I think you said a lot when referencing "humans are the only animal to do this." Which does not necessarily mean "the fence," or division, separation (of any kind) was a "smart" choice by humans. It was an intelligent choice yet not exactly "smart," and has defined and led too the "barriers" that keep people(s) apart.
Just as so many humans strive to "belong" instead of striving to be open to ongoing options and opportunities that are missing when we become focused on specific collections of thoughts and beliefs. "Fences" work to secure these perceptions and this is what has evolved to be the major underlying basis for our world problems in the last several decades.
The more humans want to harbor their ideas and possessions, the more we experience conflict. The "fence," border, flag, etc. says, "Keep out and/or Join Us."
I do not disagree that the "fence" is AN important technological development, yet it is a tool of oppression. Just ask the First Nation-People/Native Americans.