It’s a poem that involves five little piggies, each corresponding to one of our fingers or toes. From then on out, five fingers and five toes became a standard feature for the world’s inaugural tetrapods. Only one in 500 to 1,000 humans are born with extra fingers or toes. “We found that our fingers and fish fin rays use the same Hox genes and their functions to develop,” says Nakamura. In other words, fish fin rays and our fingers derive from the same genetic toolkit.