Who were the First Americans?


The Story Genetic Evidence Archaeological Evidence Clovis Combining the Evidence
The Story of the First Americans

When did modern humans colonize the Americas? From where did they come and what routes did they take? These questions have gripped scientists for decades, but until recently answers have proven difficult to find. New techniques of molecular genetic analysis, and a reinvigorated search for early archaeological sites across the western hemisphere, recently have led to some astounding results. From genetics we now know that a single population of modern humans dispersed from southern Siberia toward the Bering Land Bridge as early as ~30,000 years ago, and further dispersed from Beringia to the Americas after ~16,500 years ago. From archaeology, we know that the first Americans appeared south of the Canadian ice sheets by ~15,000 years ago, 2000 years before the emergence and spread of Clovis. The route taken by the first explorers appears to have been along the recently deglaciated north Pacific coastline.

photo of examples of Clovis lithic technologyExplaining when and how early modern humans entered the New World and adapted to its varied environments is one of anthropology's most exciting and enduring questions. Until recently, it was generally believed that about 13.5 ka (thousand calendar years ago) the first migrants spread rapidly from Beringia to Tierra del Fuego in a few centuries, passing through an interior "ice-free" corridor in western Canada, becoming Clovis, and hunting to extinction the last of the New World's mega-mammals.

Today, we realize that the peopling of the Americas was a much more complex process, because of two significant developments that occurred during the past decade. Molecular geneticists, using refined methods and an ever-increasing sample of living populations and ancient remains, are now capable of providing reliable information on the Old World origins of the first Americans, timing of their initial migration to the New World, and number of major dispersal events. Archaeologists, with renewed vigor since the acceptance of Monte Verde, Chile, as the first site to break the Clovis barrier, have found new sites and re-investigated old ones using new methods, to test whether a "Pre-Clovis" population existed in North and South America, and to explain how early populations colonized its empty landscapes. Our goals here are to provide an up-to-date interdisciplinary synthesis of the topic, especially the current molecular genetic and archaeological records, and present a working model explaining the dispersal of modern humans into the New World.

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The Genetic Evidence

Old World Origins

All human skeletal remains from the Americas are anatomically modern Homo sapiens; thus the peopling of the New World is best understood in the context of the evolution and dispersal of modern humans in the Old World. Modern human dispersal from Africa across Eurasia began by c. 50 ka and culminated with colonization of the Americas. Evidence from nuclear gene markers, mitochondrial (mt)DNA, and Y chromosomes clearly indicates that all Native Americans came from Asia. Molecular genetic diversity among modern Native Americans fits within five mtDNA (A, B, C, D, X) and two Y-chromosome (C and Q) founding haplogroups, and all of these are found among indigenous populations of southern Siberia, from the Altai to Amur regions. Of these haplogroups, only X is known from both central Asia and Europe; however, X is a large, diverse haplogroup with many lineages, and the lineage found in Amerindian populations is distinct from those in Eurasia. Ancient DNA from early American skeletal remains and human coprolites provides an important link between the present and the past; these, too, have only yielded Native American haplogroups of Asian origin. Based on the modern and ancient DNA records, then, Asia was the homeland of the first Americans, not Europe, negating the recently proposed "Solutrean hypothesis," that Clovis people were derived from an Upper Paleolithic population on the Iberian Peninsula.

Using contemporary mtDNA and Y-chromosome variation as a "genetic clock", geneticists calculate that modern humans dispersed into greater central Asia by 40 ka, setting the stage for the colonization of the Americas. Corroborating human skeletal evidence of this event, however, is still scarce. The earliest modern human remains in Siberia are from Mal'ta and date to only 24 ka. Despite this, in eastern Asia modern human fossils have been unequivocally dated to the critical period, 39-36 ka, and in Siberia archaeological evidence suggests that modern humans entered the region by 40 ka, when initial Upper Paleolithic technologies, tool forms, items of personal adornment, and art appeared for the first time. In Europe, archaeologists link the emergence of such behaviors to the spread of modern humans from southwestern Asia.

