The most consistent reading of all the evidence of archeological evidence, megafauna extinctions, ice sheet extent, land bridge openings, DNA evidence, and linguistic evidence taken together tends to favor migration dates somewhere in the range of about 14,000 to 11,500 years ago, probably towards the more recent end of that range. The migrants rapidly spread across both continents, moving particularly rapidly down the Pacific Coast.
By 10,000 years ago, the Americas were separated geographically from Asia when the Land Bridge flooded as sea levels rose. There is no evidence of contact between the New World and either Europe or Asia for another 9,000 years, and the limited contact with Europeans around 1000 A.D. in the far Northeast around Maine and the Maritime provinces of modern day Canada, had little lasting impact. From the time the land bridge flooded until 1492 when Columbus landed in the New World, it was effectively isolated.
Multiple lines of genetic evidence suggest that all Native Americans are closely related.
Genetic studies have shown that virtually all Native Americans share a set of four major mtDNA lineages, and at least two such lineages on their Y chromosome. This indicates these groups are all closely related to one another. The nearest relatives of Native Americans beyond the Americas are the native peoples of northeastern Asia. Native Americans are unrelated genetically to Europeans. Geneticists have variously estimated that peoples of Asia and the Americas were part of the same population from 21,000 to 42,000 years ago.
Genetic evidence analyzed in a 2007 scientific journal article supports, in particular, the view that the founding population of the Americas came predominantly in a single wave of migration from Asia. This is because a mutation specific to the Americas and the part of Siberia to which Native Americans have a genetic link is common in all regions of the Americas but is recent in origins and unlikely to have multiple independent origins. The frequency of this mutation in the different populations is inconsistent with either selective adaptation to New World conditions or an initial presence in only one of the several regional Native American populations.
This contradicts a controversial linguistic theory that suggested that three groupings of New World languages might represent three separate waves of migration from Asia with separate founding populations. This also strongly suggests that the large number of North American and South American languages currently classified as language isolates are really all part of a single genetic language superfamily, whose links have not been definitely established only as a result of insufficient data or inaccurate analysis of the data. Linguists currently break Native American languages (North, Central and South American) into about 85 language families and 57 language isolates.
Widely accepted linguistic links were established in 2008 between the most Northwestern most of the North American language families (Na-Dene which also includes Apache and Navajo) and the Yeniseian (or Yeniseic) languages of Siberia, the only living representative of which is the Ket language which has 537 speakers.
Similarly, there is strong circumstantial evidence to suggest that all the language families and language isolates in Australia, New Guinea and the vicinity are really descended from the language of the founding population of Australia, the founding population of Taiwan, or are creoles of languages in those two families. These languages are currently classified into 36 language families and about 20 language isolates.
If these inferences about the Americas, Australia and New Guinea are correct, the only ten likely language isolates still spoken today are:
* Korean in North Korea and South Korea with 78 million speakers
* Basque in Northern Spain with 1,063,000 speakers
* Burushaski in Northern Pakistan with 87,000 speakers
* Sandawe in Southern Africa with 40,000 speakers
* Kalto in the Maharashtra state in India with 2,000 speakers
* Nivkh in the lower Amur River basin and on the Sakhalin Islands of Russia with 1,000 speakers
* Hadze in Tanzania with fewer than 1,000 speakers
* Shompen in the Andaman Islands of India with 400 speakers (two related languages)
* Ainu on Hokkaidō Island in Japan with 100 speakers
* Kusunda in Nepal with 8 speakers
Each language listed in italics are still spoken, but only by older people; it is not being acquired by children, and without efforts to revive it will become extinct when current speakers die.
There are many language isolate sign languages and extinct languages that do not fit into known language families.
There are about 21 to 31 living language families in the rest of the world, depending on who is classifying them. Of these, depending upon classification, 7-11 are associated with Africa and the Middle East in addition to two language isolates, 10-13 are associated with Europe, North, West and South Asia (including one related to American languages) in addition to five language isolates, and 6-7 are associated with East Asia, Southeast Asia, North East India and the Pacific in addition to three language isolates (including the Andaman island languages as one language since genetic evidence from its speakers suggest is Indonesian in origins).