Which appeared first, rats or mice? - briefly
Rats evolved before mice. The earliest rat fossils date to the early Oligocene (about 34 million years ago), whereas the oldest mouse fossils appear in the Miocene (around 23 million years ago).
Which appeared first, rats or mice? - in detail
Rats and mice belong to the family Muridae, subfamily Murinae, but their evolutionary lineages diverged at different times. Molecular clock analyses of mitochondrial and nuclear DNA place the split between the genus Rattus (true rats) and the genus Mus (true mice) at approximately 12–15 million years ago, during the middle Miocene epoch. Fossil evidence supports this timing: the earliest confirmed Rattus specimens appear in the late Miocene (about 10 million years ago) in Asia, whereas Mus fossils are documented slightly earlier, in the middle Miocene deposits of Eurasia, around 13 million years ago.
Key points summarizing the chronological emergence:
- Genus Mus: earliest fossils dated ~13 Ma; molecular data suggest origin around 12–15 Ma.
- Genus Rattus: earliest fossils dated ~10 Ma; molecular estimates converge on a similar, but later, divergence time.
- Geographic origin: both groups likely originated in the Indian subcontinent and Southeast Asia, spreading westward and eastward as continents shifted.
The earlier appearance of mice is reflected in the broader diversification of Mus species compared with Rattus, which exhibits fewer extant species despite a later emergence. Consequently, the lineage leading to modern mice predates that of modern rats by several million years.