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See Inside September 2009

External Ears

They guide sound to the sensitive middle ear

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.

The tympanic membrane, or eardrum, that sits just outside the middle ear evolved separately and repeatedly in the ancestors of frogs, turtles, lizards, birds and mammals. Reptilian eardrums can do no more than crudely transmit low-frequency vibrations. To mammals, which have a fancier middle-ear setup, higher-frequency sounds are also audible; external skin and cartilage flaps, called pinnae, are thought to have evolved to capture and funnel those sounds more effectively. The entire human ear structure amplifies sounds by only about 10 to 15 decibels, but our pinnae also usefully modulate the frequency of sounds entering the ear canal. As the contours of the pinnae reflect incoming vibrations, they slightly delay the higher-frequency sounds in a way that cancels out some of them. This so-called notch-filtering effect preferentially delivers sounds in the range of human speech to the inner ear.

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