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Models of migration to the New World

thumb|300px|Schematic illustration of maternal geneflow in and out of Beringia.
There have been several models of migration to the New World (Human migration into the Americas) proposed by various academic communities. The question of how, when and why humans (Paleo-Indians) first entered the Americas is of intense interest to archaeologists and anthropologists and has been a subject of heated debate for centuries. Current understanding of human migration into the Americas derives from advances in four integrated disciplines: archeology, physical anthropology, DNA analysis and linguistics.

In recent years progress has slowed as researchers enroll familiar tools to validate or reject what have become more or less entrenched theories like Clovis First. As new discoveries come to light, past hypotheses are reevaluated and new theories constructed. The peopling of the New World has been the object of a great number of archeologic, linguistic, and genetic studies over the past century. The few agreements achieved to date are their origin from Asia, with migration and widespread habitation of the Americas during the end of the Last glacial period, or more specifically what is known as the Late Glacial Maximum, around 16,000-13,000 years before present.

Understanding the debate

Time line of important events associated with Paleoindians

The chronology of migration models is currently divided into two general approaches. The first is the short chronology theory with the first movement beyond Alaska into the New World occurring no earlier than 14,000 – 17,000 years ago, followed by successive waves of immigrants. The second belief is the long chronology theory, which proposes that the first group of people entered the hemisphere at a much earlier date, possibly 50,000 - 40,000 years ago or earlier.
One factor fueling the debate is the discontinuity of archaeological evidence between North and South America. A roughly uniform techno-complex pattern known as Clovis appears in North and Central American sites from at least 13,500 years ago onwards. South American sites of equal antiquity do not share the same consistency and exhibit more diverse cultural patterns. Thus, many archaeologists conclude that the "Clovis-first" model does not adequately explain prehistoric tool complexes appearing in South America. Some theorists seek to develop a colonization model that integrates both North and South American archaeological records.

On October 2007 one mitochondrial DNA study concluded that the initial founders of the Americas emerged from a single-source ancestral population that evolved in isolation, likely in Beringia. The isolation in Beringia might have lasted 5,000 - 20,000 years.
Migrants from northeastern Asia could have walked to Alaska with relative ease when Beringia was above sea level. But traveling south from Alaska to the rest of North America may have posed significant challenges. There were two possible routes south for human migration, down the Pacific coast or by way of an interior passage along the eastern flank of the Rocky Mountains. When the Laurentide and Cordilleran ice sheets were at their maximum extent, both routes were likely impassable. The Cordilleran sheet reached across to the Pacific shore in the west, and its eastern edge abutted the Laurentide, near the present border between British Columbia and Alberta.

Geological evidence suggests the Pacific coastal route was open for overland travel before 23,000 years ago and after 15,000 years ago. During the coldest millennia of the last ice age, roughly 23,000 to 19,000 years ago, lobes of glaciers hundreds of kilometers wide flowed down to the sea. Deep crevasses scarred their surfaces, making travel across them dangerous. Even if people traveled by boat—a claim for which there is currently no direct archaeological evidence as sea level rise has hidden the old coast line — the journey would have been difficult with abundant icebergs in the water. By 14,500 to 13,000 years ago the coast was ice-free. Additionally, by this time the climate had warmed, and coastal lands were covered in grass and trees. Hunter-gatherer groups could have readily replenished their food supplies, repaired clothing and tents, and replaced broken or lost tools.
The disagreement in the academic community about when and how the American continent was populated continues. The argument regarding the timeframe of migration of big-game hunters from Asia and Beringia who spread across the Americas is an ongoing debate. Coastal or watercraft theories have even broader implications. The first people in North America may not have been terrestrial big-game hunters, but instead already adapted to maritime or semi-maritime lifestyles.

Timeline of significant archeological, geological and genetic evidence

Land bridge theory

Northern hemisphere glaciation during the last ice ages. The set up of 3 to 4 km thick ice sheets caused a sea level lowering of about 120 m.

