Deirdre Clemente has an answer. A fashion historian at the University of Nevada Las Vegas, she says athleisure is the culmination of three long-term trends. First, technological improvements to synthetic fiber have made products like spandex more flexible, durable, and washable than natural materials. Second, the modern fixation on healthy appearance has made yoga pants an effective vector for “conspicuous consumption,” Thorstein Veblen’s term for products that confer status—like “extremely healthy person”— upon their owners. Finally, the blurring of yoga-studio fashion and office attire snaps into the long decline of formality in American fashion.

“One hundred years ago, you would have day clothes for the street, dinner clothes for the restaurant, theater clothes, and so many genres of dress,” Clemente said. “Those barriers have come down. Athleisure is the ultimate breaking down of barriers.”

To Clemente, the athleisure story doesn’t begin in the late 20th century, with the birth of Lululemon. It begins in the late 19th century, a sort of Cambrian Explosion moment for basic fashion when sports changed the way young people dressed—both on the field and in the classroom.