When did rats appear? - briefly
Rats first emerged in the Oligocene epoch, about 23–25 million years ago. Fossil evidence of the genus «Rattus» dates to the Miocene, roughly 5–10 million years ago.
When did rats appear? - in detail
Rats belong to the genus Rattus, a member of the order Rodentia. The order originated in the Paleocene, shortly after the Cretaceous–Paleogene extinction event, with the earliest known rodent fossils dated to about 66 million years ago. These primitive rodents exhibit dental characteristics that foreshadow later forms but lack the specific traits of modern rats.
The lineage leading to Rattus diverged from other murine rodents during the late Miocene, approximately 10–8 million years ago, according to molecular‑clock analyses. Fossil evidence of Rattus species appears in the Pleistocene record. The oldest confirmed Rattus fossils, identified as Rattus sp., have been recovered from sites in Southeast Asia and date to roughly 2 million years ago.
Key milestones in the evolutionary history of rats:
- Paleocene (≈66 Ma): First rodent fossils; basic gnawing dentition emerges.
- Eocene (≈56–34 Ma): Diversification of early rodent families; expansion of ecological niches.
- Oligocene (≈34–23 Ma): Appearance of murid ancestors; development of more derived molar patterns.
- Miocene (≈23–5 Ma): Split between murine and non‑murine lineages; early murines spread across Eurasia.
- Late Miocene–Early Pliocene (≈10–5 Ma): Divergence of the Rattus lineage from other murines.
- Early Pleistocene (≈2 Ma): First unequivocal Rattus fossils; presence in tropical forest environments of Southeast Asia.
- Holocene (≤12 ka): Global dispersal facilitated by human activity; establishment of commensal species such as the brown rat (Rattus norvegicus) and the black rat (Rattus rattus).
Genetic studies corroborate the fossil timeline, indicating a rapid expansion of Rattus populations during the Pleistocene, coinciding with climatic fluctuations that created new habitats. The subsequent spread worldwide aligns with the rise of agriculture and urban settlement, which provided abundant food sources and shelter.
In summary, rats emerged as a distinct genus in the early Pleistocene, with their ancestral rodent roots tracing back to the Paleocene. Their evolutionary trajectory reflects a combination of early diversification within Rodentia, later specialization in the murine clade, and recent global distribution driven by anthropogenic factors.