Also known as the Bering Strait Theory or Beringia theory, the Land Bridge theory has been widely accepted since the 1930s. This model of migration into the New World proposes that people migrated from Siberia into Alaska, tracking big game animal herds. They were able to cross between the two continents by a land bridge called the Bering Land Bridge, which spanned what is now the Bering Strait, during the Wisconsin glaciation, the last major stage of the Pleistocene beginning 50,000 years ago and ending some 10,000 years ago, when ocean levels were lower than today. This information is gathered using oxygen isotope records from deep-sea cores. An exposed land bridge that was at least 1,000 miles wide existed between Siberia and the western coast of Alaska. In the "short chronology" version, from the archaeological evidence gathered, it was concluded that this culture of big game hunters crossed the Bering Strait at least 12,000 years ago and could have eventually reached the southern tip of South America by 11,000 years ago.

Synopsis

At some point during the last Ice Age, about 17,000 years ago, as the ice sheets advanced and sea levels fell, people first migrated from the Eurasian landmass to the Americas. These nomadic hunters were following game herds from Siberia across what is today the Bering Strait into Alaska, and then gradually spread southward. Based upon the distribution of Amerind languages and language families, a movement of tribes along the Rocky Mountain foothills and eastward across the Great Plains to the Atlantic seaboard is assumed to have occurred at least some 13,000 to 10,000 years ago.

Clovis culture

Examples of Paleoindian point forms. The 2nd is a Cumberland point, the 3rd is a Clovis point, the 4th a Barnes point, and the 5th a Folsom point. These flaked-stone tools are important archaeological markers of early and Middle Paleoindian cultures in North America.

This big game-hunting culture has been labeled the Clovis culture, and is primarily identified with fluted projectile points. The culture received its name from artifacts found near Clovis, New Mexico, the first evidence of this tool complex, excavated in 1932. The Clovis culture ranged over much of North America and appeared in South America. The culture is identified by distinctive "Clovis point", a flaked flint spear-point with a notched flute by which it was inserted into a shaft; it could then be removed from the shaft for traveling. This flute is one characteristic that defines the Clovis point complex.

Dating of Clovis materials has been by association with animal bones and by the use of carbon dating methods. Recent reexaminations of Clovis materials using improved carbon-dating methods produced results of 11,050 and 10,800 radiocarbon years B.P. (before present). This evidence suggests that the culture flowered somewhat later and for a shorter period of time than previously believed. Michael R. Waters of Texas A&M University in College Station and Thomas W. Stafford Jr., proprietor of a private-sector laboratory in Lafayette, Colorado and an expert in radiocarbon dating attempted to determine the dates of the Clovis period. The heyday of Clovis technology has typically been set between 11,500 and 10,900 radiocarbon years B.P. (The radiocarbon calibration is disputed for this period, but the widely used IntCal04 calibration puts the dates at 13,300 to 12,800 calendar years B.P.). In a controversial move, Waters and Stafford conclude that no fewer than 11 of the 22 Clovis sites with radiocarbon dates are "problematic" and should be disregarded—including the type site in Clovis, New Mexico. They argue that the datable samples could have been contaminated by earlier material. This contention was received as highly controversial by many in the archaeological community.

Clovis-type artifacts seem to disappear from the archaeological record after the hypothesized Younger Dryas impact event roughly 12,900 years before the present. The effects of the event possibly caused a decline in post-Clovis human populations and shifts in culture and behavior patterns.

Problems with Clovis migration models

Significant problems arise with the Clovis migration model. If Clovis people radiated south after entering the New World and eventually reached the southern tip of South America by 11,000 years ago, this leaves only a short time span to populate the entire hemisphere. Another complication for the Clovis-only theory arose in 1997, when a panel of authorities inspected the Monte Verde site in Chile, concluding that the radiocarbon evidence predates Clovis sites in the North American Midwest by at least 1,000 years. This supports the theory of a primary coastal migration route that moved South along the coastline faster than those that migrated inland into the central areas of the Americas. Many excavations have uncovered evidence that subsistence patterns of early Americans included foods such as turtles, shellfish, and tubers. This is quite a change of diet from the big game mammoths, long-horn bison, horse, and camels that early Clovis hunters apparently followed east into the New World.

