Answer:
More resources and seasonal changes
Step-by-step explanation:
Early humans were mainly hunter-gatherers, so they traveled over vast regions of land. Depending on which place was more plentiful in food like berries and mammoth meat, the early humans moved there. In addition, seasonal changes also played a role, as shown with the crossing into North America over the Bering Strait when the glaciers were melted by the sun. For further reading, I recommend Sapiens: A History of Humankind by Yuval Noah Harari.