Earth’s vertebrate diversity may be far richer than anyone realized. A sweeping analysis of more than 300 studies suggests that for every known fish, bird, reptile, amphibian, or mammal species, there are about two nearly identical “cryptic” species hiding in plain sight—genetically distinct but visually almost impossible to tell apart. Thanks to advances in DNA sequencing, scientists are uncovering these long-separated lineages, some evolving independently for over a million years.