Model of human migration based on Mitochondrial DNA

At the Topper archaeological site (located along the banks of the Savannah River near Allendale, South Carolina) investigated by University of South Carolina archaeologist Dr. Albert Goodyear, charcoal material recovered in association with purported human artifacts returned radiocarbon dates of up to 50,000 years BP. This would indicate the presence of humans well before the last glacial period, nevertheless considerable doubt over the validity of these findings has been raised by many other researchers, and the pre-Clovis Topper dates remain controversial.

Pre-Clovis dates have been claimed for several sites in South America, but these early dates have yet to be verified unequivocally.

Recent discoveries of human coprolites (desiccated feces) found deeply buried in an Oregon Cave, indicates the presence of humans in North America as much as 1,200 years prior to the Clovis culture.

Watercraft migration theories

Earlier finds have led to a pre-Clovis culture theory encompassing different migration models with an expanded chronology to supersede the "Clovis-first" theory.

Pacific coastal models

Pacific models propose that people reached the Americas via water travel, following coastlines from northeast Asia into the Americas. Coastlines are unusually productive environments because they provide humans with access to a diverse array of plants and animals from both terrestrial and marine ecosystems. While not exclusive of land-based migrations, the Pacific 'coastal migration theory' helps explain how early colonists reached areas extremely distant from the Bering Strait region, including sites such as Monte Verde in southern Chile and Taima-Taima in western Venezuela. Two cultural components were discovered at Monte Verde near the Pacific Coast of Chile. The youngest layer is radiocarbon dated at 12,500 radiocarbon years (~14,000 cal BP) and has produced the remains of several types of seaweeds collected from coastal habitats. The older and more controversial component may date back as far as 33,000 years, but few scholars currently accept this very early component.

Other coastal models, dealing specifically with the peopling of the Pacific Northwest and California coasts, have been advocated by archaeologists Knut Fladmark, Roy Carlson, James Dixon, Jon Erlandson, Ruth Gruhn, and Daryl Fedje. In a 2007 article in the Journal of Island and Coastal Archaeology, Erlandson and his colleagues proposed a corollary to the coastal migration theory—the kelp highway hypothesis—arguing that productive kelp forests supporting similar suites of plants and animals would have existed near the end of the Pleistocene around much of the Pacific Rim from Japan to Beringia, the Pacific Northwest, and California, as well as the Andean Coast of South America. Once the coastlines of Alaska and British Columbia had deglaciated about 16,000 years ago, these kelp forest (along with estuarine, mangrove, and coral reef) habitats would have provided an ecologically similar migration corridor, entirely at sea level, and essentially unobstructed.

For the Pacific Northwest, Carlson, and others have argued for a coastal migration from Alaska pre-10,000 B.P. that predates the migration of Clovis people moving south through an ice-free corridor located near the continental divide. These people were followed by the Clovis culture, which some archaeologists believe moved south from Alaska through an ice-free corridor located between modern British Columbia and Alberta. Recent dating of Clovis and similar paleoindian sites in Alaska suggest that Clovis technology actually moved from the south into Alaska following the melting of the continental glaciers about 10,500 years ago.Dixon 1999
As the ice sheets began to melt, it became possible for riverine-adapted people who made and used microblade technology to move west to the Northwest coast. A second migration of the Denali culture at around 10,700 b.p. brought peoples down the coast from Alaska. Carlson hypothesizes that a population with a maritime adaptation could have travelled south from Alaska down the coastal islands by watercraft, settling as the ice receded, then moving up rivers to the interior. This would account for early finds at Ground Hog Bay in SE Alaska and Namu, about 800 km south of Ground Hog Bay near modern Bella Coola dating to 10,180 +/- 800 b.p. and 9700 b.p., respectively. According to the Matson and Coupland dual migration hypothesis, Namu and Ground Hog Bay represent a second migration while the initial migration route south was through the ice free corridor. Part of the difficulty is the lack of site data prior to 10,000 b.p. as well as the limited number of archaeological investigations into the coastal migration model. Other factors affecting migration models are sea level changes and the question of available land mass to support migrating groups of people.

