Sea lampreys, which arose from the jawless fish that first appeared a half-billion years ago, dramatically remodel their genomes during embryonic development. This is believed to be the first recorded observation of a vertebrate reorganizing its genome during normal development. Evolutionary biologists are interested in how and why the lamprey re-organizes its genome because the animal is a living fossil with millions of years of evolutionary history. Its closest ancestors were among the first vertebrates on earth.