Ancient Hox code reveals the developmental roots of vertebrate hind fins and limbs

Paired appendages—the pectoral and pelvic fins of fishes and the forelimbs and hindlimbs of tetrapods—were a major innovation in vertebrate evolution. Their emergence greatly expanded the ways vertebrates could move through their environments and ultimately provided the developmental foundation for the evolution of limbs. Yet how the genetic system responsible for building paired appendages was assembled during evolution remains an unresolved question.