There used to be a section of [dry land connecting Alaska and Kamchatka](https://en.wikipedia.org/wiki/Beringia) during the last ice age. You could just walk from Europe/Asia to North America on foot. A few thousand people did, then the ice age ended and the melting mega-glaciers raised sea level by a *lot* and now that area is 25-50m underwater. The people who moved over settled and gradually spread across the continent.
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