SAME AS IT EVER WAS:::Even though it is far less dangerous on the planet today in general, the individual perception of danger is directly proportional to one's exposure to too much reality, it is the SAME AS IT EVER WAS for most of the people on the planet.

Too many of those who have been privileged to have lived in the western democracies are now becoming aware of how it has always been.

I don't know when it hasn't been divided any time in recorded human history. Why would it change?

The world has been divided for centuries. We as a society, will never see eye to eye on everything so we must accept that fact. Why it seems more so now is because its constantly put in our face with technology every minute of every day if not from social media, then it's the plethora of news channels we can pick from. It is more noticeable because of these things.

The million refuges at the US Mexico border, and the million escaping Syria into Europe have caused cultural stress and adverse reactions to the refugees in destination countries. Climate Change will displace billions as sea level rises and floods coastal areas, including cities such as Miami, Venice, Bangkok, and Rio de Janeiro.

Yes, no, not a clue..

no.... there are some elements in Humanity we should not be unified with in my opinion.

no....not really...... unlikely....... I don't see how.

decimate the overwhelming majority of the human population with the plague or something.

I think various separate goals, ideals, teams unifies different groups - pretty much the best we can hope for when everything is a competition or has more than one side. The groups then band together - wait, it's what we have right now! Yes, IMHO unifying the species is a nice goal BUT boring. I would settle for total equality for all the variations of the human species.
I think it is possible.
Education, hate is learned. Interventions therapy might help. We have check ups to make sure babies are physically developing but little is really paid to the mental/emotional development. The extremes get noticed but the average persons mental health or thinking process is sort of ignored. Tribalism of course is a big factor and fear is mostly the emotion that bands the tribe together. Conquer your fears, the world's fear and FFS get government to stop ruling by fear!!

The only way to unify is to acknowledge and accept our differences

Have humans ever once been unified as a species? I know it's the classic utopian liberal goal but it seems rather shallow and uninteresting, not to mention unattainable.

I'd settle for learning to respect each other a little more and trying a lot harder not to commit atrocities.