They came in multiple waves, likely in their thousands, crossing from Asia into North America over the Bering Strait. Lower sea levels meant that a land bridge provided a direct route connecting the two continents, before the end of the last Ice Age some 12,000 years ago.
The invaders were grey wolves, which then thrived and became a dominant predator in large areas of North America.
In more recent times their population size has ebbed and flowed, dependent in part upon the public perception of these predators. For some, wolves symbolise fear, while for others they are a symbol of nature and wildness.
But despite their hold on the public imagination, how exactly grey wolves first populated North America has long been a mystery. A new study now offers a surprising answer.
The fossil evidence suggests that grey wolves were abundant in North America as long as 500,000 years ago. It was therefore assumed that today's wolves are descended from these ancient predators.
We were working with the assumption that these should be the descendants of the wolves from before
However, a genetic analysis looking at the family tree of the grey wolf has found that is not the case.
Rather, all living wolves in North America today originated from one colonisation event, researchers report in the Journal of Biogeography. The genetic diversity of all these wolves is "much younger than [was] suggested by the fossil record," says lead author Stephan Koblmüller of University of Graz in Austria.
This means that the population of ancient wolves present 500,000 years ago went extinct and the newcomers re-populated all of North America.
This was surprising. "We didn't suspect they would have disappeared, so we were working with the assumption that these should be the descendants of the wolves from before," says co-author Jennifer Leonard at the Estación Biológica de Doñana, in Seville, Spain.
The wolves came from Eurasia between 70,000 and 24,000 years ago. Bears, deer and people followed, but the exact timing of the influx is unclear.
The wolves came before North and South America was separated by a large glacier, which at its peak covered about a third of the northern parts of North America.
At first, they lived alongside another fearsome predator, the dire wolf. But this creature went extinct around the end of the last Ice Age, when most of its large-bodied prey also died out.
While it is unlikely that grey wolves contributed to the dire wolf's extinction, there are several possible reasons why the less ferocious of the two survived.
Grey wolves are smaller and more agile, so they can focus on hunting smaller prey like deer or hare. Their social nature and ability to hunt in packs also means wolves can easily outcompete larger prehistoric predators, such as sabre-toothed cats, Koblmüller says.
"Most of these other large Pleistocene predators preyed upon the large megafauna," he says. When large megafauna died out these other animals then had less to eat.
After the Ice Age, the grey wolves were well adapted to the environment and prey that was present, so they were free to dominate.
However, by the 1930s intense extermination efforts meant that the grey wolf had almost disappeared from North America. "It used to be the most widely-distributed wild carnivorous species, before it got wiped out in large parts of its original distribution range," says Koblmüller.
The grey wolf population has since rebounded, as conservationists realised their unique value to the ecosystem. Notably, the reintroduction of wolves to Yellowstone National Park in 1995 is often seen as a rewilding success story. Conservationists are calling for a similar programme in Scotland in the UK.
Now they've become inbred, so they have lots of genetic problems
Surprisingly, the Mexican wolf was long believed to have its own origin. This subspecies of the grey wolf is much smaller than its North American relative. Leonard says her study should now change this misperception.
"We showed all of the genetic diversity, including the Mexican wolves, is compatible with a single colonisation event from the Eurasian population," says Leonard.
This new finding could change conservation plans.
In an attempt to protect the Mexican wolves, they have largely been kept isolated from other wolves, to prevent hybridisation. "Now they've become inbred, so they have lots of genetic problems," says Leonard.
So it might be better to let the two subspecies meet. "This common ancestry could be used to justify genetic material from a healthier population," says Leonard.
Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.
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