True rodents are widely considered to have originated in Asia. Rodents first appear in the fossil record at the end of the Paleocene and earliest Eocene in Asia and North America, about 54 million years ago (Meng et al. 1994). These original rodents were themselves descended from rodent-like ancestors called anagalids, which also gave rise to the Lagomorpha, or rabbit group.
Murids (Muridae), the family that gave rise to present-day Norway rats, house mice, hamsters, voles, and gerbils, first appeared during the late Eocene (around 34 million years ago). Modern murids had evolved by the Miocene (23.8-5 mya) and radiated during the Pliocene (5.3-1.8 mya) (for more, see Introduction to the Rodentia).
The genus Rattus first emerged within the Muridae family about 3.5 (Furano and Usdin 1995) to 5-6 million years ago (Verneau et al. 1998). The Rattus genus was native to the Mediterranean countries, the Middle East, India, China, Japan, and Southeast Asia (including the Philipines, New Guinea and Australia) (Krinke 2000). After it arose, the Rattus genus underwent two episodes of intense speciation, one about 2.7 million years ago, and another began about 1.2 million years ago and may still be ongoing (Verneau et al. 1998).
The ancestors of Rattus norvegicus and R. rattus diverged from each other about 2 million years ago (Verneau et al. 1998). The closest relative of R. norvegicus is Rattus cf moluccarius (Verneau et al. 1998, Cabot et al. 1997, Usdin et al. 1995). The split between these two species occured around 0.5 million years ago (Verneau et al. 1998). Today, there are 51 species within the genus Rattus.
Figure 1. Phylogeny of rats. Rattus underwent two intense periods of speciation. One about 2.7 million years ago (MYA), another about 1.2 million years ago which may still be ongoing. (Verneau et al.1998)
Asian origin of the Norway rat: The Norway rat (R. norvegicus) and the black rat (R. rattus) originated in Asia. Norway rats originated on the plains of Asia, probably in what is now northern China and Mongolia, where wild rats still live in burrows today. Black rats originated further south in the in the Indo-Malayan region (Krinke 2000, Walker 1964).
Commensalism with humans: Both species of rats began to live in human homes, buildings, and ships, in a human-dependent association called commensalism. It is not clear when this association between humans and rats began: it could have started thousands of years ago (Krinke 2000). Today, both Norway rats and black rats are commensal and both species tend to settle along routes of human migration (Yoshida 1980).
Arrival in Europe: Both black rats and Norway rats traveled to Europe with humans. Black rats arrived in Europe before Norway rats. At first, it was believed that the black rat arrived in Europe at the time of the crusades, but the presence of R. rattus skeletal remains in the Mediterranean and Europe dating to antiquity indicates that the black rat actually arrived much earlier. R. rattus bones have been found in Corsica that date to a point between the fourth and second centuries BC (McCormick 2003), in Pompeii, Italy that date to the second century BC, and in London, UK from the mid-third cenutry AD (Armitage et al. 1984) and in York, UK in the fifth century AD (Rakham 1979).
Norway rats arrived in Europe several centuries after the black
rats, though the exact date of their arrival is unknown.
Norway rat bones have been discovered at the medieval settlement of
Klein Freden near Salzgitter in Northern Germany, which was occupied
from the 9th to the 13th centuries AD (König, 2007), and at
Bodenteich castle in the district of Uelzen, Lower Saxony, dating to
the medieval and post-medieval period.
The Swiss naturalist, Conrad Gesner, drew an illustration of a rat
that is thought to be a Norway rat in his 1553 book Historiae animalium (reported in
Grzmek 1968 and Krinke 2000). This illustration could have been
of a black
rat, though Gesner also mentions albino rats in his text, and albinos
are more likely to be Norway rats than black rats.
There are records of an enormous migration of Norway rats crossing
the river Volga from the province of Astrakhan in Southern Russia in
1727, as observed by the naturalist Pallas (1831), but this was
probably not their first migration.
In any case, there is little evidence to support the claim that Norway rats came to Europe from the Norwegian peninsula, as reported by Castle in 1947 (Krinke 2000).
Eighteenth century reports describe
the displacement of black rats by the more aggressive and larger
Norway rats all over Europe (Grzimek 1968, Krinke 2000).
Norway rats spread beyond Europe: Black rats reached the New World in the 16th century. Norway rats reached North America around 1755 on the ships of the new settlers (Grzimek 1968) and are reported on the east coast of the United States in 1775 (Lantz 1909, reported in Jackson 1982 and Krinke 2000). As in Europe, Norway rats displaced black rats (Krinke 2000).
Today, Norway rats have almost completely replaced black rats in Europe and America, where black rats are now rare or absent in much of their former range. In contrast, in tropical zones black rats have the edge and are more common than Norway rats there (Nowak 1991).
Earliest captive rats: In the eighteenth and nineteenth centuries in Europe, Norway rats were captured and used for food during times of famine. Rat-catchers were hired to exterminate rats and capture live ones for rat fights, rat coursing, and rat pits. Rat-catchers captured and housed wild rats in cages as well (Matthews 1898). During this time, naturally occuring albino, black, and hooded Norway rats may have preferentially captured or selected from litters of captive rats for their distinctive appearance (naturally occurring wild albino rats were first reported in Europe in 1553 by Conrad Gesner, and Castle (1947) reports the existence of wild non-agouti and hooded rats as well).
Temperament and coat color: Selection for tameness leads to changes in hormone chemistry, physiology, and development. These changes have wide-ranging effects, including changes in coat color. In particular, calmness and docility are associated with black coat color (Keeler 1942) and patches of white fur (Trut 1997). The process of domestication may therefore have increased the incidence of black and hooded rats in the domestic rat population.
The first laboratory rats: Albino rats were brought into laboratories for physiological studies as early as 1828. Crampe performed the first breeding experiments in the 1870s and 1880s (Krinke 2000). White rats of European origin were brought to America shortly after this and became the foundation stock of American laboratory rats (Castle 1947). The oldest strain of inbred rats dates from 1856, when the Jardin des Plantes reported a feeder colony of black hooded rats. That colony was still in existence 132 years later in 1988, and gave rise to a strain of inbred rats. The oldest purpose-bred strain of inbred rats, the PA strain, had been created by Helen Dean King at the Wistar institute in Philadelphia by 1909 (Krinke 2000). The Wistar rat, and to a lesser extent, the Sprague-Dawley rat, gradually became the most popular rat strains for laboratory research.