Native Americans were descendants of hunters who came from Asia by land a long time ago. This idea is now generally accepted by scientists. In its modem form, it proposes that the ancestors of Native Americans came through Beringia, a land bridge located where the Bering Strait is now between North America and Siberia, formed when the sea level was lowered during glacial times between 40,000 and 13,000 years ago. The most likely pathway for further movement into the Americas was through an ice-free corridor which was open irregularly between 36,000 and 20,000 years ago and then completely open after 9,000 years ago when the glaciation ended.

There are currently three major questions concerning the pre Columbian peopling of the Americas: (1) How many distinct groups of individuals made the journey from Asia to Americas? (2) How long ago: early (35,000 to 20,000 years ago) or late (after 14,000 years ago)? (3) Which populations in Asia share the most recent ancestors with the Native Americans?
Fig First peopling of Americas. The distribution of the three major groups of Native Americans defined by linguistic affinities is shown. The ice free corridor linking Beringia to the rest of Americas during some periods in the glacial period is also represented.

One theory, based originally on linguistic data, proposes that ancestors of present-day Native Americans came from Siberia in three separate migrations (at different times) giving rise to distinct groups, namely Amerindians (from South, Central, and most of North America, such as Yanomami), Na-Dene (from North America, or example, Navajos), and Aleut­Eskimos (from the tip of North America) [Fig. 2]. The Amerindians are thou ht to be descendants of the first migrants, also called Paleo-Indians.

However, new genet c and anthropological data are Challenging the Three Migrations theory. Recent findings of ancient bones (about 10,000 years old) in North and South America suggest a distinct migration of non-Mongoloid people, different in appearance from typical East Asians and present day Native Americans. They could have become extinct, without leaving descendants. This hypothesis cannot readily be investigated by geneticists as they usually study only the DNA of living people, but it may in the future be studied by molecular archeologists. However, analysis of the Y chromosome and mitochondrial DNA lineages is being used to tell the paternal and maternal histories of the first peopling of the Americans.

The mitochondrial DNA analysis reveals is that most present day Native American belong to only four distinct lineages, which are also found (but are rare) in parts of Asia. Thus, these Native American mitochondrial DNA can be tracted back to only four mothers, who probably lived in Asia. Sources have been proposed in the region around Mongolia and south Siberia, while suggested times for entry into the Americas range between 30,000 and 15,000 years ago.

The Y-chromosomal analysis suggests that most males, about 90%, in south America and 50-70% in North America, are descended from a single male who lived perhaps 5,000 to 30,000 years ago. In contrast to the four mitochondrial DNA lineages, this lineage is found almost exclusively in Native Americans, and the only other populations carrying it, in Siberia, display it in low frequency, perhaps due to backmigration.

The lineage therefore probably arose in the Americas or Beringia. Other studies demonstrate that there are more distantly related Y chromosomes in populations from Siberia, namely the kets from the Yenissey River Basin and the Altai from the Altai Mountains. Interestingly, Y chromosomes related to those of Native Americans are very uncommon in Asians such as Chinese and Japanese but are frequent in Europe.

The implication of these studies is that a common Y chromosome ancestor existed a long time ago (greater than 30,000 years) in Eurasia who gave rise to most European Y chromosomes, to Kets and Altai in Siberia, and finally to Native American Y chromosomes. It would imply an ancient migration route to the Americans coming from Africa and passing through northern Eurasia, Siberia, and Beringia.
These recent analyses of mitochondrial DNA and the Y chromosome, despite tracing distinct parts of the history of Native Americans, suggest that all three major groups Amerindians, Na-Dene, and Aleut-Eskimos are too similar to have entered by separate migrations. Some additional migration movements may have brought other low-frequency mitochondrial DNAs and Y chromosomes to the continent, but the findings do not support a separate entry for each of the three Native American groups. The DNA analysis indicates that most of the Native Americans today are descended from the people coming in a first and major migration movement during the Pleistocene.

Molecular dating methods are not yet accurate enough to distinguish between an early or a late Pleistocene entry, but this is an exciting area for future work.