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
The researcher selected nonrodent species that have the specific cell structures she wanted to analyze genetically, though many nonrodent mammals lack these cell structures.