When Did Humans Come to the Americas?

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For years adventurous divers had hunted fossils and artifacts in the sinkholes of the Aucilla [River] about an hour east of Tallahassee. They found stone arrowheads and the bones of extinct mammals such as mammoth, mastodon and the American ice age horse.

Then, in the 1980s, archaeologists from the Florida Museum of Natural History opened a formal excavation in one particular sink. Below a layer of undisturbed sediment they found nine stone flakes that a person must have chipped from a larger stone, most likely to make tools and projectile points. They also found a mastodon tusk, scarred by circular cut marks from a knife. The tusk was 14,500 years old.

The age was surprising, even shocking, for it suddenly made the Aucilla sinkhole one of the earliest places in the Americas to betray the presence of human beings. Curiously, though, scholars largely ignored the discoveries of the Aucilla River Prehistory Project, instead clinging to the conviction that America’s earliest settlers arrived more recently, some 13,500 years ago. But now the sinkhole is getting a fresh look, along with several other provocative archaeological sites that show evidence of an earlier human presence in the Americas, perhaps much earlier.

Which is why I found myself on the banks of the Aucilla with Michael Waters, director of the Center for the Study of the First Americans at Texas A&M University. {snip}

Waters’s team, led by Texas A&M underwater archaeologist Jessi Halligan, worked at the Page-Ladson sink, named for Buddy Page, who discovered it, and John Ladson, the property’s owner. {snip}

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The peopling of the Americas, scholars tend to agree, happened sometime in the past 25,000 years. In what might be called the standard view of events, a wave of big game hunters crossed into the New World from Siberia at the end of the last ice age, when the Bering Strait was a land bridge that had emerged after glaciers and continental ice sheets froze enough of the world’s water to lower sea level as much as 400 feet below what it is today.

The key question is precisely when the migration occurred. To be sure, there were constraints imposed by North America’s glacial history. Researchers suggest that it happened sometime after gradual warming began 25,000 years ago during the depths of the ice age, but well before a severe cold snap reversed the trend 12,900 years ago. Early in this window, when the weather was very cold, migration by boat was more likely because immense expanses of ice would have turned an overland journey into a nightmarish ordeal. Later, however, the ice receded, opening up plausible land bridges for trekkers coming across the Bering Strait.

For decades the most compelling evidence of this standard view consisted of distinctive, exquisitely crafted, grooved bifacial projectile points, called “Clovis points” after the New Mexico town near where they were first discovered in 1929. With the aid of radiocarbon dating in the 1950s, archaeologists determined that the Clovis sites were 13,500 years old. This came as little surprise, for the first Clovis points were found in ancient campsites along with the remains of mammoth and ice age bison, creatures that researchers knew had died out thousands of years ago. But the discovery dramatically undermined the prevailing wisdom that human beings and these ice age “megafauna” did not exist in America at the same time. {snip}

The idea that the Clovis people, as they came to be known, were the first Americans quickly won over the research community. {snip} Clovis sites, it turned out, were spread all over the continent, and “there was a clear association of the fauna with hundreds, if not thousands, of artifacts,” Goebel said. “Again and again it was the full picture.”

Furthermore, the earliest Clovis dates corresponded roughly to the right geological moment — after the ice age warming, before the great cold snap. {snip} “It was a very nice package, and that’s what sealed the deal,” Goebel said. “Clovis as the first Americans became the standard, and it’s really a high bar.”

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The hunters spread through the United States and Mexico, the story went, pursuing prey until too few animals remained to support them in the last cold snap. Radiocarbon dates show that most of the megafauna became extinct around 12,700 years ago. The Clovis points disappeared then as well, perhaps because there were no longer any large animals to hunt.

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{snip} In the late 1970s, the U.S. archaeologist Tom D. Dillehay and his Chilean colleagues began excavating what appeared to be an ancient settlement on a creek bank at Monte Verde, in southern Chile. Radiocarbon readings on organic material collected from the ruins of a large tent-like structure showed that the site was 14,800 years old, predating Clovis finds by more than 1,000 years. The 50-foot-long main structure, made of wood with a hide roof, was divided into what appeared to be individual spaces, each with a separate hearth. Outside was a second, wishbone-shaped structure that apparently contained medicinal plants. Mastodons were butchered nearby. The excavators found cordage, stone choppers and augers and wooden planks preserved in the bog, along with plant remains, edible seeds and traces of wild potatoes. Significantly, though, the researchers found no Clovis points. That posed a challenge: either Clovis hunters went to South America without their trademark weapons (highly unlikely) or people settled in South America even before the Clovis people arrived.

