Rodents make up roughly 40 percent of all known mammal species, so they had quite a bit to go through to solve their thumbnail mystery. “When I talk with people about this research, I always start by asking, ‘Did you know rodents have thumbnails?’ Most people don’t. The analyses also found that modern rodents descend from a common ancestor thousands of years ago that had thumbnails. This early ancestor and its thumbnails may be key to explaining how rodents eventually took over the world. “For all of the rodents that were used in this study, I bet none of the collectors would have imagined that someone someday would be studying those rodents’ thumbnails.”