In May 2010, a whale showed up on the wrong side of the world.

A team of marine biologists was conducting a survey off the coast of Israel when they spotted it. At first they thought it was a sperm whale. But each time the animal surfaced, the more clearly they could see that it had the wrong anatomy. When they got back on land, they looked closely at the photographs they had taken and realized, to their shock, that it was a gray whale. This species is a common sight off the coast of California, but biologists had never seen one outside of the Pacific before.

Aviad Scheinin, one of the marine biologists on the survey, posted the news on the web. “Nice Photoshopping,” someone replied.

Three weeks later, Scheinin got one more bit of news about the whale. It was photographed off the coast of Spain, having traveled 1864 miles. Then it disappeared.

After three years, a second gray whale appeared off the coast of Namibia in 2013. Comparing photographs, scientists could see that it was a different animal than the one that visited Israel. After lingering along the coast of Namibia for a month, the whale vanished.

These two sightings have left whale experts startled. In an interview with the Orange County Registeran interview with the Orange County Register, one scientists compared the feeling to walking down a street in California and seeing a giraffe.

But according to a new study, these two whales may be a hint of the new normal. Gray whales may be poised to move into the Atlantic, because we’re opening a path for them through the Arctic. But it’s not an unprecedented invasion. To some extent, it’s a case of history repeating itself.

View Images A feeding gray whale. Davis Melzer/National Geographic

California’s gray whales give birth each winter in the lagoons of the Baja Peninsula. Then they migrate up the west coast to the Arctic for the summer. They power these tremendous migrations–the longest of any mammal–by ramming their mouths into the sea floor and filtering out tiny crustaceans from the sediment. When they rise back up to the ocean’s surface, they bring with them wide muddy plumes.

Aside from the California population, the only other known population of gray whales is a small group of animals on the western side of the Pacific. But scientists have had hints for a long time that gray whales might once have lived in the Atlantic as well.

In the eighteenth century, whaling ships off the coast of New England chased what naturalists at the time referred to as “scrag whales.” Their descriptions of scrag whales are a match for gray whales. In the 1800s, fossil-collectors picked up whale vertebrae on the coast of England. Many years later, paleontologists found that the bones belonged to gray whales.

These findings suggested that gray whales once lived in both the Atlantic and Pacific. That’s the case today for other filter-feeding whales (known as baleen whales). Species such as humpback whales and fin whales split into Atlantic and Pacific populations a couple million years ago and have remained distinct ever since.

Scientists suspected that gray whales spread across both oceans millions of years ago. Later the planet has cooled, creating an icy Arctic that formed a barrier between the two populations. The gray whales of the eastern Pacific would migrate as far north as they could manage before reaching the ice, and then head back south. Presumably the Atlantic gray whales had a similar migration. Isolated for millions of years, the gray whales of the two oceans might well have evolved into different species. If that were true, then whalers must have driven the Atlantic gray whale species to extinction, while sparing the Pacific one.

View Images Engraving of gray whale by Charles Scammon, 1872

To explore the mystery of these whales further, a team of researchers has taken a fresh look at the fossils of Atlantic gray whales. Instead of just observing the anatomy of the bones, the scientists probed them for ancient DNA. They also measured the amounts of carbon isotopes in the bones to determine their age. The fossils ranged in age from just a few hundred years old to over 50,000 years old.

The scientists were able to use all this information to draw a family tree of gray whales, showing how Atlantic and Pacific gray whales were related to each other. They could also estimate how long ago the branches split apart.

The gray whale’s tree turned out to be different from those of other baleen whales. The Atlantic and Pacific populations of gray whales are not a pair of ancient, distantly related lineages. Instead, the Atlantic gray whales are actually made up of at least four different lineages. And each of the Atlantic branches is most closely related to a different branch of Pacific gray whales.

In other words, Pacific gray whales have periodically swum across the Arctic Ocean and into the Atlantic and established populations that survived for millennia. The scientists can identify several waves of immigration. One took place about 79,000 years ago, and then three others happened more recently, between about 10,000 and 5,000 years ago.

The timing of these colonizations is telling: the whales appear to have moved into the Atlantic whenever it was warm enough for them to get through. Between 135,000 and 70,000 years ago, the climate was so warm that the Bering Strait was open year-round, giving gray whales access to the Arctic Ocean. Once these gray whales got to the Atlantic, they then endured until at least 5,000 years ago.

Then a new ice age began. Glaciers grew, sea levels dropped, and gray whales could no longer get across the Arctic. Sixty thousand years passed before the ice age ended with a sudden burst of warmth. And that’s when new waves of gray whales came into the Atlantic. The Arctic then cooled somewhat, closing the door once more.

Now we are warming the Arctic again by releasing greenhouse gases into the atmosphere. If history is any guide, global warming in decades to come may open up the Arctic for Pacific gray whales, some of whom may wander off their regular migrations and end up in the Atlantic.