As you leave the Valley of Oaxaca and wind up the narrow switchbacks of unpaved mountain road, the sun loses its lowland sultriness and grows sharp. Roadside stands appear, selling small peaches, and bantam villages pass into and out of view, offering glimpses of hanging laundry, calla lilies in mossy streams, and men joking around in half-built houses. Farther inland, their language switches from the romantic lilt of Spanish to the tonal precision of Zapotec.

Crammed in the back of a taxi colectivo, a bare-bones Nissan Tsuru that held a few other passengers and their striped bags of city…