AUSTIN—If you walked the cobblestone streets and bustling markets of 16th and 17th century Mexico City, you would see people born all over the world: Spanish settlers on their way to mass at the cathedral built atop Aztec ruins. Indigenous people from around the Americas, including soldiers who had joined the Spanish cause. Africans, both enslaved and free, some of whom had been among the first conquistadors. Asians, who traveled to Mexico on Spanish galleons, some by choice and some in bondage. All these populations met and mingled for the first time in colonial Latin America.