Timing of Colonization

Establishing when central Asian and Native American haplogroup lineages last shared a common ancestor has proven to be a sticky subject for molecular geneticists. First coalescent estimates based on variation in extant mtDNA lineages set the event at about 40 ka, but in recent years, mtDNA coalescence has been variably calculated at 23-25 ka or less than 20 ka, after the last glacial maximum (LGM).

Initial estimates based on Y-chromosome variability suggested the divergence of the first Americans from Asians could have occurred anytime from 58 to 2 ka, but in the past five years nearly all published calculations are less than 22.5 ka, and most fall between 15 and 20 ka. The differences in calculations are the result of several issues, including potential variation in mutation rates, variable and sometimes circular techniques of calibrating coalescent times to calendar years, time-dependency of mutation/substitution rates, and effects of genetic drift on the original founding population. Nonetheless, most studies now suggest that dispersal from Siberia to America became after 30 ka.

New analyses of haplogroup sub-clades help to resolve when modern humans subsequently spread from Beringia into the rest of the Americas. Three sub-clades of mtDNA sub-haplogroup C1 are widely distributed among North, Central, and South Americans but absent in Asian populations, suggesting that they evolved after the central Asian-Native American split, as the first Americans were dispersing from Beringia. The estimated date of coalescence for these sub-clades is 16.6-11.2 ka, suggesting that the colonization of the Americas south of the continental ice sheets may have occurred sometime during the late glacial, thousands of years after the initial splitting of Asian and Native American lineages. Genetic simulation studies and analyses of the geographic structure of Native American mtDNA haplogroups further suggest that colonization from Beringia occurred earlier in this timeframe (ca. 16 ka) than later, since late-entry, rapid-spread models (like the Clovis-First model) are not capable of generating the observed genetic patterns of extant populations.

Number of Migrations

Physical anthropologists have shown that the craniometrics of the earliest Americans (i.e., "Paleoamericans" like Kennewick, Spirit Cave, Lagoa Santa) are significantly different from those of more recent Native Americans. Working under the assumption that craniometric variation is neutral and therefore phylogenetically significant, they interpret the differences to reflect two successive migrations stemming from two geographically or temporally distinct sources. Accordingly, Paleoamericans arrived in the New World and were later replaced by ancestors of modern Native Americans.

Current data from molecular genetics do not support this model of Native American replacement of Paleoamericans. All major Native American mtDNA and Y-chromosome haplogroups emerged in the same region of central Asia, and all share similar coalescent dates, indicating that a single ancient gene pool is ancestral to all Native American populations. Similarly, all sampled native New World populations (from Alaska to Brazil) share a unique allele at a specific microsatellite locus that is not found in any Old World populations (except Koryak and Chukchi of western Beringia), which implies that all modern Native Americans descended from a single founding population that was the result of a single migration.

This is further supported by ancient DNA studies showing that Paleoamericans carried the same haplogroups (and even sub-haplogroups) as modern Native groups. Thus, although the Paleoamerican sample is still small, the craniometric differences between the early and late populations are likely the result of genetic drift and natural selection, not separate migrations from different sources in Asia.

A separate but related problem is whether some modern Native American populations resulted from migrations that occurred after initial human dispersal. Phylogenetic analyses of haplogroup lineages, however, cannot easily discriminate between a single migration and multiple migrations of genetically distinct but closely related populations. For this we need identification of specific mtDNA and Y-chromosome haplogroup sub-clades through analysis of the entire molecule (as well as detailed studies of nuclear genome variation). The powerful interpretive value of this type of analysis is demonstrated by a recent study investigating mtDNA sub-clade distributions across Siberia. Two sub-clades of haplogroup D2 are recognized, one found among central Siberian groups (D2a) and the other among Chukchi, Siberian Eskimos, and Aleuts (D2b). These sub-clades share a coalescent date of 8-6 ka, suggesting that middle-Holocene ancestors of modern Eskimo-Aleuts spread from Siberia into the Bering Sea region and not vice-versa, confirming Turner's theory of a distinct Eskimo-Aleut origin. Future sub-clade analyses of other Native American populations will test whether they, too, are the result of multiple late-Pleistocene migrations.

