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

The findings support a stepwise model for how the genetic program that builds vertebrate paired fins and limbs may have evolved. An ancestral Hox9–10 program may first have helped establish posterior appendages, later expanded into a broader Hox patterning system, and eventually been redeployed in anterior paired appendages. Credit: H. Nakazawa et al., Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.2621291123. Used under CC BY-NC-ND. 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.

In this excerpt from "The Story of the Mammals in 25 Discoveries," author Donald R. Prothero looks at a time when the idea of extinction was blasphemous.