In response to assumptions that the Boston bombing suspects were of Arab or Middle Eastern descent (and the use of a lot of rude terms for people from that part of the world), brilliant Twitter account @YesYoureRacist pointed out that the suspects were the textbook definition of white guys:

So these guys, from the Caucasus, are Caucasian. Me, with my mixed Slavic and Baltic heritage, I’m also Caucasian. So are my friends whose families come from Italy, Ireland, Germany, and pretty much anywhere else in Europe. How did all these different white ethnic groups get lumped into “Caucasian?”

It goes back to German anthropologist Friedrich Blumenbach. In his work in the late 1700s and early 1800s, Blumenbach divided Homo sapiens into five distinct races based on their physical characteristics. There was the Mongolian, or “yellow,” race, the red American race, the brown Malayan race, the black Ethiopian race, and the white Caucasian race.

While he looked at a lot of physical traits to carve out his categories, Blumenbach thought characteristics of the skull—the size and angle of the forehead, jawbone, teeth, eye sockets, etc.—were especially important. He thought that the skulls of Georgians were exemplary of the characteristics of his white race and named the group after the Caucasus Mountain Range that runs along Georgia’s northern border.

All this makes Blumenbach sound like a forerunner of phrenology, and “scientific” attempts to justify discrimination, but while he categorized the races, Blumenbach didn’t put them in a hierarchy and protested any attempts to misuse his groupings to divide people or paint one group as inferior to another. “Blumenbach wrote forcefully of the kindredness of the human races…he opposed the stress on racial hierarchies of worth by more conservative colleagues in his own university and elsewhere in Europe,” historian Nell Irvin Painter writes. “Throughout his work, and especially in the definitive 1795 edition of De generis humani varietate nativa (On the Natural Variety of Mankind), Blumenbach rejected racial hierarchy and emphasized the unity of mankind.”

Blumenbach’s Caucasians weren’t even strictly white or European, as the term is commonly used today. He described this “variety” as “Colour white, cheeks rosy; hair brown or chestnut-colored; head subglobular; face oval, straight, its parts moderately defined, forehead smooth, nose narrow, slightly hooked, mouth small…To this first variety belong the inhabitants of Europe (except the Lapps and the remaining descendants of the Finns) and those of Eastern Asia, as far as the river Obi, the Caspian Sea and the Ganges; and lastly, those of Northern Africa.”

The synonymity of Caucasian and white, and the use of racial lines as discriminatory tools, came later and from other men. Painter specifically calls out “Dutch anatomist, Petrus Camper, whose ‘facial angles’ proves so useful to scientific racists' so-called ‘Great Chain of Being’,” and his “racist elaborators (like Edward Tyson, Josiah Nott, G. R. Gliddon, and even Johann Caspar Lavater) [who] placed Negroes and Kalmucks as close to apes as to Europeans.”

Today “Caucasian” lacks any real scientific meaning (though its cousin “Caucasoid” is still used in some disciplines), but hangs on in common usage as a blanket term for white/European people.