How did rats appear on Earth? - briefly
Rats evolved from early muroid ancestors that diverged in the early Eocene, about 50 million years ago. Their worldwide presence stems from natural range expansion and later close association with human habitats.
How did rats appear on Earth? - in detail
Rats belong to the order Rodentia, suborder Myomorpha, family Muridae, subfamily Murinae. Their lineage diverged from other muroid rodents during the middle Miocene, roughly 12–10 million years ago. Fossil specimens assigned to early murine genera appear in Eurasian deposits of that age, confirming the initial radiation of the group.
Key stages in the emergence of modern rats:
- Miocene diversification (≈12–8 Ma): Early murines develop the dental and cranial traits that define the subfamily Murinae—high‑crowned molars, robust incisor roots, and a flexible jaw musculature.
- Late Miocene–Pliocene spread (≈8–2 Ma): Murine rodents expand across Eurasia via forest corridors and emerging grassland habitats. Adaptive radiation produces several genera, including the ancestors of the genus Rattus.
- Early Pleistocene emergence of Rattus (≈2.5 Ma): The earliest recognizable Rattus fossils are recovered from Chinese and European sites, showing a body size and skeletal morphology comparable to extant species.
- Holocene global distribution (≤10 kyr): Natural dispersal across land bridges (e.g., Beringia) brings rats into North America and Africa. Subsequent human activity—shipping, agriculture, and settlement—accelerates their spread to virtually every continent.
Morphological adaptations that facilitated the successful colonization include:
- Continuously growing incisors that permit gnawing of diverse materials.
- Omnivorous dentition allowing exploitation of seeds, insects, carrion, and human waste.
- High reproductive output (short gestation, large litter size) enabling rapid population establishment.
- Behavioral flexibility and tolerance of varied climates, from tropical jungles to arctic tundra.
Genetic analyses of mitochondrial and nuclear DNA corroborate the fossil record, indicating a common ancestor for Rattus norvegicus and Rattus rattus around 1.5 million years ago. Phylogeographic studies trace the origin of these two species to Southeast Asia, where the highest genetic diversity is observed.
In summary, rats originated from early murine rodents that diversified in the Miocene, gave rise to the genus Rattus in the early Pleistocene, and achieved a worldwide presence through a combination of natural range expansion and extensive human-mediated transport.