In the long debate over how and when the first Native Americans arrived in the Americas, a group of scientists is arguing that multiple viable possibilities exist, according to a new study in Science Advances Wednesday.

Why it matters: Although there are many circulating theories, research over the past 20 years leans toward the idea that Native Americans arrived via a coastal route around 20,000 years ago. But, this team says they've reviewed enough evidence to indicate another theory is equally or even more strongly true — that they arrived via an inland route. Plus, they say, the earliest they arrived was closer to 16,000 years ago.

"We objected, in part, to assertions of certainty, with respect to positive arguments for coastal routes and negative arguments against interior routes. ... [T]he current data cannot definitively reject either route, and indeed, both routes may have been used."

— Ben Potter, study author

Background: The "peopling of the Americas," as scientists call it, is of great interest and debate. Advances in genome and artifact dating are transforming research but there is still no clear picture of how and when the different Native Americans, such as Amazonian Indians, the Native American tribes of North America, and Inuit tribes in Alaska and Canada, came to the Americas and when they diverged.

What they found: One of the problems is that there aren't a lot of sites that have been discovered with human remains. The oldest human remains located so far are: the 12,700-year-old Anzick Child found in Montana along with tools belonging to the Clovis culture, the 11,500-year-old remains of infants found in central Alaska, and the 8,500-year-old Kennewick Man located in Washington State (whose discovery actually led to a legal tug-of-war).

Other sites have artifacts and animal bones which can indicate human activity, such as: the discovery in Monte Verde, Chile, of 18,500-year-old stone tools and charred animal bones, the findings in the Channel Islands of California of 13,000-year-old stone tools and shell fragments, and most recently, what could be pre-Clovis artifacts found in Texas dated to around 16,000–20,000 years of age.

Some of the conflicting possibilities

1. One idea, called the Solutrean hypothesis, suggests that the first Native Americans originated from Europe at an earlier stage, specifically from what is now France and Spain.

However, the University of Alaska Fairbanks' Potter says this is unlikely:

"It’s been tested and refuted on a number of grounds, not the least of which is the wide variety of technological disconnects that we see. But probably the most damming critique has been on the genetics side where we really don’t find any connection from European Paleolithic foragers and Native American."

Another study author at the press briefing, Ripan Malhi of the University of Illinois Urbana–Champaign, says the Solutrean hypothesis has been mostly debunked. He says genetic evidence rules it out:

"[W]hen the Anzick child was analyzed genomically, the closest relationship was to present day Native Americans and not to Europeans. And so I think that pretty much closed the door on the Solutrean hypothesis."

2. Another main hypothesis is that the Native American ancestors split from East Asians around 25,000 years ago, then lived isolated for a period and entered the Americas sometime around the last ice age.

Whether they lived isolated in Asia or in a place like Alaska is a matter of active research.

Whether it was one mass migration into the Americas and then they split into groups, or whether they entered at different times also is unknown.

And whether they took the North Pacific coast or the ice-free corridor between present-day Siberia and Alaska, or both, is another debate. The viability of the ice-free corridor region — between the Laurentide and Cordilleran ice masses — also is a point of contention, as some say it hadn't yet thawed enough to traverse.

What this study says: They examined genetic, archeological and paleoecological data to identify strengths and weakness of the different routes for the peopling of the Americas. Based on their assessment, they say:

Evidence based on human remains supports the idea that Native American expansion happened 16,000 years ago, since the "earliest securely dated sites [are about] 15,000 years ago."

Based on "actual" evidence, the ancestors were likely isolated in Asia, not in Siberia or Alaska.

The route of travel could have been via the inland, coastal route or both — but the congruence of data favors the inland route. There's evidence the route was traversable at least 15,000 years ago.

While some argue that the Channel Island findings prove the coastal migration viewpoint, Potter says this could be people who came inland and moved to the coast.

On the recent Gault, Texas, findings of pre-Clovis artifacts, Potter tells Axios:

"[N]othing in the genetics rules out occupations by Native American groups after 16,000 or 15,300, and some of the Gault OSL dates overlap with this range. We await critique from the archaeological community on the new stratigraphic OSL dates at Gault which range widely from 24,500 to 14,500 at 2 standard deviations, and of course other related sites dating to the same time period establishing a pattern."

Yes, but: At least one expert, who was not part of this study, took issue with the paper's genetic findings. University of Cambridge's Eske Willerslev — who's renowned in the field for his work on ancient DNA and whose research includes a 2014 study on the Anzick child — tells Axios:

"The authors claim that divergence dates between Siberian’s and Native Americans and the deep split within native Americans are uncertain but in fact the ancient genomic literature show highly consistent results on these matters using different methods: with divergence between Siberian’s and native American ancestors around 23,000 and deep split into the north and southern branches of native Americans around 15,000."

"They also claim equal probability of first Americans entering the lower 48 states through the interior ice free corridor and the costal route ignoring recent literature showing the ice free corridor not being biological viable [until] after Clovis times."