What would it be like to live in a world where there were no borders or passports?

I believe the population centres would be more or less like what we have right now with a 10% variance chance.

Why? Because the Equator and the Tropics will still be the most hospitable locations on this planet. Without passports or borders, most of humanity will stay in these regions — just as they do right now.

What do you think might happen?