There must have been “people somewhere in the Americas 15,000 or 16,000 years ago, or perhaps as long as 18,000 years ago,” said Dillehay, now at Vanderbilt University.

Of the researchers working sites that seemed to precede Clovis people, Dillehay was singled out for special criticism. He was all but ostracized by Clovis advocates for years. When he was invited to meetings, speakers stood up to denounce Monte Verde. “It’s not fun when people write to your dean and try to get you fired,” he recalled. “And then your grad students try to get jobs and they can’t get jobs.”

The Monte Verde site gained wider acceptance after a panel of well-known archaeologists visited it in 1997 and reached a consensus. Dillehay was pleased that the panel had verified the integrity of his team’s work, “but it was a small group of people,” he said, meaning others in the profession continued to harbor doubts.

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At the Buttermilk Creek Complex archaeological site north of Austin, Texas, in a layer of earth beneath a known Clovis excavation, researchers led by Waters over the past several years found 15,528 pre-Clovis artifacts — most of them toolmaking chert flakes, but also 56 chert tools. Using optically stimulated luminescence, a technique that analyzes light energy trapped in sediment particles to identify the last time the soil was exposed to sunlight, they found that the oldest artifacts dated to 15,500 years ago — some 2,000 years older than Clovis. The work “confirms the emerging view that people occupied the Americas before Clovis,” the researchers concluded in Science in 2011. In Waters’ view, the people who made the oldest artifacts were experimenting with stone technology that, over time, may have developed further into Clovis-style tools.

Waters recently landed other blows to the Clovis orthodoxy in collaboration with Thomas Stafford, president of the Colorado-based Stafford Research Laboratories. In one series of experiments using accelerator mass spectrometry (AMS), a dating technique that is more precise than earlier radiocarbon measurements, they reanalyzed a mastodon rib from a skeleton previously recovered in Manis, Washington, and found to have a projectile point lodged in it. The original radiocarbon tests had surrounded the discovery in controversy because they showed it to be 13,800 years old — centuries older than Clovis. The new AMS tests confirmed that age estimate date, and DNA analysis showed that the projectile point was mastodon bone.

Deploying AMS technology, Waters and Stafford also retested many known Clovis samples from around the country, some collected decades earlier. The results, Waters said, “blew me away.” Instead of a culture spanning about 700 years, the analysis shrunk the Clovis window to 13,100 to 12,800 years ago. This new time frame required the Siberian hunters to negotiate the ice-free corridor, settle two continents and put the megafauna on the road to extinction within 300 years, an incredible feat. “Not possible,” Waters said. “You’ve got people in South America at the same time as Clovis, and the only way they could have gotten down there that fast is if they transported like ‘Star Trek.’ “

But Haynes, of the University of Nevada-Reno, disagrees. “Think of a small number of very mobile people covering a lot of ground,” he suggests. “They could have been walking thousands of kilometers per year.”

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The discovery of numerous artifacts that pre-date Clovis has, over the years, required scholars to come up with different ideas about not only when people arrived in the Americas but how they got here. For instance, if they were already established 14,800 years ago, they must not have used the famed ice-free corridor through North America: Researchers say that it would not appear for another 1,000 years.

Maybe the first Americans didn’t walk here but came in small boats and followed the coastline, some researchers say. {snip}

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Jon Erlandson, a University of Oregon archaeologist, and Torben Rick, an anthropologist at the Smithsonian National Museum of Natural History, propose a pre-Clovis “kelp highway” for coast-hugging seamen skirting the southern edge of the Bering land bridge on their way from northeast Asia to the New World. “People came between 15,000 and 16,000 years ago” by sea, and “could eat the same seaweed and seafood as they moved along the coastline in boats,” Erlandson said. “It seems logical.” The notion that ancient people could travel great distances by boat isn’t far-fetched; many anthropologists believe that humans voyaged from the Asian mainland to Australia 45,000 years ago.

