Although anatomically modern humans have existed for tens of thousands of years, we have no record of civilizations prior to about 5,000 BC, when civilizations grew up in places like the Tigris/Euphrates, Nile and Ganges. There is certainly adequate time for civilizations to have risen and fallen. Any number of quack theories purport to identify Atlantis. My personal favorite explanation is that England and France were to blame, as every century or so they send ships via the Pillars of Hercules to go invade.
But no matter. For years I have been curious about the Ainu people, who were the original inhabitants of Japan. They are more Caucasian in their appearance than the Japanese people, and moreover some researchers think their language may have some similarities to Gaelic (Irish). What? How did such people get to Japan? In Japan, there are only a few hundred Ainu left, as most of married into Japanese society.
Recently I found out about another small group of people. The Kek are a group which lives in Siberia, near the Yenesei river valley. They look like Asian people, and live much like the Eskimos, herding reindeer and lving like Arctic nomads. Their language is thought by some to resemble the Basques and even the North American Athabaskan Native Americans. How could this be? Well, if you are willing to imagine that there might have been periods of warming during earth's recent history, similar to the Medieval Warming period which allowed the Vikings to survive and to build settlements in Nova Scotia and Greenland, well perhaps the Ket people were able to sail between North America and Eurasia as well.
Another possibility, possibly, would be to have the Ket travel frrom Siberia to North America via the Aleutian Islands to Alaska and then cross the Rocky Mountains somehow to mix with the Athabaskans. Somehow that seems even harder to me than sailing the Atlantic.
Are the Ket the remnants of a once widespread civilization that included travel between the North Coast of Siberia and the North America? I think it's a possibility worth considering, at least.
This Ket family seems well adapted for life in the Siberian Arctic.
This picture from 1914 shows Ket people in houseboats, probably near the mighty Yenesei River in Siberia.