A Fishy Tale of Limb Development
By Hannah Isom
The origins of paired appendages, (that's arms and legs to you and me), was a major step on the evolutionary path as it allowed new types of coordinated locomotion. Now an article published in advance on Nature online has revealed that human limbs and the median fins of fishes share a common developmental mechanism.
The hypothesis that paired fins and limbs developed from median fins (fins along the midline of the fish) has been swimming around since the nineteenth century, but has not been proved until now. Early vertebrate fossils have shown well developed median fins, suggesting that all limbs evolved from this position. However, the fact that these fins were not on the side of the body, like our arms and legs are, has until now challenged this view.
To investigate, Renata Freitas, Guang Jun Zhang and Martin J. Cohn from the University of Florida marked genes in the embryos of Catshark, so they could be tracked during development. They discovered, that the same genes that control limb and paired fin development (Hox and Tbx18) also drive the formation of Catshark median fins.
This theory was confirmed when the team discovered that lampreys, a much more primitive fish, also used the same mechanism for fin development. According to Cohn; "our own arms and legs have their evolutionary roots in the dorsal, caudal and anal fins of our fishy ancestors". The team now plan see whether cephalocordates, which appeared before vertebrates, also share a similar mechanism.
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