[10/16/08 Correction appended: see end of post]

When our ancestors moved ashore some 360 million years ago, they underwent a lot of changes as they evolved from ocean-swimming fish to land-walking tetrapods. For one thing, they needed feet instead of fins. Paleontologists have discovered a series of fossils that document the early evolution of limb bones in our aquatic ancestors, showing how long bones first evolved, then parts of the wrist and digit-like bones, and finally full-blown feet. But lots of other changes happened at the same time, producing traits in tetrapods not found in other animals. Tetrapods, for example, are good at hearing airborne sounds, thanks to small bones that can vibrate in their ears. The earliest of