![]() The tympanic middle ear is the assemblage of tiny bones that connects at one end to an eardrum and at the other end to the oval window, an aperture in the bone of the inner ear. Independently, but within just 20 million to 30 million years of one another, all three amniote lineages evolved a tympanic middle ear from parts of the skull and the jaws. ICONS: ISTOCK.COMDuring the Triassic period, some 250 to 200 million years ago, a truly remarkable thing happened. ![]() See full infographic: JPG ILLUSTRATIONS: PHEBE LI FOR THE SCIENTIST. Despite the independent origin of hearing structures in the three lineages, the outcomes were functionally quite similar, serving as a remarkable example of convergent evolution. It is unclear what kind of stimuli could have existed that would have led to the retention of such hearing organs for such a long time.ĬONVERGING ON THE EAR: Starting around 250 million years ago, the three amniote lineages-lepidosaurs (lizards and snakes), archosaurs (crocodilians and birds), and mammals-separately evolved a tympanic middle ear, followed by evolution of the inner ear, both of which served to increase hearing sensitivity. As such, ancestral amniotes most likely perceived only sounds of relatively low frequency and high amplitude that reached the inner ear via the limbs or, if the skull were rested on the ground, through the tissues of the head. They had not yet evolved any mechanism for absorbing sound energy from air they lacked the middle ear and eardrum that are vital for the function of modern hearing organs. For a period of at least 50 million years after amniotes arose, the three main lineages were most likely quite hard of hearing. Good high-frequency hearing did not exist from the start, however. ![]() What remains is the watery macromolecular gel known as the tectorial membrane, which assures that local groups of hair cells move synchronously, resulting in greater sensitivity. Moreover, the early evolution of these dedicated auditory organs in land vertebrates led to the loss of the heavy otolithic membrane that overlies the hair-cell bundles of vestibular organs and is responsible for their slow responses. This change is attributable in part to modifications in the ion channels of the cell membrane, such that each cell is “electrically tuned” to a particular frequency, a phenomenon still observed in some modern amniote ears. Low-frequency vestibular hair cells became specialized to transduce higher frequencies, requiring much faster response rates. The amniote hearing organ evolved as a separate group of hair cells that lay between two existing vestibular epithelia. ![]() Between these stereovilli are proteinaceous links, most of which are closely coupled to sensory transduction channels that respond to a tilting of the stereovilli bundles caused by sound waves. On their apical surface, all hair cells have tight tufts or bundles of large, hairlike villi known as stereovilli (or, more commonly stereocilia, even though they are not true cilia), which give hair cells their name. Initially, the hearing organ only responded to low-frequency sounds. These rudimentary structures evolved from the hair cells of vestibular organs, which help organisms maintain their balance by responding to physical input, such as head rotation or gravity. By comparing the skulls of the extinct common ancestors of these three lineages, as well as the ears of the most basal modern amniotes, researchers have concluded that ancestral amniotes had a small (perhaps less than 1 millimeter in length) but dedicated hearing organ: a sensory epithelium called a basilar papilla, with perhaps a few hundred sensory hair cells supported by a thin basilar membrane that is freely suspended in fluid. 1 New paleontological studies and comparative research on hearing organs have revealed the remarkable history of this unexpected diversity of ears.Īmniote vertebrates comprise three lineages of extant groups that diverged roughly 300 million years ago: the lepidosaurs, which include lizards and snakes the archosaurs, which include crocodilians and birds and mammals, which include egg-laying, pouched, and placental mammals. As a result, external, middle, and inner ears of various amniotes are characteristically different. Although it seems obvious that the ability to process nearby sounds would be enormously useful, modern amniote ears in fact arose quite late in evolutionary history, and to a large extent independently in different lineages. Two ears complete the picture, allowing animals’ brains to localize the source of the sounds they hear by comparing the two inputs. Together, these hair cells and nerve fibers encode a wide range of sounds that enter the ear on that side of the head.
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