To move is human. Humankind has spent the past 70,000 years spreading out of Africa onto all continents. Even our past 2,000 years of recorded history features enormous migration waves due to conquest and crusades, colonialism and conflict. In the past century, the share of the world population living outside of one's home country held steady at 3 per cent; today that translates into roughly 300 million migrants. Now we stand at the beginning of another era of global mass migrations, one that could witness several billion people moving. We are not talking about the distant future; this is happening in our lifetimes. As we follow the movements of billions of people over the next decade and beyond, we will witness humankind's progress not just from one place to another, but from one model of civilization to another. There are many books about migration - but at its most fundamental level, this is not one of them. Activists and scholars often make the case for more open borders on humanitarian or economic grounds. But today's migrations are just one piece of a far grander narrative of the evolution of human civilization. This is the ultimate story: the future of our geography.