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The link between the Channel Islands artifacts and the western stemmed tradition recently acquired greater significance: Inside the Paisley Caves in Oregon, scientists excavated similar points that were associated with organic material yielding radiocarbon dates 13,000 years old — contemporary with Clovis.

The University of Oregon’s Dennis L. Jenkins, who led the Paisley excavation, re-examined a site first explored in the 1930s. The earlier documentation (photographs and some film) was insufficient to show a definite association between the bones and artifacts. But soon, he said, “we had artifacts and we had the bones” from the site. To determine if the human artifacts were the same age as the animal remains, the researchers conducted radiocarbon dating tests on human coprolites — petrified feces — extracting carbon residue from the organic material digested long ago. They also analyzed human mitochondrial DNA in the sample, probably shed from the intestinal wall, and determined that it came from a modern human with an apparently Asian genome. The toolmakers had lived 13,000 years ago.

“And there is nothing connecting this to any Clovis site,” Jenkins said. “You have two technologies existing at the same time in North America, and there is no direct immediate relationship.”

To answer critics, Jenkins and his team tested the DNA of project participants, to make sure they had not contaminated the coprolites, and tested the sediments surrounding the coprolites for modern rodent urine and other telltale signs the area had been tainted. They found no evidence of contemporary animal or human DNA.

Jenkins and his colleagues published the final results this year and closed the site: “We’ve gotten to the bottom,” he said. “We’ve convinced the people who are willing to be convinced that the caves are as old as Clovis, if not older.”

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Perhaps the most radical scholarly work suggests the Americas were colonized first by immigrants from Europe several thousand years before Clovis. The theory is the brainchild of Dennis Stanford, a curator of North American archaeology at the National Museum of Natural History, and Bruce Bradley, an archaeologist at Britain’s University of Exeter. In their 2012 book Across Atlantic Ice, they suggest that these Europeans reached the New World more than 20,000 years ago, settled in the eastern United States, developed the Clovis technology over several thousand years, then spread across the continent.

This theory is based partly on similarities between Clovis points and finely crafted “laurel leaf” points from Europe’s Solutrean culture, which flourished in southwestern France and northern Spain between 24,000 and 17,000 years ago. Stanford and Bradley argue that artifacts found at Page-Ladson, as well as other pre-Clovis sites, including the Meadowcroft Rock Shelter in western Pennsylvania and the sand dunes of Cactus Hill in southeastern Virginia, have similarities to Solutrean technologies.

The Solutreans, whose territory on the European continent was apparently rather compact, may have been forced by encroaching glaciers and extreme cold to cluster on the Atlantic coast. At some point, Stanford and Bradley say, the stresses of overpopulation may have forced some Solutreans to escape by sea. They headed north and west beneath the Atlantic ice sheet to nudge into North America at the Grand Banks of Newfoundland.

Stanford and Bradley say evidence for the Solutreans’ presence in America includes stone artifacts gathered by archaeologists at several sites on the eastern shore of Chesapeake Bay, all producing dates more than 20,000 years old. Most of the dates were derived from organic material found with the artifacts. The exception was a mastodon tusk with attached bone and teeth netted by a fisherman in 1974, along with a laurel leaf-shaped stone knife. Stanford found the tusk to be 22,760 years old. Among other things, the Solutrean hypothesis provides context not only for the Clovis people, but also for North America’s pre-Clovis sites. And it does not rule out Bering Sea migrations — those could have happened, too.

“Solutrean evolved into Clovis over close to 13,000 years,” Stanford said, and the Clovis hunters began migrating westward when the cold snap brought dry, windy, inhospitable weather to the East Coast.

But the archaeological evidence found so far in support of a European migration more than 20,000 years ago has raised skepticism. {snip}

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None of the doubts expressed by critics have stopped Stanford and Bradley, veterans of the Clovis wars, from pushing forward. “Solutrean people became more and more efficient in exploiting the rich sea margin resources,” they write in Across Atlantic Ice. “Eventually their range expansion led them to a whole new world in the west.”

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