Evolutionary constraints impose multiple checkpoints that block mammalian sensory hair cell regeneration.

Irreversible damage to auditory hair cells is a primary etiology of sensorineural hearing loss. Across the vertebrate phylogenetic lineage, the regenerative capacity of these cells exhibits a marked decline: from robust regeneration in fish and amphibians, to functionally limited regeneration in birds, culminating in the permanent loss of this ability upon hair cell maturation in mammals. To elucidate the molecular logic underlying this evolutionary divergence and explore viable pathways for regeneration, this article employs a systematic cross-species comparative analysis. We propose that regenerating species share a highly conserved regenerative framework, whose most representative core components include: supporting cells as the key progenitor source, transcriptional reprogramming centered on genes such as Atonal homolog 1 (Atoh1), and a switch to a permissive microenvironmental state. The variation in regenerative capacity primarily stems from species-specific regulatory networks superimposed upon this core framework (e.g., the Forkhead box G1a (Foxg1a)/SRY-related HMG-box (SOX)/Sine oculis homeobox (Six) network in fish, the Fibroblast growth factor (FGF)-Extracellular signal-regulated kinase (ERK)-SOX2 cascade in birds), which fine-tune the efficiency and mode of regeneration. Based on this, we advance a "Multiple Checkpoints" hypothesis, positing that the acquisition of a regeneration-silent state in mammals is not due to the loss of the core framework. Instead, evolutionary processes have erected physiological barriers requiring coordinated overcoming at these key junctures, including: deep quiescence of progenitor cells, failure to initiate core reprogramming programs, and degeneration of the permissive microenvironment. This framework suggests that future therapeutic strategies aimed at achieving functional hearing restoration will likely require the concerted targeting of these multiple barriers. Through temporally precise interventions, microenvironmental remodeling, and combinatorial therapies, it may be possible to reactivate this evolutionarily dormant regenerative potential into a clinically viable new pathway.