For a family of pathogens that has likely been with humanity since the dawn of agriculture, Treponema has a funny way of keeping its secrets. Its fingerprints show up everywhere in the archaeological record in the form of bowed tibias in desert burials, childhood skull lesions in the Caribbean, and scarring of the lining around bones from the Andes to Oceania. And yet, one form, venereal syphilis, showed up abruptly in Europe in 1495 like it fell out of the sky. That juxtaposition, with the ancient ubiquity of treponemes conflicting with the sudden appearance of a sexually transmitted version, created a puzzle that has annoyed paleopathologists, geneticists, and historians for more than a century. How can a disease family be so old and global, b…