Biologists agree that human beings evolved from a fish, but they disagree about which species of fish. Since biologi...

Gabriela-Diaz on July 8, 2020

Why isn't the answer C? D says it is an assumption. How? Scientists confirm this to be true so it is a fact not an assumption?

Morad on September 28, 2020

Also hoping for an answer! I thought LSAT Language had an 'assumption' to mean an unstated premise, but this premise is actually stated.

jingjingxiao11111@gmail.com on October 27, 2020

Hi I am not an instructor but I was able to get to the right answer D by the following strategy.

I broke the argument stimulus into premises and conclusion

Premise1: biologists agree that frogs are related to the species of fish from which human beings evolved
Premise 2: on the basis of a close match between the mitochondrial DNA of lungfish and that of frogs Dr. Stevens–Hoyt claims that this ancestor must be lungfish.
Premise 3: Dr. Grover disagrees with the conclusion drawn from Dr. Stevens-Hoyt’s study by asserting that mitochondrial DNA evolves too rapidly to be a reliable indicator of the relationships between species over long periods of time
Premise 4: Instead, Dr. Grover cites the close chemical match between the hemoglobin of coelacanths (a saltwater fish) and that of tadpoles as the basis for her claim that human beings must be descended from coelacanths.

Conclusion: Biologists agree that human beings evolved from a fish, but they disagree about which species of fish.

C is wrong because the argument is not arguing that human beings must be descended from either lungfish or coelacanths. The conclusion of the argument in the stimulus concludes simply that biologists agree that human evolved from a fish but cannot agree upon which species of fish we came from. The author never tries to argue that human beings must came from either lungfish or coelacanths since other species of fish are possible contenders for human ancestor according to this argument.

E is wrong because the argument never implies that a match of mitochondrial DNA or a match of hemoglobin between lungfish and coelacanths would definitively show that human must have evolved from one of these two species. The author simply states human must came from one species of fish and does not specify which species of fish our ancestor must be. Lungfish and coelacanths are not the only possible contenders for human ancestor, though a match of mitochondrial DNA or match of hemoglobin are good evidence to support their possible candidacies as human ancestor.

D is correct because it refers to premise 1, which states that biologists agree upon the fact that frogs are definitely related to the species of fish from which human beings evolved. In fact, the reason Dr. Grover and Dr. Stevens–Hoyt test coelacanths and lungfish respectively is due to their shared biologist belief that frogs are related to the species of fish from which human beings evolved, as coelacanths are matched to tadpole (a type of immature frog) and lungfish are matched to frogs, according to the stimulus. The reasons these two biologists both chose fish species closely related to frogs or tadpoles is due to their shared biologists’ belief that frogs must be related to the species of fish that human came from. Thus both biologists, dr. grover and dr stevens-hoyt, specifically targeted fish species that are closely related to tadpoles or frogs.

I hope that I explained it correct and that you find this useful. Please feel free to correct me. Thank you very much.