Since the work of Charles Darwin, it has been more or less accepted that people, horses, porpoises, and bats share a common ancestor — one that's arms and digits developed over many, many years to suit its needs. But only recent research has revealed that the human hand is actually extremely similar to the fin of a fish.
Although you wouldn't know it comparing your hand side-by-side with a trout (and might only guess it watching Michael Phelps swim), human fingers and fish fins "follow some of the same rules," The New York Times reports. In hands, we have a kind of tissue called "endochondral bone"; fish, however, have only small endochondral bones at the tip of their fin, the rest of the fin being made of rays of dermal bone. But by toggling with two genes related to limbs, scientists realized that the same cells in fish and in tetrapods either ask the body to develop dermal bones or endochondral bones.
The new discovery could help make sense of the intermediate fish with limb-like fins that [evolutionary biologist Dr. Neil H. Shubin] and his colleagues have unearthed. These animals still used the molecular addresses their ancestors used. But when their cells reached their addresses, some of them became endochondral bone instead of fin rays. It may have been a simple matter to shift from one kind of tissue to another. [The New York Times]
"Here we're finding that the digits and the fin rays have some sort of equivalence at the level of the cells that make them. Honestly, you could have knocked me over with a feather — it ran counter to everything that I was expecting after working on this problem for decades," Dr. Shubin told The New York Times.