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.
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These fins were made for walking. Nature 478, 159 (2011). https://doi.org/10.1038/478159a
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DOI: https://doi.org/10.1038/478159a