Q16 - Biologists agree that human beings

[opulence2001]

I was wondering why C was incorrect. The correct answer D says that the proposition is an assumption, but how is this assumed when the stimulus offers two pieces of evidence explaining why this is true?

[cyruswhittaker]

The proposition is used as an agreed-upon point, and each of the claims are developed in order to align with this proposition, rather than support or be supported by it. The claim is not used as explicit support, ie "evidence," of either of the propositions.

I thought about it like the following:

"We both agree that X is true. Now here are two different explanations that can provide an explanation for a phenomena Y while at the same time not conflicting with X."

So between (C) and (D), the proposition would more closely be described as an "assumption" rather than as "evidence."

[opulence2001]

That makes a lot of sense! Thank-you!

[taaron]

Can someone please explain why E is incorrect? I have a hard time understanding what E is getting at. Thank you!

[WaltGrace1983]

Let's break this down.

(1) Scientists all agree that humans came from fish
(2) Biologists all agree that frogs are related to the kind of fish humans came from

Dr. Stevens-Hoyt

Close match between mitochondrial DNA of lungfish and frogs
→
Ancestor must be lungfish

Dr. Grover:

Mitochondrial DNA is not a reliable indicator (weakening S-H's point)
+
Close chemical match between hemoglobin of coelacanths and frog tadpoles
→
Ancestor much be coelacanth

So basically, the "frogs are definitely related to the species of fish from which humans evolved" is not used as evidence at all. It is what the scientists take as true and then they form their hypotheses with this in mind. They use the statement as a jumping off point it seems.

(A) It is not used as evidence and it is also not used as evidence AGAINST anything. No one is even arguing AGAINST anything; both doctors are simply arguing for their hypothesis. It would be different if one doctor said "Nope! You are wrong because _______" and didn't outline his/her own hypothesis.

(B) Same as (A)

(C) Not exactly. The doctors definitely only argue for lungfish or coelacanth. However, this doesn't mean that the statement is used to say that "well because frogs are definitely related to the fish from which human beings evolved, its gotta be that humans only came from either X or Y." The argument is stating something more along the lines of "well because THIS is true, humans COULD HAVE come from X or Y."

(D) This is right because it shows how the statement is used as a "starting point." It is not evidence and it is not meant to contradict anything. It simply is what both scientists accept to be true and develop their ideas around.

(E) This is super weird. We don't know how reliable mDNA is (as shown by Grover's point) and we don't know if hemoglobin is reliable either. This just goes way beyond the scope of the argument. Either way, the statement doesn't really "imply" anything.

[magic.imango]

I am a bit confused by answer choice (D). I thought assumptions are "unstated premises" but the statement in question is explicitly stated in the stimulus so how can it be an assumption?

[Mab6q]

I am a bit confused by answer choice (D). I thought assumptions are "unstated premises" but the statement in question is explicitly stated in the stimulus so how can it be an assumption?

When we get an argument, we always take the premise to be true, as you stated. In this case, the fact that we are given is that all biologist agree that frogs are related to the species of fish from which humans evolved. The fact that all biologist believe this is not up for debate, that's the part that's taken to be true. However whether those biologist are right is the assumption in question, and both doctors believe that assumption to be true.

[LukeM22]

A couple of more questions:

1) Just confirming: I marked C as wrong because there never was a single contention that humans were from either X or Y; rather there were two separate arguments: one for X, one for Y. Is this correct?

2) According to previous posts, it appears another reason why C is wrong is because the proposition does not constitute "evidence" and is instead an assumption. What would this stimulus look like if that proposition was actually evidence? Would it say "XYZ studies have definitively confirmed that humans are descended from... bla bla bla"? Should this be something that is immediately clear to the reader?

[ohthatpatrick]

Yes, your first reason for (C) is exactly right.

I don't really agree with the "assumption, not evidence" part of the 2nd reason. I wouldn't call this an "assumption" in the LSAT sense, because it is explicitly stated (and assumptions aren't). But obviously the correct answer calls it that.

In a sense, I think they just chose that noun because the "fact" that frogs are related to the human-ancestral fish is a 'given', but it's still an opinion.

It wouldn't be worth getting hung up on "evidence vs. assumption" in the sense that they both play the same role of supporting an argument.

In the context of this paragraph, which prevents two different hypotheses, the fact that frogs are related to the old human-fish is common to both of them.

Thus it feels more apt to call each Dr's "EVIDENCE" the fact that took them in two different directions (the mitochondrial match vs. the hemoglobin match)

You can also see clearer argument indicator words attached to these ideas.

ON THE BASIS OF [prem: mitochondrial match], Dr. Stevens claims [conc: lungfish]

Dr. Grover claims [conc: coelacanths], CITING [prem: hemoglobin match]