To help his readers fathom evolution, Charles Darwin asked them to consider their own hands.

“What can be more curious,” he asked, “than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include similar bones, in the same relative positions?”

Darwin had a straightforward explanation: People, moles, horses, porpoises and bats all shared a common ancestor that grew limbs with digits. Its descendants evolved different kinds of limbs adapted for different tasks. But they never lost the anatomical similarities that revealed their kinship.

As a Victorian naturalist, Darwin was limited in the similarities he could find. The most sophisticated equipment he could use for the task was a crude microscope. Today, scientists are carrying on his work with new biological tools. They are uncovering deep similarities that have been overlooked until now.

On Wednesday, a team of researchers at the University of Chicago reported that our hands share a deep evolutionary connection not only to bat wings or horse hooves, but also to fish fins.