PerspectiveArchaeology

Finding the first Americans

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Science  03 Nov 2017:
Vol. 358, Issue 6363, pp. 592-594
DOI: 10.1126/science.aao5473

For much of the 20th century, most archaeologists believed humans first colonized the Americas ∼13,500 years ago via an overland route that crossed Beringia and followed a long and narrow, mostly ice-free corridor to the vast plains of central North America. There, Clovis people and their descendants hunted large game and spread rapidly through the New World. Twentieth-century discoveries of distinctive Clovis artifacts throughout North America, some associated with mammoth or mastodon kill sites, supported this “Clovis-first” model. North America's coastlines and their rich marine, estuarine, riverine, and terrestrial ecosystems were peripheral to the story of how and when the Americas were first settled by humans. Recent work along the Pacific coastlines of North and South America has revealed that these environments were settled early and continuously provided a rich diversity of subsistence options and technological resources for New World hunter-gatherers.

Confidence in the Clovis-first theory started to crumble in the late 1980s and 1990s, when archaeological evidence for late Pleistocene seafaring and maritime colonization of multiple islands off eastern Asia (such as the Ryukyu Islands and the Bismarck Archipelago) accumulated. By the early 2000s, the Clovis-first theory collapsed after widespread scholarly acceptance that the Monte Verde locality near central Chile's Pacific Coast was occupied at least ∼14,500 years ago (and possibly 16,000 to 18,000 years ago), a millennium or more older than Clovis and the opening of a viable ice-free corridor no earlier than ∼13,500 years ago (1, 2). Several more pre-Clovis sites in North America's interior dated between ∼14,000 and 16,000 years ago have gained broad scholarly acceptance (36), along with possible evidence for human presence in eastern Beringia ∼24,000 years ago (7).

A coastal route for the first Americans

Recent archaeological finds show that pre-Clovis people arrived in the Americas before 13,500 years ago, likely via a coastal route along the Pacific Coast. Higher sea levels make finding direct evidence difficult.

GRAPHIC: J. YOU AND N. CARY/SCIENCE

In a dramatic intellectual turnabout, most archaeologists and other scholars now believe that the earliest Americans followed Pacific Rim shorelines from northeast Asia to Beringia and the Americas (8). According to the kelp highway hypothesis, deglaciation of the outer coast of North America's Pacific Northwest ∼17,000 years ago created a possible dispersal corridor rich in aquatic and terrestrial resources along the Pacific Coast, with productive kelp forest and estuarine ecosystems at sea level and no major geographic barriers (9, 10). Kelp resources extended as far south as Baja California, and then—after a gap in Central America, where productive mangrove and other aquatic habitats were available—picked up again in northern Peru, where the cold, nutrient-rich waters from the Humboldt Current supported kelp forests as far south as Tierra del Fuego.

But finding proof for this dispersal route has remained elusive (8). Archaeological evidence for early maritime activity has been growing in several areas along the Pacific Coast of North America, including the ∼13,000-year-old Arlington Man skeletal remains from California's Santa Rosa Island. But no definitively pre-Clovis coastal sites in North America have been well documented or widely accepted.

Testing the kelp highway hypothesis is challenging because much of the archaeological evidence would have been submerged by rising seas since the last glacial maximum (LGM) ∼26,500 years ago. The earlier such a dispersal took place, the further offshore (and at greater depth) the evidence may lie, enlarging already vast potential search areas on the submerged continental shelf. Although direct evidence of a maritime pre-Clovis dispersal has yet to emerge, recent discoveries confirm that late Pleistocene archaeological sites can be found underwater. Recent discoveries at the Page-Ladson site, for example, produced ∼14,500-year-old butchered mastodon bones and chipped stone tools in the bottom of Florida's Aucilla River (3). Several multidisciplinary studies are currently mapping and exploring the submerged landscapes of North America's Pacific and Gulf of Mexico coasts, searching for submerged pre-Clovis sites (8).

With Clovis-first's demise, debate has shifted to whether colonization occurred well before the last deglaciation (before 25,000 years ago) or after it. Currently, most archaeological and genomic data suggest that the Americas were colonized between ∼25,000 and 15,000 years ago (11), probably in the latter half of that range, by anatomically modern humans (Homo sapiens) who followed a Pacific Rim coastal corridor from northeast Asia into the New World.

The uncertainty left by the collapse of the Clovis-first paradigm, however, has opened a Pandora's box of alternative scenarios for the peopling of the Americas, with some scholars and members of the general public quick to accept implausible claims based on limited and equivocal evidence. For example, a recent report on the Cerutti Mastodon Locality (CML) in California would dramatically extend initial occupation of the Americas to ∼130,000 years ago, possibly by a hominin other than Homo sapiens (12). The CML claim hinges on ambiguous artifacts associated with broken mastodon bones and provides minimal evidence for their geological and stratigraphic context (13). The CML claim—similar to a handful of previous assertions for human occupation of North and South America before the LGM—is at odds with most archaeological, paleoecological, and genomic evidence. And despite considerable effort, scientists have found no clear evidence that humans were even in far northeast Asia before ∼50,000 years ago.

Answers to the questions of how, when, and where humans first reached the Americas remain tentative. The small sample of pre-Clovis sites has yet to produce a coherent technological signature with the broad geographic patterning that characterizes Clovis. Distinctive fluted Clovis, other fluted Paleoindian, and fishtail points previously provided a roadmap that archaeologists used to trace the spread of Paleoindians throughout the Americas. Such a roadmap is lacking for pre-Clovis sites. Assemblages with distinctive stemmed (“tanged”) chipped-stone projectile points, crescents (lunate-shaped), and leaf-shaped bifaces found in Japan, northeast Asia, western North America, and South America (see the figure) have been proposed as potential markers of a pre-Clovis coastal dispersal (14) that seems generally consistent with genomic data, which suggest a northeast Asian origin for Native American ancestors some time in the past 20,000 years. But more data are needed to close substantial spatial and temporal gaps between these far-flung finds and trace a dispersal route from Asia to the Americas. Work on early coastal localities along the Pacific Coast from Alaska to Baja California (8), Peru (10), and Chile (1) is helping to fill these gaps.

If the first Americans followed a coastal route from Asia to the Americas, finding evidence for their earliest settlements will require careful consideration of the effects of sea level rise and coastal landscape evolution on local and regional archaeological records (15). Around the globe, evidence for coastal occupations between ∼50,000 and 15,000 years ago are rare because of postglacial sea level rise, marine erosion, and shorelines that have migrated tens or even hundreds of kilometers from their locations at the LGM. Overcoming these obstacles requires interdisciplinary research focused on coastal areas with relatively steep offshore bathymetry, formerly glaciated areas where ancient shorelines have not shifted so dramatically, or the submerged landscapes that are one of the last frontiers for archaeology in the Americas. Methodological and analytical advances are moving us closer than ever toward understanding when, how, and why people first colonized the Americas. Coastal regions are central to this debate.

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