Question

The best explanation for how many humans populated the Earth is

Answer 1

Many thousands of years ago, not a single human being lived in the Americas.
This only changed during the last Ice Age. It was a time when most of North America was covered with a thick sheet of ice, which made the Americas difficult to inhabit.
But at some point during this time, adventurous humans started their journey into a new world.
They probably came on foot from Siberia across the Bering Land Bridge, which existed between Alaska and Eurasia from the end of the last Ice Age until about 10,000 years ago. The area is now submerged by water.
There is still debate about when these first Americans actually arrived and where they came from. But we are now getting closer to uncovering the original narrative, and finding out who these first Americans really were.

Answer 2

The baby boom in the 2thousands