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The Archaeological Evidence

Colonizing Beringia

To colonize the Americas, modern humans had to learn to subsist in one of the most extreme environments ever faced by humans in the prehistoric world-the Siberian Arctic. They did this by 32 ka. The evidence comes from the Yana RHS site, which is located along the lower Yana River in Siberia's high Arctic and contains a frozen, well-preserved cultural layer with stone artifacts and remains of extinct fauna. Most interesting are bi-beveled rods on rhinoceros horn and mammoth ivory, signs of a sophisticated Upper Paleolithic technology. Sites of similar age occur in subarctic central Siberia (Nepa, Alekseevsk) and arctic European Russia (Byzovaia), suggesting people had become well-equipped to handle life in the far north shortly after arriving in south Siberia.

Did the early people of Yana RHS explore farther east onto the Bering Land Bridge and cross into Alaska and northwest Canada? As yet, no unequivocal traces of such an event have been found, but hints of an early human presence may include the 28-ka mammoth-bone core and flake recovered from Bluefish Cave and even older bone materials from along the Old Crow River. Many archaeologists, however, have attributed this evidence to natural bone breakage because of the absence of associated stone artifacts. The next time we see evidence of humans in Beringia is closer to 14 ka. These late-glacial Beringian sites share many technological qualities with late Upper Paleolithic sites in central Siberia (e.g., Studenoe-2, Nizhnii Idzhir, Khaergas Cave, Diuktai Cave). The dating of the Beringian sites, and their potential role in understanding the peopling of the Americas, has undergone significant revision in the past few years.

In 1993, a new synthesis of Beringian archaeology highlighting the Nenana complex was introduced. The Nenana complex was defined from a group of central Alaskan sites dating to 13.8-13 ka. Assemblages contained bifaces and unifaces made on blades and flakes, but lacked microblades and burins diagnostic of later Alaskan complexes. The age and character of Nenana suggested that it was part of the same rapid expansion of humans from Siberia to America as Clovis. The abrupt transition to distinctive microblade technology in Beringia after 13 ka was the result of a second movement of people from Siberia. Evidence for these two late-glacial migrations could be found at the Ushki site in Kamchatka, where a "pre-microblade" culture dating to about 16.5 ka occurred stratigraphically below a microblade culture dating to 12.7 ka.

Based on the re-evaluation of known sites and excavation of new sites, we now know that the late-glacial Beringian record is more complicated than previously thought. The presumed 16.5-ka, pre-microblade assemblage at Ushki actually dates to 13 ka, so it is not ancestral to Nenana. In central Alaska at Swan Point, microblades and burins date to 14 ka, and at Bluefish Caves they may be as old as 13.6 ka. These sites document the dispersal of microblade-producing humans from Siberia to Beringia much earlier in the late glacial than previously thought. Clearly, the Nenana complex was not first and not pre-microblade. In northwest Alaska, a rich bifacial industry called Sluiceway-Tuluaq is contemporaneous with Nenana but technologically distinct from it, confirming that lanceolate point industries appeared in Beringia by 13.2 ka. Also, a small number of undated fluted points similar to Clovis occur in Alaska, but their relationship to Clovis south of the continental ice sheets is unknown and may represent the backward flow of technologies (or people) from America to Beringia at the very end of the Pleistocene. Thus, Alaska may have been home to multiple human populations during the late glacial. Some may have been new to Beringia, having come from Siberia or America during the late glacial; others may have descended from a more permanent Beringian ancestor.

Paleogeographic Constraints

Since 40 ka, the Cordilleran and Laurentide ice sheets covered much of Canada, but during warmer periods they retreated sufficiently to create ice-free corridors along the Pacific coast and Plains east of the Canadian Rockies. The closing and opening of these corridors is of utmost importance, because they were the conduits through which the first humans spread from Beringia to the Americas. When humans arrived at Yana RHS 32 ka, contracted ice sheets left wide-open corridors through which humans could have passed, but by 24 ka the ice sheets had grown sufficiently to clog both passageways. Although isolated ice-free refugia probably existed in both corridors throughout the LGM, humans probably did not occupy these areas until the corridors re-opened during the late glacial.

