Most of what I'm going to say comes from reading Cavalli-Sforza's chapter on the ancient Americans several years ago.
I'm glad to run into someone else who knows about Cavalli-Sforza. I haven't read his work directly. But I encountered a PBS series in which some of his colleagues followed the migration paths he had charted, checking DNA at regular intervals.
The most touching segment was a crew entering the enormous Navajo Reservation in Arizona and New Mexico. The Navajo teach their children that the tribe has always been right where it is now; that this is where the Creator put them. While the chief was arguing with the scientists, his son was riffling through the photos they had brought from other regions, when he stopped dead, pulled one out, and said, "Dad, this guy looks just like Uncle Ernie."
He inspected it for a long time. Then he turned to the camera, put on his most solemn face, and said, "Then I guess what you white men have been trying to tell us is true. We really are all brothers."
The man in the photo was a member of the Yenisei tribe in Siberia. Linguists have spent years trying to find a solid relationship between Yenisei and the Na-Dene language family. Na-Dene languages are spoken in western North America, from Arizona (Navajo) to Alaska (Tlingit).
Dating these kind of traces is very difficult and there seems to be a lot of controversy about whether these older dates are really accurate. But evidence does seem to be accumulating that human beings might have been present in the Americas, including South America, before the end of the last ice age. The best evidence I've heard of for very early ancient Americans comes from Chile. Interestingly, that's near the southern tip of South America. Which suggests that humans were widely dispersed in the Americas by that time. (Unfortunately, the date remains controversial.)
One anthropologist pulled out a calculator and determined that for this to have happened, the migrants had to be walking south at an average rate of about three miles per year. That leaves them plenty of time to leave some folks behind in new, permanent habitations with plenty of food around, but still it's quite a feat.
Cavalli-Sforza is of the opinion that examination of the genetics of pre-Columbian American populations shows evidence of three different migrations. These are associated with linguistic divisions, first, the bulk of the American Indian population, second, the Na-Dene speakers in NW Canada down to Arizona, and third, the Eskimos and Aleuts.
Yes, this is a well-established hypothesis that's supported by geography and by the style of tools left along the way.
The Eskimos and Aleuts appear to be the most recent arrivals.
Yes, they can still recognize their cousins in Siberia, and linguists can easily identify the familiar relationship of their languages.
The other two arrived in two waves associated with the last ice-age, lower sea-levels, and the Bering land-bridge.
There's a contingent of anthropologists who remind us that boat technology was well advanced in Japan, Korea, the other islands off the shore of Asia, and even in (what is now) China itself. It's not unrealistic to hypothesize people sailing to Alaska.
I recently read (in either National Geographic or the Smithsonian magazine) of an ancient site in Alaska that had been excavated. As usual, the human remains were in good condition and they were able to analyze the DNA. The anthropologists were shocked to discover that the people who comprised this community had ancestors from four or five different regions in Asia. They might have arrived many centuries apart.
Cavalli-Sforza suggests that crossing this land-bridge would have been most likely at the end of the ice-age, or at its beginning. During the height of the ice-age, climactic conditions might have made the crossing more difficult.
During a warming period, of course, it would have been much easier to approach by water.
And he speculates that while it's probable that the Na-Dene speakers came across at the end of the ice age...
This correlates with the hypothetical relationship between the Yeniseian and Na-Dene people.
Some question exists whether the rest of the American Indians' ancestors came across in a separate wave at roughly the same time (give or take a thousand years, maybe) or whether they came across much earlier, towards the beginning of the last period of massive glaciation, instead of its end.
We have plenty of evidence for other people arriving from different places. The Vikings are the most recent prior to the Spaniards, but there's evidence of at least one Atlantic coastal village built around 14000BCE by the Solutreans, a people whose technology places their origin firmly in western Europe. It wasn't discovered until recently, because in 14000BCE, sea level was much lower. So the village is several miles out to sea and well below today's sea level.
Apparently lots of work (genetic and otherwise) has been done to try to resolve this, but I got the impression that it's kind of inconclusive at this point.
Yes, it's a shame that those folks didn't leave their cellphones behind.
