Your Hand in a Fin

The bones in your arm and hand were not new inventions. Lobe-finned fish already carried the same set of skeletal elements inside their fleshy fins — one upper bone, two forearm bones, then a cluster of smaller ones, just shaped and spaced differently.

Evolution reworked an existing fin design instead of starting over. That is why a bat's wing, a whale's flipper, and your hand are all the same bones in different proportions.

One Gene, Two Worlds

The same gene that helps lay out the bones in a fish fin — known as Sonic hedgehog — also helps lay out the bones in a human hand. Switch the fish version on in a mouse embryo and it still works.

This molecular toolkit has been doing similar jobs for hundreds of millions of years. Fins and hands look nothing alike, yet they are built from the same deep genetic instructions, carried unchanged across an almost unimaginable stretch of time.