Cochlear mechanisms from a phylogenetic viewpoint

TLDR

The inner‑ear hearing organ, the last of the amniote paired sense organs to evolve, originated as a small, simple structure in early amniotes but already possessed hair cells with cochlear amplification, electrical frequency tuning, and nascent micromechanical tuning. This review compares structure and function across taxonomic groups and evaluates how phylogenetic selection pressures shaped fundamental hearing mechanisms. Parallel middle‑ear improvements across lineages drove papillar elongation, increased sensory cell numbers, enhanced micromechanical tuning, and lineage‑specific hair‑cell specializations, producing distinct morphological configurations.

Abstract

The hearing organ of the inner ear was the last of the paired sense organs of amniotes to undergo formative evolution. As a mechanical sensory organ, the inner-ear hearing organ's function depends highly on its physical structure. Comparative studies suggest that the hearing organ of the earliest amniote vertebrates was small and simple, but possessed hair cells with a cochlear amplifier mechanism, electrical frequency tuning, and incipient micromechanical tuning. The separation of the different groups of amniotes from the stem reptiles occurred relatively early, with the ancestors of the mammals branching off first, approximately 320 million years ago. The evolution of the hearing organ in the three major lines of the descendents of the stem reptiles (e.g., mammals, birds-crocodiles, and lizards-snakes) thus occurred independently over long periods of time. Dramatic and parallel improvements in the middle ear initiated papillar elongation in all lineages, accompanied by increased numbers of sensory cells with enhanced micromechanical tuning and group-specific hair-cell specializations that resulted in unique morphological configurations. This review aims not only to compare structure and function across classification boundaries (the comparative approach), but also to assess how and to what extent fundamental mechanisms were influenced by selection pressures in times past (the phylogenetic viewpoint).