The first Americans: How and when were the Americas populated?
But the story is not so simple. The once-dominant “Clovis First” Hypothesis has been overturned in recent years by discoveries of an array of pre-Clovis tools and campsites throughout North and South America that date to as early as 16,000 years ago. But how is it that people colonized the Americas so much earlier than once thought? From where did they come? And what routes did they take?
Recent work investigating the eastern slope of the Rockies suggests that an ice-free corridor in this area may not have opened until about 13,000 years ago, thousands of years later than the earliest settlement sites now known in North and South America. That leaves a coastal route as the most likely passage, but which coast the earliest settlers followed is up for debate.
Entry into North America from Asia — either along the coast or through an ice-free corridor — has long been the favored colonization route, but early settlements have been found in Florida and the mid-Atlantic, closer to Europe than East Asia. This has left researchers to wonder whether colonizers came in multiple waves, and from more than one direction. And, as sea levels are higher now than they were thousands of years ago, archaeologists searching the coasts for hidden secrets of the first Americans have been forced out onto beaches, into intertidal zones and even underwater.
Closing the Ice-Free Corridor
“The ice-free corridor is an integral part of the Clovis First model, and yet it wasn’t until about 10 years ago that people started investigating it to see if it was viable,” says Quentin Mackie, an anthropologist at the University of Victoria in British Columbia. “For decades, archaeologists took it for granted that it existed,” he says, while simultaneously suggesting that coastal routes would themselves have been impassable due to ice.
Two studies, both published in August 2016, that looked into the timing and ecological viability of the inland ice-free corridor suggested that, even after the corridor opened about 13,400 years ago, it didn’t become habitable for plants, animals and humans for another 400 years or more. “After the ice retreated, the land would have been initially uninhabitable, without any plants or a functioning food web,” says Peter Heintzman, a paleogeneticist at the University of California, Santa Cruz, and lead author of one of the new studies, published in Proceedings of the National Academy of Sciences. “Ecological succession would have gradually brought in plants and animals as grasslands returned to the corridor. Big game, including bison, would have followed the grass, and people could have followed the big game.”
Heintzman and his colleagues focused on bison fossils for their study, in part because they are plentiful and also because genetically distinct populations of bison are known to have lived to the north and south of the ice sheets. By collecting and dating fossils throughout the corridor region, as well as sequencing DNA pulled from them to determine whether they came from the northern or southern populations, the team determined that the corridor was open to travel for bison by about 13,000 years ago. “When we found fossils from northern and southern bison together again in the same place, we figured the corridor must have been open.”
Using the shotgun approach to determine what species were around at different points in time within the cores, the team found that the first terrestrial plant life dated back to 12,600 years ago, with large grazing animals — and perhaps humans — following soon after.
“Given that we know humans were in South America by 14,700 years ago [at the Monte Verde site in southern Chile], we can definitely say that the first humans [in the Americas] did not use this corridor,” says Mikkel Pedersen, a paleoecologist at the University of Copenhagen in Denmark and lead author of the Nature study. “People migrating later, from the north and from the south, could have used it, but not the first colonizers. They must have taken an alternate route, most likely along the Pacific Coast.”
If Not Inland, Then Along the Sea
The challenge to proving the first colonizers took a west coast route is the lack of archaeological evidence along the coasts: rising sea levels over the last 15,000 years have drowned whatever scant evidence was left by the first colonizers. Only in a few places have rising sea levels and post-glacial isostatic rebound balanced out to keep coastlines relatively stable.
There is plenty of evidence to be found, if you know where to look for it, Mackie says. “Archaeologists have traditionally been very pessimistic about finding evidence along the coast,” he says. “And that has led a lot of people to turn away from the coastal route, even though most would admit that it was a viable route.”
About 15,000 years ago, sea level along the Pacific margin of North America was 150 meters lower than today, and finding artifacts can feel like searching for a needle in a drowned haystack, Mackie says. But you can’t just draw a line a few hundred meters offshore and call that the paleo sea level: Topography, isostatic rebound and erosion all complicate the reconstruction of past shorelines. “It takes a lot of grunt work to establish sea-level history, but the payoff is being able to read the ancient landscape, which gives you a much better chance of finding stuff,” he says.
In some places, where the weight of glacial ice pushed the land down during the last ice age, isostatic rebound of the crust since has actually raised the land surface at about the same rate that sea level has risen locally, creating a zone of relatively stable shorelines. “Reconstructing sea-level histories is one of the most time-consuming, but also one of the most important, steps for coastal archaeologists,” says Duncan McLaren, an archaeologist at the University of Victoria. “We’ve been looking for places that lie at what we call the sea-level hinge — where sea-level rise and isostatic rise have conspired to keep [the relative local] sea level relatively stable.”
The upside of working along the sea-level hinge is that the team doesn’t have to deal with the challenges of underwater archaeology (see sidebar). The downside is that they often have to work in the intertidal zone, where 5-meter-high tides come and go twice a day. Excavating in the intertidal zone requires speed and efficiency to work within the windows of time when tides aren’t too high, McLaren says. “Conditions are difficult, but not as difficult as excavating underwater.”
