In the four-legged ancestors of dinosaurs, wrists were robust, weight-bearing joints with as many as 11 bones. When two-legged dinosaurs evolved some 230 million years ago, though, the wrists, no longer supporting such weight, became comparatively dainty. The forelimbs of the meat-eating raptors took on the task of manipulating prey, and the number of wrist bones shrank to just three. Among those that disappeared was a knobby bone called the pisiform. 
Alexander Vargas of the University of Chile and his team decided to investigate the next steps in this evolutionary tale by studying not just fossils but also embryos of today’s birds, including chickens, pigeons and parakeets. Ancestral features are often visible in a developing embryo; human and chicken embryos, for example, have folds in the neck similar to those that become gills in fish.
When meat-eating dinosaurs evolved into birds, the wrist joint in the wing, between the middle and final segments, morphed again—increasing flexibility so the wing could fold back against the body. Birds also evolved a bone in the same place as the pisiform, to transmit force to the wing. Anatomists deemed it a new bone, the ulnare. 
The 19th-century biologist Louis Dollo taught that evolution is irreversible; once a structure is lost, that pathway is closed forever. It’s a principle now known as Dollo’s law. But in analyzing the development of the ulnare, Vargas showed that it is, in fact, the re-emergence of the pisiform. “While the physical expression of a gene may be suppressed, it doesn’t mean the possibility of generating that structure has disappeared,” says Luis Chiappe, director of the Dinosaur Institute at the Natural History Museum of Los Angeles County. “The gene is still there, it’s just dormant.” 
This isn’t the first time Dollo’s law has been challenged. Some mites have returned to their free-roaming ways after countless millennia living on animal hosts. And a tree frog from South America lost its lower teeth only to re-evolve them after 200 million years. Within the human embryo, there is similar potential. Perhaps the very bone you are sitting on, your coccyx, is ready to re-evolve a tail at some future moment when humans might need it again to hang from trees.