The “when,” as it turns out, is pretty easy to answer, though the answer is surprising, in light of how conditioned we are to think of race as something that always was and always will be. The concept of race, says Painter, dates from about the mid-18th Century. It is less than 300 years old.

Though there were always a few people, she says, who attempted to impute character or worth from a stranger’s color, it was more likely in that era for people to make those judgments based upon a stranger’s religion or wealth. “So if you were a light-skinned person and you met a dark-skinned person in rags, and you weren’t [in rags], you’d feel superior,” says Painter. “But if you were in rags and that other person was in rags, you’d be trying to figure if that person had something that you could get.”

Then came race — that which would allow one person in rags to feel superior to another person in rags. Early on, it was defined not by appearance but as a function of climate. Greek scholars believed people from places where the seasons do not change were placid. Those from places of dramatic seasonal shifts were wild and unsociable. Those from hot places were impulsive and hot-tempered. Those from cooler climes were stiff and intellectual.

Race was also defined geographically. Hippocrates, the great Greek physician, thought people who lived in low-lying areas tended to be dark-skinned, fat, cowardly, ill-spoken and lazy. Those who lived in flat, windy places would be large in stature, “but their minds will be rather unmanly and gentle.”

And then, there were those who decided the key to race lay in measuring the size and shape of people’s skulls.

“American scientists and anthropologists get ahold of this concept by the early 1800s,” says Wray. “For the whole of the 19th Century, they’re refining and tweaking their models to really give scientific weight and authority to the notion that these racial differences can be empirically verified. In other words, they’re out there in the world. We just have to figure out whether it’s the distance between the eye sockets and the bridge of the nose, or some combination of measurements from the back of the skull to the chin divided by the circumference of the head that will give us the kind of golden ratio we’re looking for — in other words, the one that will enable us to definitively say, this person is African, this person is Caucasian and so forth.”

The great Harlem Renaissance author Zora Neale Hurston was fascinated by this thinking. She was known to stand on New York City street corners with a pair of calipers, asking passersby for permission to measure their skulls.

In 1895, D.B. Brinton, an American anthropologist, published a complicated chart purporting to categorize all the races of humankind. He ended up with more than a hundred kinds of Caucasian alone.

Brinton and Hurston were never able to quantify race. No one was. And yet, the nation — indeed, the world — was never able to give it up. Race was, and is, too useful.

Says Wray, “It has enabled in the United States for us to justify and legitimate the conquest of Indian land and the near genocide of Native American tribes. It enabled us for such a long time to justify slavery and when we got done with that justification, when people called B.S. on that, we said, ‘Well, this is how we can explain Jim Crow.’ When the Civil Rights Movement happens in the 1950s and ’60s, when African Americans rise up and say, ‘Enough Jim Crow,’ then we use it to justify mass incarceration of black Americans. We find the idea of race and inherent racial differences and the idea that some people are frankly, just better than others, to be indispensible.”