On the Mammalian Ear

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Science  09 Oct 2009:
Vol. 326, Issue 5950, pp. 243-244
DOI: 10.1126/science.1181131

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During their development, embryos of many species repeat evolutionary stages of their ancestors (1). For example, in human embryos gill pouches are formed during early developmental stages. Developmental heterochrony—that is, the differing timing of developmental processes during embryonic growth—can lead to a premature fixation of ancestral character states and the retention of embryonic patterns in the adult. This process is believed to be an important driving force for evolution (2, 3). One of the key innovations in mammalian history is the evolution of the mammalian ear, leading to the most efficient hearing among vertebrates. On page 278 of this issue, Ji et al. (4) use an analysis of the Early Cretaceous mammal Maotherium to show how heterochrony routed the evolution of the definitive mammalian middle ear (DMME).