Much later, a wave of brown rats spread northeast, into Japan and Siberia. Another emigrated west, eventually reaching Europe in what appear to have been three major arrivals on the Continent. These rats may have traveled on overland routes, or perhaps hidden on ships that sailed along the coasts of Asia and Europe.

The new study suggests that brown rats were slower to spread around the globe than our other familiars, the black rat and the house mouse. Geography may be the reason: House mice originated in the Fertile Crescent, and black rats in India.

Farming societies and widespread trade arose in those places much earlier than in northern China, giving the black rat and the house mouse early opportunities to travel.

But in the past three centuries, the brown rat has more than made up for lost time.

Brown rats in Alaska and along the Pacific Coast of the United States and Canada can trace much of their ancestry to Russia, Dr. Puckett and her colleagues found. Their ancestors may have stowed away aboard ships that traveled to fur-trapping communities in the New World in the 1700s and early 1800s.

But the brown rats of Europe became the true globe-trotters. As Western European countries colonized other parts of the world, they took the rodents with them.

The brown rats of New York and other Eastern American cities trace their ancestry to those in Western Europe. So do brown rats in South America, Africa, New Zealand, and isolated islands scattered across the Atlantic and Pacific.