The existence of rats stretches back millions of years. To understand this timeline, it is important to first define the term “rat” in a scientific context, which primarily refers to members of the genus Rattus, distinguishing them from hundreds of other large rodents. Tracing their lineage requires looking at the deep history of their family tree, pinpointing the moment the Rattus genus emerged, and observing how their recent history became linked with the global spread of human civilization. This successful timeline is rooted in a remarkable degree of biological adaptability.
Deep Ancestry: The Muridae Family
The evolutionary journey of the rat begins with the Muridae family, an expansive group of rodents that includes mice, gerbils, and voles. This family’s origins are rooted in the Miocene epoch, with the earliest known fossils of the Murinae subfamily appearing around 14 million years ago in northern Pakistan. The Muridae family likely evolved from earlier, hamster-like ancestors in tropical Asia, gradually spreading throughout Eurasia, Africa, and Australia.
By approximately 11 million years ago, these ancestral forms had begun to colonize Africa from Asia, demonstrating an early capacity for dispersal. This initial diversification across continents provided the genetic foundation from which the specific genus Rattus would later diverge.
The Emergence of the Rattus Genus
The emergence of the true Rattus genus occurred much later than the initial Muridae diversification. Molecular phylogenetic studies suggest the deepest divergence within the Rattus genus occurred around 3.5 million years ago. This split, separating the early New Guinean lineages from the Asian lineages, is thought to have been centered in Southeast Asia and the Indo-Malayan region.
This timeline places the origin of the core Rattus group firmly in the Pliocene epoch. Within the Asian clade, speciation continued, with the lineage leading to the Brown Rat (Rattus norvegicus) diverging around 2.9 million years ago. The diversification of the genus was rapid, and the ancestors of the Black Rat (Rattus rattus) and other species split off around 2.2 million years ago.
The Great Migration: Rats and Human Civilization
The global expansion of rats began by leveraging human networks, marking a relatively recent chapter in their history. The two most widespread species, the Black Rat (Rattus rattus) and the Brown Rat (Rattus norvegicus), originated in Asia but rapidly became commensal, thriving alongside human settlements. The Black Rat, also known as the roof rat, originated in the Indian subcontinent and Southeast Asia.
This species was the first to achieve widespread distribution, colonizing Europe as early as the first century AD via Roman trade routes. Following a decline after the collapse of the Roman economy, a second wave of Black Rats spread throughout Europe during the Medieval period, linked to the re-emergence of long-range trade and urbanization.
The Brown Rat, which originated further north in the plains of Asia, began its major migration much later. This larger and more aggressive species began its expansion across Europe and the Americas starting around the 18th century, utilizing faster shipping routes. Its tendency to burrow favored the brick and stone structures of industrializing cities, allowing it to rapidly displace the more arboreal Black Rat in temperate regions. In North America, the Brown Rat appeared later than the Black Rat, achieving dominance in urban coastal areas around the 1700s. This human-assisted dispersal transformed both species into globally dominant mammals.
Evolutionary Success and Global Presence
The longevity of the Rattus genus is rooted in a suite of highly adaptive biological traits. Rats are defined by their omnivorous diet, which allows them to exploit almost any food source in diverse environments, from tropical forests to urban sewers. Their high intelligence and behavioral flexibility enable them to quickly learn and adapt to changing conditions, a trait that proved useful in navigating human environments. Their ability to reproduce rapidly ensures population survival even under intense pressure. These biological advantages explain why rats have achieved a truly cosmopolitan distribution and remain among the most widespread mammals on Earth.