Chicago - The first human migration into the Americas was blocked by two massive glaciers which kept early settlers on the doorstep of the New World for about 20,000 years, according to a study published on Wednesday.

The study, which used DNA analysis to trace genetic changes over generations and then compared it with the geological and archaeological record, is the latest to challenge the standard story of how the Americas were populated.

This data shows a gradual and interrupted expansion by a much larger group of between 1,000 and 5,400 people in three distinct stages. The early generations began to move out of Siberia into Beringia, which is now under the icy waters of the Bering Strait, about 40,000 years ago.

They settled into the dry grasslands, which was productive enough to support large mammals but not rich enough to support significant population growth, after their eastward progression was blocked by two massive glaciers which covered much of what is now Alaska and northern Canada.

They were moving out of Asia and finally reached a landmass that was exposed because of lower sea levels during the last glacial maximum, but two major glaciers blocked their progress into the New World. So they basically stayed put for about 20,000 years.

Two passages opened up as the ice sheets started to melt about 15,000 years ago and the initial settlers expanded rapidly into the more fertile lands in the Americas.

Within about four of five thousand years the land bridge across the Bering Strait was swallowed by the ocean, blocking further expansion out of Asia.
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Interesting.
I've often wondered why the first Siberians into North America didn't continue south. This would explain it.
Of course, most Native American tribes believe they have always been here. They dispute that their ancestors came from Siberia. They believe they were created where their traditional tribal lands are, or moved to those lands shortly thereafter.
Of course, they have no proof.
Far more proof exists for the theory that Siberians populated what is now Alaska and northern Canada, then eventually moved south to populate the rest of the Americas.
It's a dispute that will likely never be resolved due to stubborness, beliefs, politics, money, prestige and social pressure -- on both sides of the issue.
But if it can be proven that no human remains are to be found south of these glaciers, until after a passage around them was available, then it would be strong evidence.
And if human remains are found south of the glaciers before this passage --- there goes the "wandering Siberians" theory!
This new evidence could make or break what we assume about the populating of North and South America.