Timing of the re-opening of the coastal and interior corridors is still debated, because of imprecise dating and because the various Cordilleran glaciers reacted differently to climate change. Nonetheless, the coastal corridor appears to have become deglaciated and open to human habitation by at least 15 ka, while the interior corridor may not have opened until after 13.5 ka. The archaeological records of both corridors are still inadequate for addressing questions about the initial peopling of the Americas; however, the presence of human remains dating to 13-13.1 ka at Arlington Springs, on Santa Rosa Island off the coast of California, indicates the first Americans did use watercraft.

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Clovis and Its Contemporaries

Discussion of the early archaeological record south of the Canadian ice sheets starts with Clovis, the best-documented early complex in the Americas. Our understanding of Clovis chronology, technology, and subsistence has changed significantly over the last ten years.

Radiocarbon dates obtained over the last 40 years from Clovis sites across North America suggested that the complex ranged in age from 13.6 to 13 ka; however, careful evaluation of the existing date record and analysis of new 14C samples reveal that Clovis more precisely dates from 13.2-13.1 to12.9 ka. This not only is a much shorter time span for Clovis than earlier proposed, but also indicates Clovis is four to five centuries younger than previously thought. While older Clovis sites may someday be found and dated, current evidence suggests Clovis flourished during the late Allerød interstadial and quickly disappeared at the start of the Younger Dryas stadial. The apparent simultaneous appearance of Clovis across much of North America suggests a very rapid expansion across the continent, but the overlap in 14C dates between regions of North America presently makes it impossible to determine a point of origin or direction of movement for this event.

photo of examples of Clovis lithic technologyAnother recent development has been in the area of Clovis lithic technology. With newly excavated Clovis assemblages, especially from the southeastern U.S., we now know that Clovis is characterized by not only bifacial technology but also distinctive Upper Paleolithic blade technology. The principal diagnostic artifact of Clovis is its lanceolate fluted projectile point, not just because of its form but also the technology used to produce it. Other tool forms were equally important, especially formal stone tools like end scrapers, as well as cylindrical rods made on ivory, antler, or bone. These rods were beveled at one or both ends and functioned as fore-shafts or projectile points, respectively.

Traditionally, Clovis has been thought to represent a population of mobile hunter-gatherers because individual Clovis tools had multiple functions and were highly curated, suggesting that they were part of a conveniently transported tool kit. Many Clovis tools were made on high-quality stones like chert and obsidian procured hundreds of kilometers from where they were eventually discarded. Clovis sites are small and typically represent mammoth or mastodon kills, short-term camps, or caches. This concept of high Clovis mobility, however, has been challenged in the southeastern U.S., where enormous scatters of Clovis artifacts have been found that possibly represent quarry-habitation sites habitually used by Clovis people, from which they did not range great distances. At the Gault site (Texas), of 650,000 excavated artifacts (mostly debitage), 99% are made on local, on-site cherts, while rare non-local materials are from sources only 70 km away.

Clovis points have long been known to be associated with remains of now-extinct mammoth and mastodon, but archaeologists continue to debate the importance of proboscideans in Clovis subsistence. Optimal foraging theory has been used to predict that humans would not become proboscidean-hunting specialists, and certainly the recurrence of bison, deer, hares, reptiles, and amphibians, indicates that in some contexts Clovis people did more than hunt mammoth and mastodon. However, the 14 known unequivocal Clovis proboscidean kill and butchery sites is an unusually high number for such a short period of time, especially in light of the number of kill sites known for the Old World Paleolithic. In most areas of North America, Clovis people hunted mammoth and mastodon regularly, and they likely played some role in their extinction. Not surprisingly, they also subsisted on a variety of other foods.

