[Photo] A Caribbean reef shark. A new study finds that the genetic blueprint for building limbs has its evolutionary origin in the median fins of early fishes. Credit: George Ryschkewitsch/University of Florida

The triangular shark fin that sends frightened swimmers scrambling to shore is made using the same genes that help form the arms and legs of humans, a new study reports.

Researchers found that about a dozen genes that help give rise to a shark's median fins???those that run along its back and belly???also determine where paired side fins will form on its body. These genes are known to play important roles in the development of paired limbs in humans and other land animals.

The genes come from an ancient ancestor shared by sharks and humans.

"It shows that this genetic program for building limbs has its origins in the median fin structures of very early vertebrates," or animals with backbones, study leader Martin Cohn of the University of Florida told LiveScience.

The study, detailed in the July 27 issue of the journal Nature, also found that the genes are vital for the formation of the ribbon-like fins on the back of lampreys, a primitive jawless fish that does not have paired side fins.

Sharks and lampreys belong to groups of fish that diverged many millions of years ago, so the new finding suggests genes important for the development of fins, and eventually limbs, were in place long before the different kinds of fishes evolved and went their separate ways.