by Welkin E. John­son — Per­haps more than any other bi­o­log­i­cal dis­ci­pline, the study of an­i­mal viruses is con­fined to the present. Viri­ons are sim­ply not the stuff of which ro­bust fos­sils are made. Phy­lo­ge­netic analy­sis can help by re­veal­ing deep re­la­tion­ships be­tween ex­tant vi­ral lin­eages, yet such re­con­struc­tions lack de­tail (telling us noth­ing about…