The density of Clovis sites indicates that it is a unique North American phenomenon. Very few Clovis artifacts have been found in Central and South America. Instead, a different complex of archaeological sites may mark the "Clovis era" south of Panama. The 14C evidence indicates that at least six South American sites (Cerro Tres Tetas, Cueva Casa del Minero, and Piedra Museo in Argentina, and Fell's Cave, Quebrada Santa Julia, and Quebrada Jaguay in Chile) are synchronous with Clovis, but lack Clovis tools and technologies. For the most part they contain undiagnostic flake tools and bifaces, but distinctive Fishtail points (some with fluted bases) have been found in deposits dating to 13.1-12.9 ka at Fell's Cave and Piedro Museo. While it has been suggested that Fishtail points post-date Clovis and were derived from it, the two may have shared an earlier, as yet unidentified progenitor. Among the newest Clovis-aged localities in South America is Quebrada Santa Julia, a stratified site with a well-preserved living floor and hearth dating to 13.1 ka. Associated with the hearth were a broken, non-diagnostic, fluted biface, several flake tools, a core, and nearly 200 flakes, as well as remains of extinct horse. Quebrada Santa Julia provides the first unambiguous association of fluting technology and extinct fauna in South America.

Earlier Occupations

Since the discovery and definition of Clovis, researchers have searched for evidence of even older occupation of the Americas, but most sites dating before Clovis have not held up to scientific scrutiny. Even the 14.6-ka occupation at Monte Verde has not been immune to criticism, but its acceptance by most archaeologists means synchronous and possibly earlier sites in North America likely exist. In the past decade we have seen a reinvigorated search for early sites, and a number of localities in North America dating between 14 and 15 ka now seem to provide compelling evidence of an occupation before Clovis.

In the northern U.S., the Schaefer and Hebior sites (Wisconsin) provide strong evidence of human proboscidean hunting or scavenging near the margin of the Laurentide ice sheet between 14.2 and 14.8 ka. At each site, disarticulated remains of a single mammoth were sealed in pond clay and associated with unequivocal stone artifacts. The bones bear consistent signs of butchering-cut and pry marks made by stone tools. Critics suggest that the bone breakage and surface marring is the result of natural processes; however, it is difficult to reject the evidence from these sites because of the consistent patterning of the marks, low-energy depositional context, and associated stone tools. Even earlier evidence of humans in Wisconsin is suggested by the cut and pry marks on the lower limb bones of a mammoth recovered from Mud Lake. These bones date to 16 ka, but stone tools are absent.

Three other sites-Meadowcroft Rockshelter (Pennsylvania), Page-Ladson (Florida), and Paisley Cave (Oregon)-may provide additional evidence of humans in North America by about 14.6 ka, but these sites have unresolved issues.

At Meadowcroft Rockshelter, artifacts occur in sediments that may be as old as 18 to 22 ka. The assemblage includes about 700 pieces, including a small lanceolate biface from the upper part of stratum IIa, which is specifically bracketed by dates of 13.4 and 15.1 ka. Objective review and unequivocal acceptance of the site, however, hinges upon resolution of dating issues and publication of a comprehensive report of the excavations.

At Page-Ladson, early materials occur in a buried, submerged context in a sinkhole within the Aucilla River. Seven pieces of chert debitage, one expedient unifacial flake tool, and a possible hammerstone occurred in association with extinct faunal remains, including a mastodon tusk with six deep grooves at the point were the tusk emerged from the alveolus of the cranium. These grooves are interpreted to have been made by humans as the tusk was removed from its socket. Seven 14C dates for this horizon average about 14.4 ka, suggesting human occupation of the sinkhole occurred during the late Pleistocene when the area's water table was lower than it is today. Page-Ladson may contain evidence of Pre-Clovis humans, but a lack of detailed reporting about artifact contexts and site formation processes at the site prevent definitive evaluation.

