Looking more like a baby salamander than anything else, a six-week-old human embryo has tiny paddles for hands, dark dots for eyes and on either side of its shallow mouth slit, half a dozen small bumps destined to form an ear. By nine weeks, these “hillocks” will migrate up the face as the jaw becomes more pronounced and start taking on the recognizable shell shape so handy for holding up eyeglasses. Because development often reprises stages of evolution, the growth of embryonic ears in tandem with the jaw is no accident: the sound-transmitting middle ear bones that are a distinguishing feature of mammals evolved from what used to be gill arches in fish and jawbones in reptiles.