Cruising the Kelp Highway
The Channel Islands, off the coast of Southern California, have never been connected to the mainland, even when sea levels were much lower. And yet, artifacts and human remains dating back to 13,000 years ago have been found on the islands — evidence that the earliest colonizers probably traveled by boat, says Jon Erlandson, an archaeologist at the University of Oregon.
The insistence that the first colonizers were strictly land-dwellers is left over from the Clovis First model, which was enamored with the idea of spear-wielding hunters as the first colonizers, says James Adovasio, an archaeologist at the Senator John Heinz History Center in Pittsburgh. “For some ridiculous reason, we have all but ignored the role of watercraft and boats in the colonization of the New World,” he says.
The Channel Islands are just one waypoint along what Erlandson and his colleagues have dubbed the “kelp highway” — a nearly continuous and abundant coastal ecosystem supported by underwater kelp forests. “Kelp forests used to run all along the Pacific coast, up into the high Arctic and around the Pacific Rim to Japan,” he says. A long list of edible plants, shellfish, seaweeds, birds and mammals potentially could have sustained people on long coastal voyages around the Pacific Rim.
A North Atlantic Coastal Route?
About 15,000 years ago, lower sea levels would have made the landmass of Florida twice as large as it is today. The Page-Ladson site, which now sits 5 kilometers from the Gulf of Mexico, would have been more than 240 kilometers from the coast, hinting at a culture of hunter-gatherers who were adapted to making their living inland, away from the coast. How people got to Florida so early is an open question, says Jessi Halligan, an underwater archaeologist at Florida State University and lead author of a study published last May in Science Advances announcing the find. “Florida is pretty far from the initial point of entry into the Americas, no matter what model you follow,” she says.
Fossils found in Florida from exotic species such as sloths, gomphotheres, jaguars and capybaras — better known from Mexico, Central and South America — point to a possible migration pathway from the Southwest, rather than across the Midwest, as was assumed under the Clovis model.
Florida has long been one of the most active locations for coastal underwater archaeology (see sidebar), but the mid-Atlantic coast and Chesapeake Bay region have been getting more attention in recent years, and findings from these locations may point to a potential initial migration route across the North Atlantic.
The Solutrean Hypothesis, named for the region of the Pyrenees where Solutrean-type tools were first found, suggests that some colonizers may have come by boat along the North Atlantic pack-ice margin (where they would have more easily found sustenance than in the open ocean) between 20,000 and 16,000 years ago. They would have then spread throughout the mid-Atlantic and westward, possibly planting the seeds of the Clovis culture in the New World. Clovis points and Solutrean points are both thin, bifacial (carved on both sides) and made using similar techniques.
For example, in 1974, a Solutrean-style knife known as the Cinmar biface was dredged from the seafloor 100 kilometers off the coast of Virginia, along with a mastodon skull that was dated to about 22,000 years old. But many question whether the knife should be assumed to be the same age as the skull. Still, the landscape where the tool was found has been underwater for at least 16,500 years, providing an impressively old minimum age for the artifact.
More recent excavations on Parson’s Island on the Chesapeake Bay have turned up a cluster of Solutrean-type points and tools in layers of soils dated to 21,000 years old. “Here, we have good stratigraphic context, really good dating, with artifacts found in place. If that material were found in Alaska, it would be the holy grail,” says Bruce Bradley, an archaeologist at the University of Exeter in England. “The only problem here is location, location, location. People are very resistant to the idea of an Atlantic migration, even though it would have been just as feasible as a Pacific migration,” he says, assuming explorers likely would have followed the edge of the northern ice pack, subsisting by fishing and hunting marine mammals like the Inuit do today.
Bradley first proposed the Solutrean Hypothesis at a conference in 1999 “as something interesting that should be tested. But it quickly became an idea that had to be defended in the face of a lot of negativity,” Hemmings says. “I’m not a huge fan of the hypothesis myself, but I do think it deserves proper vetting. With all the glaring gaps in our understanding of the peopling of the Americas, it’s dangerous to dismiss an idea without due process. We have to remember that we aren’t writing the story. It has already been written. It’s our job to figure it out based on the clues we find.”
America: The Original Melting Pot?
“Nothing says there was just one route. We need to entertain all hypotheses,” Collins adds. “My personal view is that waves of people have been coming into the Americas for a long time. Some came on foot, some came by boat across the Pacific and some came across the Atlantic. It was a long-term process, not a single event.” Studies of modern and ancient DNA have also hinted that the influx was likely complicated, involving multiple waves of people, from multiple gene pools, and possibly, from multiple directions.
Finding early sites, especially along the now-drowned coastlines, will always be a challenge because the footprint of early people was slight. Sometimes, all they left behind was a single tool or a few footprints.
“We have to remember that all we have are snapshots — very disconnected moments in time. But when you look at all these early sites, what’s most striking is their diversity. This diversity is something we are just now coming to appreciate,” Adovasio says. These sites represent a variety of successful lifestyles, he says, suggesting “there is more than one way to colonize a continent.”