I love this graphic

  

It's an animation from the American Museum of Natural History showing the spread of anatomically modern man, Homo Sapiens, starting about 100,000 years ago. 

 

https://www.youtube.com/watch?v=PUwmA3Q0_OE&t=149s

 

 

Northward migration out of Africa wasn't stopped by the equatorial jungles, but it was interrupted by the Saharan Desert.  When we/they migrated out of Africa, they had to go out the top right into the Middle East; the Fertile Crescent.  Then westward movement in Europe paused at what are now France and Spain until about 30,000 years ago.  That area had already been occupied for 300,000 years by Neanderthal Man, the result of an earlier migration of their ancestor out of Africa.  It may have been intimidating for modern man to encounter another species of humans already well-established there.  I wish there were enough evidence we could track earlier migrations like this one.  By the time Homo Sapiens migrated out of Africa there were already several other species of human living across Eurasia .  The family tree isn't simple and linear.  There were several different species of humans all living at the same time.

 

 

The migration then flows northeast across Asia.  There must be geographical pattens of least resistance there to govern that flow.  Then it stalls at the Bering Strait.  Once the climatic conditions are right for that crossing, the entire western edge of the Americas are populated almost instantly on a timescale like this.  It took a lot longer for humans to migrate east across North and South America.  They were probably hindered by the barriers of the Rocky Mountains, southern deserts, and the Andes.  North America was slowly settled from west to east, then the Europeans arrived and quickly worked their way back from east to west.

 

Before the arrival of the Europeans in the Americas, we can see the Incas on the western slope of South America (Peru), the Aztec in Mesoamerica (Southern Mexico) and the Maya on the Yucatan Peninsula. 

 

A band of population growth happened all across Eurasia in the middle ages.  Relatively unimpeded by physical barriers, the great civilizations were able to exchange information and build on each other's accomplishments when they weren't killing and conquering each other.

 

The human migration made it to Australia 50,000 years ago, but that continent never achieved significant population until Europeans arrived.  The aboriginal people stayed hunter/gatherers and remained dispersed.  It's the efficiency of farming that allowed for dense populations.  As crowded as it can seem in the U.S. at times, we've got nothing like the population density of India and China though.

 

What I describe may not be an exact rendering of what happened, I wasn't actually there, it's just what this looks like to me…