OP stated:

For pretty much the entireity of our existence, we didn't have borders. [...]

Please point me towards a part of our existance where we didn't have any types of borders. I deliberatly went all the way back to our beginnings as a species to point out that even then borders existed in some form.

As for the "shift and flow" part, I clearly stated:

[...] We just evolved the means to make them somewhat permanent. [...]

So, what exaclty is there pseudo-intellectual in my statement?

Edit: Here is a source to back up my claims.
Alexander C. Diener, Joshua Hagen. Borders: a very short inteoduction (2012), page 19, bottom half:

The earliest humans lived within small bands of nomadic foragers, know as hunter-gatherers. Migrations of these groups were generally patterned by changing seasons and enviromental conditions that affected the availability of food and other resources. Given their nomadic way of life, one might assume these early peoples lacked notiins of borders, territoriality, or land ownership. Yet research on contemporary hunter-gathere groups strongly suggests that these prehistoric humans possessed surprisingly elaborate tertitorial arrangements.

The passage isn't very long and anybody can sail the high seas to read for themselves.
There are exceptions, as with anything in life, but it seems that borders did in fact exist since the dawn of humans.