Evidence from Southeast Alaska and Queen Charlotte Islands in British Columbia, provides some data about food and land resources during the last glacial maximum. Fedje and Christensen (1999) have identified several sites on Haida Gwaii that date to post 9000 b.p. (642). The oldest human remains known from Alaska or Canada are from On Your Knees Cave, which is on Prince of Wales island in Southeast Alaska. The individual, a young man in his early twenties when he died, has been dated to 10,300 years ago and isotopic analyses indicate the individual was raised on a diet primarily of marine foods. These data suggest that there are a number of submerged sites just beyond the shorelines of Haida Gwaii (Fedje & Christensen, 1999) and along the coast of Southeast Alaska. Paleoecological evidence suggests that travel along the coast would have been possible between 13,000 and 11,000 b.p. as the ice sheets began retreating. Between 13,000 and 10,500 b.p. Haida Gwaii had more than double its current land area (Fedje & Christensen, 1999:638). This area was flooded as the ice sheets began to melt between 11,000 and 9,000 b.p. (Ibid). Therefore any evidence of human occupation would now be below sea level. Conversely, older sites that are located near modern shorelines would have been approximately 15 miles from the coast (Ibid). The antiquity of the lithic scatters that Fedje and Christensen (1999) have found in intertidal zones along the Haida Gwaii coast suggests an early human occupation of the area.

Fedje and Christensen (1999) support Carlson (1990), and Fladmark's (1975, 1979 & 1989) initial coastal migration model rather than the ice free corridor model proposed by Matson and Coupland (1995) through their investigations of intertidal zones on Haida Gwaii. The coastal region was quite hospitable by 13,000 b.p. to peoples with watercraft and a maritime adaptation. Furthermore, Fedje and Christensen (1999) argue that the coast was likely colonized before 13,000 b.p. (648). This assertion is based largely on watercraft evidence from Japan and Australia before 13000 b.p. If maritime peoples colonized Island Southeast Asia, Australia, western Melanesia, the Ryukyu Islands, and Japan between 50,000 and 35,000 years ago, they may well have been capable of migrating from Northeast Asia into the Americas as the North Pacific Coast warmed and deglaciated after about 16,000 years ago. Although no boats have been recovered from early Northwest Coast archaeological sites, this may be due to poor preservation of organic materials and the inundation of coastal areas mentioned above. We can still infer water travel based on the presence of artifacts made by humans found in island sites.

Other evidence comes from zooarchaeological finds along the Northwest coast. Goat remains as old as 12,000 b.p. have been found on Vancouver Island, British Columbia as well as bear remains dating to 12,500 b.p. in the Prince of Wales Archipelago, British Columbia. Even older remains of black and brown bear, caribou, sea birds, fish, and ringed seal have been dated from a number of caves in Southeast Alaska by paleontologist Timothy Heaton. This means that there were enough land and floral resources to support large land mammals and theoretically, humans. Further intertidal and underwater investigations may produce sites older than 11,000 b.p.. Coastal occupation prior to 13,000 b.p. would allow for people to migrate further south and account for the early South American sites.

Anecdotal evidence comes from the surviving Bella Bella oral tradition as recorded by Franz Boas in 1898. "In the beginning there was nothing but water and ice and a narrow strip of shoreline". Some believe this story describes the Northwest Coast during the last glacial maximum and that the story suggests that the Northwest Coast colonization occurred during the last ice age.

Further south, California's Channel Islands have produced even earlier evidence for seafaring by Paleoindian (or Paleocoastal) peoples. Santa Rosa and San Miguel islands, for instance, have produced five sites dating to the Terminal Pleistocene, including the Arlington Man site dated to ~11,000 radiocarbon years (13,000 cal BP) and Daisy Cave occupied about 10,700 radiocarbon years ago (~11,500 cal BP). Erlandson and his colleagues have also identified several early shell middens located near sources of chert, which was used to make stone tools. These quarry/workshop sites have been dated between about 10,800 and 10,500 RYBP (~12,000-11,500 cal BP) and contain crescents and finely made stemmed projectiles points probably used to hunt birds and sea mammals, respectively. Significantly, the Channel Islands have not been connected to the mainland coast during the Quaternary, so maritime peoples contemporary with the Clovis and Folsom complexes in the interior had to have seaworthy boats to colonize them. The Channel Islands have also produced the earliest fishhooks yet found in the Americas, bone bipoints (gorges) that date between about 10,000 and 9500 cal BP.

