Fijian iguanas pose a conundrum to biologists: How did these lizards normally found in the Americas wind up thousands of miles away on an isolated tropical island in the South Pacific?

By floating on a raft of downed trees and broken branches, according to a study published Monday in the journal PNAS. The voyage made by these inadvertently intrepid iguanas would represent the longest transoceanic migration of any nonhuman land vertebrate.

“We often forget how vast the Pacific Ocean is,” says Mozes Blom, a biologist at the Museum für Naturkunde in Berlin who wasn’t involved in the study. “But it’s really when flying across the Pacific, and not seeing anything but ocean for hours, that it dawns how unique it is that some animals manage to disperse across it.”

Biologists have long disagreed over how iguanas got to Fiji. While some supported the raft idea, others argued that such a long journey was too far-fetched. Instead, iguanas could have traveled there more gradually, island-hopping and walking across ancient land bridges to populate parts of Asia and the Pacific before dying out everywhere except Fiji.

But there wasn’t enough evidence to distinguish between these possibilities, said Simon Scarpetta, an evolutionary biologist at the University of San Francisco. “Part of the problem was that we didn’t really know how the Fijian iguanas were related to other iguanas. And we didn’t really know how old they were.”

To find that out, he and his colleagues used genomic data from 14 different species to build an iguana family tree and estimate when Fijian iguanas diverged from their cousins. The results surprised Scarpetta.

“The iguanas that live on Fiji were most closely related to a group of iguanas that I knew very well from the United States called desert iguanas,” he says.

The team estimates that those two groups split about 34 million years ago, a timeline that doesn’t quite add up with the land-bridge idea, as clear routes from the Americas via the Bering Strait or Antarctica would have been mostly underwater by then. They argue that a single dispersal event, likely more than 5,000 miles, from North America to Fiji is the most likely scenario.

The study “does an excellent job cracking such a tough phylogenetic question,” says Blom. He generally buys their argument, though he cautions that alternatives can’t be completely ruled out because “these scenarios played out over such a long timescale.”

Scientists have observed iguanas making similar, albeit shorter, trips in the Caribbean. In the mid-’90s, researchers actually tracked a group of about 15 iguanas hitching a ride on a tangle of downed trees after a hurricane. The iguanas safely made it from Guadeloupe to Anguilla, Scarpetta said, a distance of about 186 miles.

Reaching Fiji would have been nearly 30 times as far. But if any creature could make such a journey, it’s iguanas, said Scarpetta.

“They’re large, so they have a good amount of body mass to sustain them,” said Scarpetta. “And they’re herbivorous, so if they’re floating around on vegetation, they may even have a source of food to eat.”

Given their desert ancestry, these iguanas would’ve been ideally suited to withstand the dehydration and intense sun of such a long journey.

Even with those advantages, surviving such a journey still defies belief. But this analysis suggests that from the perspective of evolutionary timescales, Blom said, “it may not be that improbable.”