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 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 colonization model that integrates both North and South American archaeological records.
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 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.
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.
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 and subsequent coastal migration.
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 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.
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 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. 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 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 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 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, however, 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 used to 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.
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. 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 land bridge model.
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.
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. Another explanation that has been suggested for the presence of Haplogroup X among the Beothuk is that it resulted from the intermarriage of Vikings or Icelanders with Beothuk natives. The Vikings are known to have settled in Newfoundland during the early 11th century, and their writings discuss interactions with the Skraelings who are thought to be synonymous with the Beothuks.