Researcher: It is commonly believed that species belonging to the same biological order, such as rodents, descended from a single common ancestor. However, I compared the genetic pattern in 3 rodent species -- guinea pigs, rats, and mice -- as well as in 13 nonrodent mammals, and found that while rats and mice are genetically quite similar, the genetic differences between guinea pigs and mice are as great as those between mice and some nonrodent species. Thus, despite their similar physical form, guinea pigs stem from a separate ancestor. Which one of the following, if true, most seriously undermines the researcher's reasoning? ['Some pairs of species not having a common ancestor are genetically more similar to each other than are some pairs that do have a common ancestor.', 'For some genuine biological orders, the most recent common ancestor dates from later epochs than does the most recent common ancestor of other biological orders.', 'Peculiarities of body structure, such as distinctive teeth and olfactory structures, are shared by all rodents, including guinea pigs.', 'The researcher selected nonrodent species that have the specific cell structures she wanted to analyze genetically, though many nonrodent mammals lack these cell structures.']