To find the first Americans, search below the sea

We still don't know how humans got to the Americas. Many researchers are arguing we need to ditch the land-first hypothesis of human migration, and look for underwater evidence of a seafaring past. It's an idea that's gaining momentum, though the evidence is complex and the proposal isn't unanimously accepted.

Data: Sites compiled from academic papers & information provided by Braje; Map: Lazaro Gamio / Axios

This map shows some of the oldest archaeological sites in the Americas. Many in South America are either along the coastline or major waterways, supporting the coastal migration hypothesis. But North America looks a lot messier and lacks coastal sites.

Why it matters: "We're trying to answer one of the most fundamentally important questions in American archaeology," Todd Braje, an author of the paper that appears today in Science, tells Axios. Recent finds have challenged the old, accepted models of a single migration over a single route.

The history of America used to be obvious: during the Ice Age, sea levels dropped and humans crossed a land bridge between Russia and Alaska. From there, they supposedly followed massive animal migrations down a narrow, ice-free corridor that eventually took them all the way to South America.

The mystery: Most evidence indicates the ice-free corridor opened between 13,500 and 15,000 years ago. But scientists have found a handful of archaeological sites in South America that are around 14,500 years old — and some disputed sites that may be thousands of years older — leaving little time for human migration across two continents to happen. Major finds include:

A solution? Braje and his collaborators make the case that early humans instead moved rapidly along shorelines, following a 'kelp highway' of food-filled fisheries. Crossing land means travel over difficult and varied terrain. Following the coast by boat may have been simpler.

Yes, but archaeologist Gary Haynes, who wasn't involved in the study, isn't convinced, "There is still a possibility the interior route was open to recolonizing animals, pioneering humans, and migrating waterfowl early enough to account for the first human presence in the Americas," he says.

Braje and his colleagues are the first to admit crucial data for the kelp highway are missing. There are very few archaeological sites along the North America coast. And, any coastal settlements that did exist are likely underwater because sea levels rose when the Ice Age ended.

"Archeologists have spent decades looking for early sites which are now inundated on the Pacific side of the continents," says Haynes. "I wish they'd find one, so we'd finally have empirical evidence." Braje and his collaborators have expeditions currently planned.

Go deeper: The above map outlines Clovis-associated sites with a black circle. For a long time, archaeologists believed the Clovis people, who appear to share a culture, were the first humans in the Americas. We now know that many archaeological sites predate Clovis. Still, Clovis settlements appear to follow the ice-free corridor, so some suspect they, at least, migrated on land. However, the 15,500 year old Friedkin site in Texas shows stone tools that, although they are not precisely Clovis, do share some characteristics with the later tech. This has led some to suggest that Clovis culture arose near Texas and spread from there. It's also possible that multiple groups independently invented the tools through a natural progression of trial and error.

Get philosophical: Sometimes new discoveries can raise more questions than they answer. It's very possible neither idea is correct — or that both are. "We now know less of the answer than we did 20 years ago," says Braje — and that's a good thing.