Department of Biology, University of Maryland, College Park, MD 20742, USA.
Department of Biology, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark.
J Exp Biol. 2022 Jun 15;225(12). doi: 10.1242/jeb.244130. Epub 2022 Jun 20.
The ability to sense and localize sound is so advantageous for survival that it is difficult to understand the almost 100 million year gap separating the appearance of early tetrapods and the emergence of an impedance-matching tympanic middle ear - which we normally regard as a prerequisite for sensitive hearing on land - in their descendants. Recent studies of hearing in extant atympanate vertebrates have provided significant insights into the ancestral state(s) and the early evolution of the terrestrial tetrapod auditory system. These reveal a mechanism for sound pressure detection and directional hearing in 'earless' atympanate vertebrates that may be generalizable to all tetrapods, including the earliest terrestrial species. Here, we review the structure and function of vertebrate tympanic middle ears and highlight the multiple acquisition and loss events that characterize the complex evolutionary history of this important sensory structure. We describe extratympanic pathways for sound transmission to the inner ear and synthesize findings from recent studies to propose a general mechanism for hearing in 'earless' atympanate vertebrates. Finally, we integrate these studies with research on tympanate species that may also rely on extratympanic mechanisms for acoustic reception of infrasound (<20 Hz) and with studies on human bone conduction mechanisms of hearing.
感知和定位声音的能力对生存非常有利，以至于我们很难理解早期四足动物的出现与中耳阻抗匹配的出现之间相隔近 1 亿年 - 我们通常认为这是陆地敏感听力的前提 - 在它们的后代中。对现存无中耳脊椎动物听力的研究为陆地四足动物听觉系统的祖先状态和早期进化提供了重要的见解。这些研究揭示了一种在“无耳”无中耳脊椎动物中检测声压和定向听觉的机制，该机制可能适用于包括最早的陆地物种在内的所有四足动物。在这里，我们回顾了脊椎动物中耳的结构和功能，并强调了描述这种重要感觉结构复杂进化历史的多次获得和丧失事件。我们描述了向内耳传递声音的鼓室外途径，并综合了最近研究的结果，提出了“无耳”无中耳脊椎动物听觉的一般机制。最后，我们将这些研究与对可能也依赖于鼓室外机制来接收次声（<20 Hz）的有中耳物种的研究以及对人类骨导听力机制的研究相结合。
