Overview

• An examination of 433 rodent genera shows that roughly 86–90% include species with flat nails on the first digit.
• Comparative analyses infer a nail‑bearing common ancestor about 60 million years ago, supported by fossils ~50 million years old with thumb bones shaped for nails.
• Nail‑bearing thumbs correlate with hand‑based food handling and aboveground or arboreal living, while fossorial lineages more often have claws or lack a thumb unguis.
• Rodents share thumbnail‑like digits with primates only, pointing to convergent evolution of a manipulation‑friendly first digit.
• The team combined museum specimens with sources like iNaturalist to map traits and behaviors, and they plan high‑speed video and skeletal studies to directly test dexterity.