerged (conclusion).


The argument assumes that if the islands had entirely submerged, the species could not have survived. What if the ancestral reptiles survived at sea? If the ancestral reptiles survived at sea, we’d have living descendant of the ancestral reptiles. In such a case, we’d have no basis to say that a part of Marlandia remained unsubmerged.

What if the ancestral reptiles migrated elsewhere, and then the descendants came back and resettled? In such a case too, we wouldn’t be able to arrive at the conclusion of the argument.

The argument also assumes that if an ancestral species goes extinct, it cannot have subsequent descendants. What if, there were some interim species that eventually led to the living descendants?


We’re looking for additional support for the scientists’ conclusion that the entire area must not have submerged. We can strengthen the argument by supporting any of the assumptions we figured above.


A. Reptiles in Marlandia have adapted to many environmental changes since the sea-level rise.
Incorrect. Good for the reptiles! Did some adaptation come about from a complete submerge? Was there a complete submerge? Nothing here gives us a concrete answer for either of these questions.

B. Marlandia separated from a much larger landmass about eighty million years ago.
Incorrect. How the islands came into being has got nothing to do with the argument at hand.

C. No fossils that prove the relationship between the present-day species and the ancestral species have been found anywhere other than Marlandia.
Incorrect. Let’s consider the following statement:
Fossils of the ancestral species have been found at places other than Marlandia. (This statement is quite different from the one above, but stay with us a bit.)
Does this statement WEAKEN the argument?

It does. It indicates that the ancestral species perhaps relocated elsewhere and then the species or their descendants came back to Marlandia, thus making us doubt the hypothesis that the species must have survived in Marlandia itself, and thus weakening the conclusion that the islands must not have completely submerged.

Ok, let’s consider the following variation next:

Fossils that prove the relationship between the ancestral species and the present day species have been found at places other than Marlandia.

Does this statement WEAKEN the argument?

What does finding such fossils that ‘prove the relationship’ tell us? Do we know what type of fossils prove the relationship? Can only fossils of ancient reptile species prove this relationship? Couldn’t fossils of some other species help prove the relationship between the present-day and ancestral species? For example, couldn’t there be some indicative gene discovered in a fossil of species Z that helps prove a relationship between species X and Y? We do not know anything about what kind of fossils could prove the relationship. Thus this variation does not weaken the argument.

Now, let’s consider the option mentioned and discuss whether it STRENGTHENS the argument.

The fact that fossils that ‘prove the relationship’ haven’t been found does not increase our belief that the ancient species did not migrate (and as a consequence the belief that the sea-level rise left at least part of Marlandia unsubmerged.)

In addition, even if archaeologists were looking for such fossils, not finding does not equate to not being present. Fossils are difficult to find. Maybe they are out there, just that they haven’t been found yet.
D. The present-day reptiles are able to thrive on very tiny Marlandia islands.
Incorrect. Good for them! Happy to hear that the present-day reptiles are able to thrive. The argument is about whether the islands submerged and how the ancestral reptiles survived. This option does not support the argument at all.

E. The ancestral reptiles could not have survived long at sea.
Correct. Had the islands been completely submerged, this option suggests, the ancestral reptiles could have gone extinct (since the possibility of surviving at sea is taken away). Since, in such a case, we wouldn’t find their descendants, an expectation that doesn’t match with the reality, it is likely that the islands were not completely submerged. This option does support the conclusion.


Remember: ‘To support’ does not mean ‘to confirm’. Option E does not confirm that the islands were not completely submerged. It does increase the probability, though.

If you have any doubts regarding any part of this solution, please feel free to ask.