Human limbs evolved from transformation of gill arches

Sharks, skates, and rays have appendages growing out of the gill arch, a small cradle of bones that supports the gills. This anatomical peculiarity has led to the proposal that the paired limbs of humans, and before that the paired fins of fish, evolved from the transformation of gill arches in early fish.

The study demonstrates striking similarities in the genetic mechanism used to pattern gill arch appendages (called branchial rays) and fins/limbs.

Studying embryos of the little skate, Gillis focused on the gene Sonic hedgehog, which produces a signaling protein whose function is well understood in the mammalian limb.

Remarkably, he found that Sonic hedgehog's role in branchial rays closely parallels its role in the limb: it sets up the axis of development and, later, maintains growth of the limb skeleton.

Ongoing studies comparing the function of other genes during branchial ray and fin/limb development will help to resolve this.

The study has been published in Development.