Look, ma, no fins! (Image: Freitas et al/Developmental Cell)

PERHAPS the little fish embryo shown here is dancing a jig because it has just discovered that it has legs instead of fins. Fossils show that limbs evolved from fins, but a new study shows how it may have happened, live in the lab.

Fernando Casares of the Spanish National Research Council and his colleagues injected zebrafish with the hoxd13 gene from a mouse. The protein that the gene codes for controls the development of autopods, a precursor to hands, feet and paws.

Zebrafish naturally carry hoxd13 but produce less of the protein than tetrapods – all four-limbed vertebrates and birds – do. Casares and his colleagues hoped that by injecting extra copies of the gene into the zebrafish embryos, some of their cells would make more of the protein.


One full day later, all of those fish whose cells had taken up the gene began to develop autopods instead of fins. They carried on growing for four days but then died (Cell, DOI: 10.1016/j.devcel.2012.10.015).

“Of course, we haven’t been able to grow hands,” says Casares. He speculates that hundreds of millions of years ago, the ancestors of tetrapods began expressing more hoxd13 for some reason and that this could have allowed them to evolve autopods.