How did animals like sharks and skates evolve claspers—paired penislike organs found on male pelvic fins—like the ones seen on the male skate on the left? For years, their origin has remained an evolutionary mystery, but now, a new study suggests that regulation of the genetic circuit known as the Sonic hedgehog (Shh) pathway by sex hormones may be the answer. A team of researchers started by rearing little skate (Leucoraja erinacea) eggs in the lab. After comparing male and female embryos, they observed that claspers start to form in males during a late phase of pelvic fin development. They zeroed in on the Shh network, which, among other functions, drives appendage development in vertebrates, and found it remained active in the fins’ clasper-forming zones a month longer in males than females. Inhibiting Shh activity in the fins of male skates significantly reduced clasper development, and activating it in female skates created claspers where there had been none, the team reports this month in Nature Communications. With that realization, the researchers turned to understanding what kept the males’ Shh switch turned on longer and discovered that the sex hormone androgen was responsible. At some point in their evolutionary history, chondrichthyans—which include sharks, skates, rays, and chimeras—evolved a way for sex hormones to control the genes responsible for appendage development by extending the length of time they are activated, thus spurring the development of male sex organs. The same mechanism could explain how claspers developed in placoderms—the extinct, armored fish with the earliest known vertebrate sex organs—as well as the evolution of vertebrate penises, researchers say.