Australia/Oceania model

As early as 1787 Chilean naturalist Juan Ignacio Molina mentioned the possibility of South America being populated from south Asia through the "infinite island chains" of the Pacific while north America could have been populated from Siberia. Some anthropologists such as Paul Rivet have proposed that peoples of Oceania or southeast Asia crossed the Pacific Ocean and arrived in South America long before the Siberian hunter-gatherers. These hypothetical Pre-Siberian American Aborigines populated much of South America before being nearly exterminated and/or absorbed by the Siberian migrants coming from the north. Some of the theories involve a southward migration from or through Australia and Tasmania, hopping Subantarctic islands and then proceeding along the coast of Antarctica and/or southern ice sheets to the tip of South America sometime during the last glacial maximum.

There have been well-dated stratigraphic studies that point to people entering Australia some 40,000 years ago. At this period Australia was not connected to another continent, which leads to the assumption that it was reached by watercraft. If Australia was reached in this fashion, some reason that the New World could have been reached in the same way. Proponents of this model have pointed to cultural and phenotypical similarities between the Aboriginals of Australia and the Selknam and Yaghan tribes of southern Patagonia. The theory of Australoid migration to the Americas has earned little scientific support as there is no genetic evidence matching indigenous Australians with South American populations. This model is taught in Chilean schools together with the land bridge model.

A recent study claimed that the Mapuche pre-Columbian Araucana chicken came from Polynesia by analysing their DNA; this suggests a more recent contact between the Mapuche and Polynesia. Another recent study has contradicted this claim stating that the DNA found in the chicken bone was closer to post colonial European chickens.

One of the earliest known sites of human occupation in the Americas, Monte Verde, lies within what was later to become Huilliche territory, although there is currently no demonstrated link between the Monte Verde people and the Mapuche.

Southeast Asians: Paleoindians of the Coast

The boat-builders from Southeast Asia may have been one of the earliest groups to reach the shores of North America. One theory suggests people in boats followed the coastline from the Kurile Islands to Alaska down the coasts of North and South America as far as Chile [2 62; 7 54, 57]. The Haida nation on the Queen Charlotte Islands off the coast of British Columbia may have originated from these early Asian mariners between 25,000 and 12,000. Early watercraft migration would also explain the habitation of coastal sites in South America such as Pikimachay Cave in Peru by 20,000 years ago and Monte Verde in Chile by 13,000 years ago [6 30; 8 383].

"'There was boat use in Japan 20,000 years ago,' says Jon Erlandson, a University of Oregon anthropologist. 'The Kurile Islands (north of Japan) are like stepping stones to Beringia,' the then continuous land bridging the Bering Strait. Migrants, he said, could have then skirted the tidewater glaciers in Canada right on down the coast." [7 64]'

Atlantic coastal model

Archaeologists Dennis Stanford and Bruce Bradley champion the coastal Atlantic route. Their Solutrean Hypothesis is also based on evidence from the Clovis complex, but instead traces the origins of the Clovis toolmaking style to the Solutrean culture of Ice Age Western Europe. They have hypothesized that Solutrean hunters and fishers may have worked their way along the southern margins of the Atlantic sea ice to North America. Their argument is based on technological analysis of the similarities between Solutrean and Clovis flint-knapping techniques. Their book on the Solutrean Hypothesis is scheduled for publishing in 2009.

