A Mexican castaway who says he was at sea for 13 months steps off the "Lomor" Sea Patrol vessel in Majuro this week. (Giff Johnson/AFP/Getty Images)

Here's what Jose Salvador Alvarenga says happened: On Dec. 21, 2012, he and a teenage acquaintance got into a 23-foot fiberglass boat for a day of shark fishing along Mexico's Pacific coast, where he lives. Then the boat was blown adrift by a storm. They drifted westward for 13 long months. Alvarenga, 37, survived by eating raw fish, birds and turtles. The teenager died after a month. The boat finally drifted near the Marshall Islands, 5,500 miles away in the western Pacific, where locals found and rescued Alvarenga.

It is an amazing, improbable story. As the Associated Press put it, "If true, the man’s ordeal would rank among the greatest tales ever of survival at sea."

But maybe the most astounding thing about Alvarenga's story is that it might not be new at all. A growing number of archaeologists are suggesting that ancient Pacific Islanders may have made the journey to the Americas centuries earlier, maybe more than a thousand years ago, and with far more primitive technology. Imagine if Alvarenga had set out on his journey willingly, in a wooden boat, with only his family and some supplies and an unconfirmed hope of land on the other side.

The conventional wisdom is that humans arrived in the Americas, thousands of years ago, by crossing a now-molten ice bridge from Siberia to Alaska. Much, much later, a small group of Norse explorers landed at Newfoundland around 1000 A.D. Five hundred years later, the Italian explorer Christopher Columbus made it to the Caribbean, and we know how the story goes from there.

Meanwhile, in the much wider Pacific, the almost impossibly brave explorers of today's Polynesia may have been "discovering" the Americas on their own. The science is far from settled; these are just theories, many of them contested and none of them 100 percent certain. But a growing body of research suggest that Alvarenga, unwittingly, may have been retracing one of the most fantastic migratory routes in human history.

One of the most discussed and debated pieces of evidence that Pacific islanders may have sailed all the way to the Americas is a pile of chicken bones. Scientists found the bones in southern Chile and dated them, using a special radiocarbon method, to the 14th century. The bones belonged to an animal almost identical to chickens that, at the time, were found only in the Pacific Islands. This may have been the first American chicken, and it appeared to come from modern-day Polynesia.

It's just a chicken, but there are other little hints in southern Chile. The Mapuche, a tribe indigenous to southern Chile, has a suspicious number of words in common with far-away Polynesians. They wore similar jewelry and rode in similar boats, they both made unusual half-moon clubs that resembled birds, both groups even played a similar stick game.

Far north of Chile, in present-day Southern California, archaeologists found two indigenous tribes that used highly distinctive sewn-plank canoes around the year 700 A.D. At that time, that was highly advanced technology. It was also the same kind of boat used by Polynesians at the other end of the Pacific – and no one else. The archaeologists found other common traits, such as special grooved fishhooks, dated to about 1300 A.D.

There are even signs of possible journeys back: sweet potatoes, native to the Americans, were somehow planted in the Polynesian Cook Islands around 1000 A.D.

If Pacific Islanders really made these journeys, if they set out voluntarily on the same route that almost killed present-day Albarengo despite his many advantages, it demands two questions: how and why?

The "why" is as straightforward as it is tragic. Polynesians spread far and wide, over many centuries, partly because these islands were so ripe and untouched and partly because the explorers had a tendency to exhaust new islands of their resources quickly. Over-fishing, over-hunting and under-planting seemed to drive them ever outward.

The "how" has to do with the ancient Polynesians' remarkable bravery, their leaps in boat-making and navigation, as well as an assist from nature. Those ancient Pacific explorers would have been pushing against the prevailing tail winds, which travel from east to west. That was a challenge but also an insurance policy. As University of Auckland professor Geoff Irwin told National Geographic in 2008, "They could sail out for days into the unknown and reconnoiter, secure in the knowledge that if they didn't find anything, they could turn about and catch a swift ride home on the trade winds. It's what made the whole thing work." It's the same wind that Alvarenga may have ridden from Mexico to the Marshall Islands.

To jump from the Pacific Islands to the Americas would have been far beyond anything these explorers had done before, but not totally without precedent. The journey to Fiji, which is known to have been populated around 1100 B.C., is 500 miles from the nearest shore. Ancient Polynesian history is filled with these journeys: days or weeks in small wooden boats, with no compass and no sure knowledge that there would be land on the other side.

Climate scientists also point out that this burst in trans-Pacific exploration would have happened around the same time as a series of El Niños. (An El Niño is a massive climate event that causes Pacific waters to warm, which also creates lots of high air pressure, which creates wind.) As Roff Smith wrote in National Geographic, this may have even created "Super El Niños" that reversed the trans-Pacific trade winds for "weeks at a time," so that they flew from west to east, possibly sending Polynesian seafarers on journeys planned and unplanned to the Americas.

Not so unlike Alvarenga. They would have lacked his sturdy fiberglass boat, whatever technology he had on board and, perhaps most crucial, the knowledge that there was land and rescue on the other side of the ocean. But if anyone was going to make such a journey successfully – much less willingly – it was going to be the ancient Polynesians.