These fins were made for walking

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The first fish to emerge onto land a few hundred million years ago were equipped with pelvic-fin muscles that would eventually help their descendants to walk.

Nicholas Cole and Peter Currie at the Universities of Sydney and Monash in Australia and their co-workers charted pelvic-fin-muscle development in three extant species of bony fish, including lungfish. These creatures' ancestors gave rise to tetrapods — four-legged creatures. The authors also studied two shark species, which are more distantly related to tetrapods.

In bony fish, the pelvic-fin muscles start as extensions of body-wall muscles, just as they do in the shark species. However, only in bony fish do the cells in the developing fin muscle express a gene that is involved in hind-limb muscle development in tetrapods. The authors visualized this process by labelling cells and transplanting them from one strain of zebrafish embryo to another.

PLoS Biol. 9, e1001168 (2011)

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