How did the first rats appear? - briefly
The earliest rats originated from primitive murid rodents that diverged in the early Eocene, roughly 40 million years ago, with fossil records indicating Asian ancestors resembling modern species. These early forms later dispersed across continents, giving rise to the worldwide distribution of rats seen today.
How did the first rats appear? - in detail
The earliest members of the murine lineage emerged during the early Miocene, roughly 20 million years ago, as small, nocturnal mammals adapted to forest floor habitats. Fossil specimens attributed to the genus Pseudomys and related forms show dental and skeletal traits that bridge primitive muroid rodents with later rat species. These early forms possessed ever‑growing incisors, a characteristic that facilitated gnawing on seeds and bark, and a flexible jaw musculature that allowed efficient processing of diverse plant material.
Key evolutionary steps that produced the first true rats include:
- Divergence from other muroids in the Oligocene‑Miocene transition, driven by climatic cooling that created new niches in open woodlands and scrub.
- Development of a robust cranial structure supporting stronger bite forces, evident in fossil skulls from the Siwalik Hills.
- Expansion of the auditory bulla, enhancing low‑frequency hearing for predator detection in dense underbrush.
- Genetic mutations identified in ancient DNA extracts that accelerated reproductive cycles, allowing rapid population growth in fluctuating environments.
Paleontological sites in Eurasia, such as the Baikal region and the Siwalik deposits, yield teeth and jaw fragments dated by argon‑argon methods to 17–15 million years ago, confirming the presence of rat‑like rodents well before modern Rattus species. Molecular clock analyses, calibrated with these fossils, estimate that the lineage leading to today’s Rattus norvegicus and R. rattus split from other murines approximately 12 million years ago.
Subsequent dispersal followed the spread of grassland and agricultural ecosystems during the Pliocene, providing abundant grain resources. Adaptations such as omnivorous diet, high reproductive output, and social flexibility facilitated colonization of new territories, ultimately leading to the global distribution of contemporary rats.