Models of migration to the New World

There are several popular models of migration to the New World proposed by the anthropological community. The question of how, when and why humans first entered the Americas is of serious interest to anthropologists and has been a subject of heated debate for centuries. As new discoveries come to light, past hypotheses are reevaluated and new theories constructed.

Understanding the Debate
The chronology of migration models is divided into two general schools of thought. One school believes in a “short chronology,” with the first movement into the New World occurring no earlier than 14,000 – 16,000 years ago. The “long chronology” camp posits that the first group of people entered the hemisphere at a much earlier date, possibly 20,000 years ago or earlier, and may have been followed by successive waves of immigrants.

One factor fueling the debate is the discontinuity of archaeological evidence between North and South America. A more or less 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 or greater 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 are seeking to develop a Pan-American colonization model that integrates both North and South American archaeological records.

Land bridge theory
Also known as The Bering Strait Theory or “short chronology” theory, Land Bridge theory has been widely accepted since the 1930s. This model of migration into the New World proposes that people wandered 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 60 m 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. 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 the height of 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 some 10,000 years ago.

Clovis culture
This big game hunting culture has been labeled as 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 even appeared in South America. The culture is identified by distinctive "Clovis point", a notched flute in flint spear-points where a shaft was inserted. 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. However, 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 Younger Dryas impact event roughly 12,900 years before the present. The effects of the event are thought to have caused a decline in post-Clovis human populations and shifts in culture and behavior patterns.

Recent Scholarship
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 of 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, and certain genetic diversity patterns from West to East may suggest at least some coastal migration events.

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 by at least 1,000 years. This makes it difficult to defend the theory of a singular north-to-south population movement. 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.

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; however, considerable doubt over the validity of these findings has been raised by 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.

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.

A study by Brian Kemp and colleagues published in The American Journal of Physical Anthropology reports new DNA-based research that uniquely 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. 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.

Pacific coastal models
Pacific models propose that people reached the Americas via water travel. While not exclusive of land-based migrations, the Pacific coastal models help to explain how early colonists reached areas extremely distant from the Bering Strait region. These arguments include the examples of sites such as Monte Verde in southern Chile and Taima-Taima in western Venezuela. Two cultural components were discovered at Monte Verde. The youngest layer is radiocarbon dated at 12,500 years, while the older component possibly dates back as far as 33,000 B.P. However, the older dates associated with the site are still debated.

Other coastal models deal specifically with the peopling of the Pacific Northwest coast and have been advocated by archaeologists Knut Fladmark, Roy Carlson, Ruth Gruhn, James Dixon, and Daryl Fedje. Carlson, and the others argue 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. However, 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.

As the ice sheets began to melt, it became possible for riverine-adapted people who made and used microblades 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 indicates 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 mass (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 15m from the coast (Ibid). The antiquity of the lithic scatters that Fedje and Christensen (1999) have reported finding in intertidal zones along the Haida Gwaii coast is suggestive of 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. It is speculated that if peoples elsewhere at 13,000 b.p. could build boats then the possibility exists that migrating human groups could have produced watercraft to travel south from Beringia. There have been no water vessels recovered along the Northwest coast from this time. This may be due to poor preservation of organic materials, the inundation of coastal areas mentioned above, or that boats were not common 13,000 years ago. We can only infer water travel based on the presence of stone tools manufactured by humans found on 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.

Australia/Oceania model
Some anthropologists propose 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 point to cultural and phenotypical similarities between the Aboriginals of Australia and the Selknam and Yaghan tribes of southern Patagonia. However, 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 landbridge model.

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 into the New World. Their argument is based on similarities between Solutrean and Clovis flint-knapping techniques, and is indirectly supported by the pre-Columbian presence of the European mitochondrial DNA Haplogroup X in North America.

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 McMaster University anthropologist Hendrik Poinar, dental samples from a Beothuk chief yielded positive results for mtDNA Haplogroup X.

Problems with coastal migration models
The coastal migration models have provided a new look at migration in the New World, but they are not without their own problems. One of the biggest problems is collecting data for these theories. The coastline of the Pleistocene is now under 60 meters of water. This makes excavation rather difficult and probably unreachable until the utilization of underwater technology advances. If there was an early pre-Clovis coastal migration, there is always the possibility of a “failed colonization.” Of course as mentioned, evidence of this would be under 60 meters of water. Another problem that arises is the lack of hard evidence found for a “long chronology” theory. No sites have been able to produce a consistent date that is older than 12,500 years, but the entirety of South America is gravely undersampled for early periods. There is also the possibility that archaeologists are not identifying the tool technology of pre-Clovis sites. Early tools might have been crude stone flakes, edge-trimmed cobble tools, and tools of perishable bone that North and South American archaeologists could easily overlook.