LESS THAN AN HOUR after Barack Obama was elected president for the first time, I was standing outside our former building at Broad and Callowhill streets and watched as hundreds of young people marched toward City Hall. They were black, brown and white, waving their cellphones, chanting and cheering Obama's victory. Here it was: a new "post-racial" age in which old prejudices and hatreds were part of history, not part of the present.

Yes, as a matter of fact, I am naïve.

But for a while, it was easy to believe that social and cultural evolution was real. Here was proof: We elected a black man! Soon, we might have gender equality, too!

The reality as played out over the past eight years is far more checkered, of course. Obama embraced many progressive ideals, and, under his leadership, women and people of color held positions of power - Sonia Sotomayor and Elena Kagan on the U.S. Supreme Court, and Eric Holder, Hillary Clinton, Loretta Lynch in top Cabinet roles, among others.

But for all that progress, we also saw a disturbing rise in racial violence, including police shootings of black men, so much so that we had to be reminded that black lives matter.

For a minute, our defining slogan was "Yes, we can." But then it became "I can't breathe."

No one thought during Obama's two terms that we had entered an Age of Aquarius when it came to racial tolerance. But I thought we at least had progressed a bit on the spectrum, moving a little closer to enlightenment. So when Donald Trump began drawing crowds who responded to his disdain for non-whites - whether they were immigrants, native-born, brown and black, I was disturbed, but not unduly worried.

Until he won the presidential election.

The day after he won, swastikas appeared spray-painted in Philadelphia, and, by the end of the week, the Southern Poverty Law Center was fielding hundreds of reports of harassment of blacks and Muslims around the country.

The recent desecration of tombstones in Jewish cemeteries in Missouri and Philadelphia and a Catholic cemetery in Philadelphia, bomb threats in Jewish centers across the country, and the recent slaying of Indian IT worker Srinivas Kuchibhotla in Kansas have slammed us back into an ugly past. Surely we abandoned this mindset long ago - and not only those in the "bubble."

What worries me is that the reappearance of these incidents says something ugly about all of us - and not only those who are filled with hate.

We've told ourselves for generations that we are a tolerant nation, that we welcome outsiders, that we are a melting pot of cultures and nationalities. I've believed it. But now I have to wonder: Was that tolerance only skin deep? Was collective hatred so close to the surface that all it took was implied (and explicit) permission from someone who encourages suspicion and fear?

To some degree, tolerance and acceptance must be learned. We learn it from the collective messages of leaders and society at large. Without it, we have no civil society, after all. But if it is learned, that means it can be unlearned.

If tolerance is "unlearned," what we are left with? What worries me is the possibility that hatred and suspicion are our default settings. After all, how many centuries have people been persecuted for their religion? How many eons have people discriminated against black- and brown-skinned people? Well before 9/11, people were suspicious of those wearing head scarves. Hostility and hatred are as old as civilization itself. And if that's so, does that mean tribal hostility is innate, and that acceptance is a thin veneer?

That's an ugly thought. Because it means I have to question my own beliefs, my own tolerance. I can't take it for granted, can't consider it "innate."

Many who supported Trump were fearful that their place in society has been lost, lost to "the other" who took their jobs or their status. His rhetoric validated those fears, and his election seems to have unbottled them into the kind of hatred we're now witnessing. But Trump didn't create this hatred. This kind of hatred is as old as time. You could argue that it's human nature. But that doesn't mean we should not fight against it, in ourselves or others.

This week, in his address to Congress, Trump, in talking about protectionism, quoted Abraham Lincoln. But it's this Lincoln quote, from his first inaugural address, that I wish Trump would take to heart, and begin communicating himself:

"We are not enemies, but friends. We must not be enemies. Though passion may have strained, it must not break our bonds of affection. The mystic chords of memory, stretching from every battlefield and patriot grave to every living heart and hearthstone all over this broad land, will yet swell the chorus of the Union, when again touched, as surely they will be, by the better angels of our nature."

We have to serve those better angels. Even if it goes against our darker, uglier natures.

Sandy Shea is the editorial page editor of the Daily News.