At Paisley Cave, three human coprolites are directly 14C dated to about 14.1 ka. The human origin of the coprolites is supported by ancient mtDNA analyses that showed they contained haplogroups A and B, but a complete report on these genetic studies as well as the stratigraphic and archaeological context of the coprolites is not yet available. Given the evidence of mixing in the cave's late-Pleistocene deposits, it is not possible to evaluate this evidence until it is fully reported.

The evidence for humans in the Americas even earlier than 15 ka is less secure, but recently has been presented for four sites: Cactus Hill (Virginia), La Sena (Nebraska), Lovewell (Kansas), and Topper (South Carolina). Cactus Hill is a sand-dune site with late prehistoric, Archaic, and Clovis levels. Potentially older artifacts, including small prismatic blade cores, blades, and two basally thinned bifacial points were recovered 10-15 cm below the Clovis level. Three 14C dates ranging from 18 to 20 ka are reported from the levels below Clovis, but there are also dates of 10.3 ka and later.

Charcoal samples were not recovered from hearth features, but occur as isolated fragments at the same level as the artifacts. The younger charcoal clearly indicates some bioturbation at the site, and the older charcoal could be derived from older sediments underlying the cultural layer, but luminescence dates on the aeolian sands correlate with the 14C results and indicate minimal mixing of the sediments. Even though much information has yet to be published about this site, one thing seems clear-a biface and blade assemblage appears stratigraphically below the site's Clovis assemblage.

An even older occupation of the Americas has been proposed based on taphonomically altered mammoth bones at the La Sena and Lovewell sites that date from 19 to 22 ka. Neither site has yielded stone tools or evidence of butchering; however, many of the leg bones display percussion impact and flaking, suggesting that they were quarried and flaked by humans while they were in a fresh, green state, within a few years of the death of the animals. Clovis people periodically flaked bone in this fashion, as did Upper Paleolithic Beringians; however, in those contexts humans left behind stone tools, whereas at La Sena and Lovewell stone tools remain absent.

Currently, the oldest claim for occupation of North America is made at the Topper site, located on a Pleistocene terrace overlooking the Savannah River. Clovis artifacts at Topper are found at the base of a colluvial deposit, and older artifacts are reported to occur in underlying sandy alluvial sediments dated to about 15 ka. The oldest assemblage is a smashed core and microlithic industry. Cores and their removals show no negative bulbs, and flakes and spalls were modified into small unifacial tools and "bend-break tools" possibly used for working wood or bone. In 2004, similar-looking material was found in older alluvial deposits dating in excess of 50 ka. Given that the assemblage was not produced through conventional Paleolithic technologies, and that the putative artifacts could have been produced through natural processes, specifically thermal spalling, evaluation of this site must await complete reporting.

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Recombining the Genetic and Archaeological Evidence

After decades of research, some fundamental questions about the first Americans (who they were and whence they came) have been answered at a very general level, while other questions (when did they arrive and how did they get here) remain less certain.

Unquestionably, the human skeletal evidence across the Americas shows that the New World was populated by Homo sapiens. While the crania of these early people look different from modern Native Americans, modern and ancient DNA studies show that they were genetically related. The earliest inhabitants of the Americas hailed from south Siberia (between the Altai Mountains and Amur valley) and ultimately descended from a population of modern humans who dispersed from Africa by 50 ka and appeared in central Asia by 40 ka. Thus, a maximum limiting age can be placed on the entry of people into the New World of no earlier than 40,000 BP. Any claims for an earlier migration should be viewed with skepticism.
graph of combined genetic and archaeological data

Genetic studies alone are unclear on when and how people migrated to the Americas. Current molecular evidence implies that members of a single population left Siberia and headed east to the Americas sometime between 30 and 13 ka.

The majority of studies suggest this event occurred after the LGM, less than 22 ka. Initial analysis of mtDNA haplogroup sub-clades further suggests humans spread south from Beringia after 16.6 ka. The genetic record has not revealed multiple late-Pleistocene migrations, but does distinguish a Holocene dispersal of Eskimo-Aleuts from northeast Asia.

