Biologists agree that human beings evolved from a fish, but they disagree about which species of fish. Since biologists agree that frogs are definitely related to the species of fish from which human beings evolved, 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. Dr. Grover, on the other hand, contends that mitochondrial DNA evolves too rapidly to be a reliable indicator of relationships between species over long periods of time, and citing the close chemical match between the hemoglobin of coelacanths (a saltwater fish) and that of tadpoles, claims that human beings must be descended from coelacanths. Which one of the following most accurately describes the role played in the dispute above by the proposition that frogs are definitely related to the species of fish from which human beings evolved? ['It is offered as evidence for the contention that human beings must be descended from either lungfish or coelacanths.', 'Since it implies that human beings are not descended from lungfish, it is cited as evidence against the claim that humans are descended from lungfish.', 'It is an assumption that both parties to the dispute use as a starting point for their arguments about human evolution.', 'It implies that either a match of mitochondrial DNA or a match of hemoglobin between lungfish and coelacanths would show that human beings evolved from one of these two species.']