Other Atlantic migration proponents include the French archaeologist Remy Cottevieille-Giraudet, who in the 1930s suggested a European Cro-Magnon origin of the Algonquian peoples. In 1963, Emerson Greenman proposed a hypothetical Atlantic migration during the Upper Paleolithic, also citing New World similarities with Solutrean tools as well as art. He suggested that the Beothuk people of Newfoundland, among others, may have been at least partial descendants of that migration. According to a research report on Beothuk DNA published in 2007, " the data do not lend credence to the proposed idea that the Beothuk (specifically, Nonosabasut) were of admixed (European-Native American) descent."
Problems with evaluating coastal migration models
The coastal migration models provide a different perspective on migration to the New World, but they are not without their own problems. One of the biggest problems is that global sea levels have risen over 100 metres since the end of the last glacial period, and this has submerged the ancient coastlines which maritime people would have followed into the Americas. Finding sites associated with early coastal migrations is extremely difficult—and systematic excavation of any sites found in deeper waters is challenging and expensive. If there was an early pre-Clovis coastal migration, there is always the possibility of a “failed colonization.” Another problem that arises is the lack of hard evidence found for a “long chronology” theory. No sites have yet produced a consistent chronology older than about 12,500 radiocarbon years (~14,500 calendar years) , but South America has still seen only limited research on the possibility of early coastal migrations.
The Dyukhtai of Northeast Asia
Archaeological sites found in Dyukhtai Cave and other sites in the Aldan River valley have yielded remains of a culture that may be a potential Paleoindian ancestor. This culture occupied the region from 35,000-12,000 years ago. The Dyukhtai or similar Northeast Asian cultures may have entered the New World through Beringia and spread into British Columbia [1 140]. It is thought that they pursued Pleistocene mammals such as the giant beaver, goats, elk, ancient reindeer (early caribou), horses, Yukon camels, steppe bison, musk ox, mastodons, and woolly mammoths.

The chief characteristic of the Dyukhtai was their manufacture of microliths or microblades. Microblades are small flakes less than 1 1/4 inches long, with a sharp edge and a "backed" or blunted edge that could be guided with the index finger to sever meat from a carcass. Microblades could also be incorporated into composite tools such as an arrow or sickle. Thousands of microblades have been found at upper Paleolithic Stone Age sites. They have been found north of Mongolia together with projectile points and hand-carved ivory statuettes. The earliest of several sites there has been dated at 45,000 years ago. Microblades appeared in Japan by 20,000 during the LGM when the island was still a peninsula and reachable by land [1 144, 202; 3 189-191].

Microblade manufacture was an important event in human history and its appearance corresponds roughly to the end of the Middle Paleolithic 60,000 years ago. Over 98% of all human history is encompassed by the period of time that began with the appearance of Australopithecus afarensis [e.g. "Lucy"]--a maker and user of Oldowan chopper tools—and ended with the manufacture of microblades by lower-upper Stone Age cultures such as the Dyukhtai.
Recent scholarship
mtDNA-based chart of possible large human migrations.
A study by Brian Kemp and colleagues published in The American Journal of Physical Anthropology reports new DNA-based research that links the DNA retrieved from a 10,000-year-old fossilized tooth from an Alaskan island, with specific coastal tribes in Tierra del Fuego, Ecuador, Mexico and California. Los Angeles Times (accessed September 11, 2006) Unique markers found in DNA recovered from the Alaskan tooth were found in these specific coastal tribes, and were rare in any of the other indigenous peoples in the Americas. This finding lends substantial credence to a migration theory that at least one set of early peoples moved south along the west coast of the Americas in boats. A previous study (Eshleman et al. 2004) showed that mtDNA (human mitochondrial DNA) from indigenous populations in coastal British Columbia showed similarities to coastal populations in Southern California, while inland populations in both localities differed markedly.

A recent molecular genetics study suggests that the Amerindian population in the Americas may be derived from a theoretical founding population with an effective size as small as 70. The Hey study is restricted to 9 genomic regions (or loci) in the Americas and Asia, and excludes speakers of Na-Dene and Eskimo-Aleut languages. The study does not address the question of separate migrations for these groups, and excludes other DNA datasets not sampled in the source literature.

An October 2007 study suggested "that the initial founders of the Americas emerged from a single source ancestral population that evolved in isolation, likely in Beringia... the isolation in Beringia might have lasted up to 15,000 years. Following this isolation, the initial founders of the Americas began rapidly populating the New World from North to South America."

To be sure, Amerindian groups in the Bering Strait region exhibit perhaps the strongest DNA or mitochondrial DNA relations to Siberian peoples. The genetic diversity of Amerindian groups seems to increase with distance from the assumed entry point into the Americas. Certain genetic diversity patterns from West to East may suggest at least some coastal migration events.

A more recent article in the American Journal of Human Genetics'' states "Here we show, by using 86 complete mitochondrial genomes, that all Native American haplogroups, including haplogroup X, were part of a single founding population, thereby refuting multiple-migration models." The study also argues for a Beringian isolation, with a subsequent coastal route of expansion into the Americas.