Rats, often perceived as recent inhabitants of human environments, possess a deep and ancient lineage. Their evolutionary journey spans millions of years, tracing back to some of the earliest mammalian groups. While the “rats” familiar to us today have a more recent history, their distant ancestors roamed the Earth long before human civilization emerged. This long past underscores their adaptability and enduring presence across diverse ecosystems.
The Deep Ancestry of Rodents
The story of rats begins approximately 60 to 54 million years ago, during the Paleocene and Eocene epochs, with the emergence of the earliest rodent-like mammals. These ancient creatures, part of a group called Glires which also includes lagomorphs (rabbits and hares), were small and likely insectivorous. Fossil evidence suggests their initial appearance occurred in regions now encompassing Asia and North America.
These early forms represented the foundational stock from which all modern rodents, including the Rattus genus, eventually diversified. The family Muridae, which encompasses present-day rats, mice, hamsters, and gerbils, first appeared around 34 million years ago in the late Eocene period.
Further diversification within the Muridae family continued through the Miocene and Pliocene epochs. This period saw the development of various adaptations that allowed these mammals to thrive in different ecological niches.
Emergence of True Rats
The genus Rattus, commonly known as “true rats,” emerged within the Muridae family between 3.5 and 6 million years ago. These ancestral Rattus species originated primarily in Asia, spanning Southeast Asia, India, China, the Middle East, and Mediterranean countries. This region served as a center for their early diversification.
Approximately 2 million years ago, the black rat (Rattus rattus) and the brown rat (Rattus norvegicus) diverged. The black rat originated in the Indo-Malayan region or India, while the brown rat’s origins are traced to the plains of northern China and Mongolia.
Black rats typically have a slender body, pointed snout, large ears, and a tail often longer than their body. They are skilled climbers and prefer warmer climates, often nesting in elevated structures. In contrast, brown rats are more robust, with a blunt snout, smaller ears, and a tail usually shorter than their body. They excel at digging extensive burrow systems and are better suited to cooler environments, possessing a more omnivorous diet.
Rats and Human Civilization
The global distribution of black and brown rats is closely tied to their association with human activities. Both species exhibit a commensal relationship with humans, living in close proximity and benefiting from human environments. Their adaptability allowed them to thrive alongside human settlements, utilizing resources provided by human populations.
Rats spread across continents primarily through human migration, trade routes, and urbanization. The black rat, for example, expanded its range with the Roman Empire, reaching Great Britain as early as the 1st century AD. It then experienced another significant spread during the Medieval period, likely facilitated by the re-emergence of long-distance trade. Its journey continued globally as Europeans embarked on voyages during the Age of Exploration.
The brown rat followed a similar, though later, pattern of global dissemination. It arrived in Europe by the mid-1500s and reached North America around 1750, often as stowaways on ships. In many cooler and urbanized regions, the brown rat gradually displaced the black rat due to its larger size, more aggressive nature, burrowing capabilities, broader dietary preferences, and greater resistance to extreme weather conditions. The expansion of urban areas and human populations continues to provide opportunities for these rodents.