The discovery of the early and late waves of Hox gene activity in the fin buds of shark embryos suggests that both phases were present in the common ancestor of sharks and bony fishes, Cohn said.
"Thus, the late wave is not uniquely associated with the origin of tetrapod digits," he added.
The reason sharks carry the genes but don't actually grow digits, the team speculated, is because the gene expression occurs briefly and only on a narrow band of cells.
Genetic Tweaks
An earlier study had demonstrated that the patterns of Hox genes—once thought to be unique to the arms and legs of land animals—are in fact much more ancient.
"We demonstrated this pattern in a primitive living fish related to sturgeons, called the paddlefish," said Marcus C. Davis, a researcher on the earlier study, which was published in May in the journal Nature. Davis was not involved in the PloS One study. (Related: "China's Giant River Fish, the World's Largest, Feared Extinct" [July 26, 2007].)
"What [Cohn's] group has done is confirmed these results in an animal considered even further down the family tree of vertebrates."
These studies demonstrate that the genesis of hands, feet, fingers, and toes in animals did not require new genes or even new patterns of gene expression.
"Dramatically different ways of being—new forms, new functions—may evolve through relatively minor adjustments to existing genes and gene functions," Davis said.
"It only requires modifications—'tweaks'—if you will, to previously existing genetic systems," he said.
"A symphony can play dramatically different compositions by changing the role each musician plays, [but] only on occasion are instruments added or lost."
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