Only if we were forced to and had our free will taken away.

No, not if we existed for another million years. It seems pretty fundamental to how we work, and how animals work in general. We basically discriminate along most possible lines. Few enough people even aspire to anything else.

Doubtful. Climate change and our own ignorant stupidity will wipe us out long before we’ll ever evolve past idiocracy.

when aliens arrive

There's a book I read a few years ago named "Tribe: On Homecoming and Belonging" that delvs into this a bit and why humans are so tribal instinctively. Would highly recommend.

https://www.amazon.com/Tribe-Homecoming-Belonging-Sebastian-Junger/dp/1455566381?ref=d6k_applink_bb_dls&dplnkId=2999a0a3-f1d3-4c19-b97a-6215a1e3c695

I don't think it's an instinct, because it can absolutely be taught.

I encourage my kids to get along with everyone, but at the same time I can see how some of their peers are taught to be racists and other clique behaviours from home by parents who are just like that and don't even think about it when they pass it on.

But by default, nobody is like that from birth. Babies aren't racists or afraid of different kinds of people. The fear of others is taught.

It will take many generations to change.

The way I see it that instinct is the cause behind so much suffering and injustice in the world.

That's just what they want you to think.

Outside perspective. Only when we meet another other.

Bold to assume that it's an instinct and not a taught and learned behavior.

The "Us vs. Them" mentality is also called the "in-group bias", in which you tend to align with other members of a perceived group (with little to no logical reason, it can be as simple as belts vs. suspenders). Like many other fallacies or biases, it is a built-in feature of our caveman-brains that no longer benefits us. When used in propaganda, it is often paired with the "strawman fallacy" to build the perception of an enemy that is barely even human.

You can learn to recognize these biases in yourself and in others - This is called critical thinking. I recommend the podcast "You Are Not So Smart" to everyone to get more insight on this subject.

I think we could if enough effort was put forth into making it happen. The problem is that very same "instinct," or rather the plethora of different experiences and ideals held by individuals seems to make it harder if not impossible to ever come to a global united consensus on anything.

No

The news feeding propaganda over and over isn't helping.

This current version of humans? No. But could it ever happen? Absolutely, if we assume our future evolutionary human descendants survive and provided we can supply everyone's needs.

I hope so. Knowledge and curiousity feed intelligence feed knowledge feed curiousity. A highly educated society with healthy education sytem and good working socioeconomy (concurency in news coverage) can theoretically get over "us vs. them". Until we someday maybe lose it as evolutionary trait.

I don't think it needs to be overcome, just applied differently. A more global "us" vs problems like global warming or poverty would be fantastic.

It's also a self preservation instinct - sometimes there's just too much going on and you gotta narrow your focus to the people around you.