Malleus & Incus 4th of December, 2007 ANTE·MERIDIEM 12:44

My favorite example of this phenomenon [that particular traits can take very non-intuitive paths through evolutionary history] involves the three little bones in our middle ear—the malleus, incus and stapes. Now used for hearing, two of these bones (the malleus and incus) were originally part of the lower jaw of our reptilian ancestors, who used them for chewing. Reptiles needed flexible, multielement, multihinged jaws so they could swallow giant prey, whereas mammals preferred a single strong bone (the dentary) for cracking nuts and chewing tough substances like grains. So as reptiles evolved into mammals, two of the jawbones were co-opted into the middle ear and used for amplifying sounds (partly because early mammals were nocturnal and relied largely on hearing for survival.) This such an ad hoc, bizarre solution that unless you know your comparative anatomy well or discovered fossil intermediates, you never could have deduced it from simply considering the functional needs of the organism.

(Dr. V.S. Ramachandran and Sandra Blakeslee, ‘Phantoms in the Brain’, p. 210)