The archaeological record provides more clues about when the Americas were colonized. Humans occupied western Beringia by 32 ka, but the earliest unequivocal occupation of eastern Beringia is 14 ka. South of the continental ice sheets is Clovis, which first appears 13.2-13.1 ka, and Monte Verde, Schaefer, and Hebior, which point to a human presence in the Americas by 14.6 ka. Human occupations at Meadowcroft, Page-Ladson, and Paisley Cave may also date to this time, but all have shortcomings in reporting. Together these sites may represent the new basal stratum of American prehistory, one that could have given rise to Clovis. Other sites like Cactus Hill and La Sena may be even older, but issues related to their formation need to be clarified before they can be accepted as incontrovertible proof of an early human presence in the Americas.

When combined, what do the existing genetic and archaeological records tell us about when and how people first arrived in the Americas?

Clovis has long been considered to represent the first people to enter the Americas. Newly reported coalescence estimates for mtDNA haplogroup sub-clade C1 imply that such a late-glacial dispersal could have occurred, and much of what we know about Clovis implies it represents a rapid population migration. Redating of Clovis to 13.2-13.1 to 12.9 ka indicates it is not only centuries younger than the late-glacial complexes of Alaska, but also younger than even the most conservative estimate for the opening of the interior Canadian corridor. The "Clovis-First" model, however, requires that all American sites older than Clovis be rejected, and this appears to be no longer possible. The Clovis-First model does not explain the apparent synchroneity between Clovis and the early Paleoindian sites of South America. Finally, a late-entry and rapid dispersal of humans across the New World is inconsistent with the distribution of genetic variation observed in Native American populations today. Thus, alternative models to explain the peopling of the Americas need to be developed.

Humans likely colonized the Americas around 15 ka, immediately upon deglaciation of the Pacific coastal corridor. Most mtDNA and Y-chromosome haplogroup coalescence estimates predict such an event, and it may correlate to the post-LGM dispersal of microblade-producing populations into northern Siberia and their eventual appearance in Beringia during the late glacial. The first Americans did use boats, as the evidence from the Channel Islands, California, attests. Once reaching the Pacific Northwest, humans could have continued their spread southward along the coast to Chile, as well as eastward along the southern margin of the continental ice sheets, possibly following traces of mammoth and mastodon to Wisconsin.

Clovis could have originated south of the continental ice sheets, and the dense Clovis quarry-campsites in the southeastern U.S. may be the result of a longer time-depth there than in other regions. Alternatively, Clovis could have originated in the north, as part of a second dispersal event from Beringia to America as the interior "ice-free" corridor opened, 13.5 ka. To us, this model offers the most parsimonious explanation of the evidence at hand today. It is compatible with the emerging genetic and archaeological records-the re-dating of Clovis, the contemporaneity of fluted-point complexes in North and South America, and the presence of Pre-Clovis sites south of the Canadian ice sheets.

Perhaps humans colonized the Americas even earlier, before the LGM, by 24 ka. There is clear evidence to show that people were capable of surviving in arctic western Beringia by 32 ka, and the Canadian ice sheets do not appear to have coalesced until 24 ka. To prove a pre-LGM dispersal event, unequivocal archaeological sites predating the opening of the Pacific coastal corridor (15 ka) are required in the Americas, and so far no such sites have been found. Further, current genetic estimates imply that the dispersal of modern humans from Beringia to the Americas occurred after the LGM.

These are only possibilities, however, and they require rigorous testing. Continued archaeological work in the Americas, especially in Beringia and the western Canadian corridors, as well as more detailed studies of modern and ancient haplogroup sub-clades in combination with full mtDNA genome sequencing and identification of patterns of nuclear DNA variation, will provide the evidence needed to explain the origins of the first Americans. This is an exciting time to be studying the peopling of the Americas. We are confident that through continued empirical research and active interdisciplinary dialog, we will soon know precisely when and how humans dispersed across the New World.

This text has been adapted from the Science Magazine publication
The Dispersal of Modern Humans in America during the Late Pleistocene
by Ted Goebel, Michael R. Waters, and Dennis H. O'Rourke Download pdf