Convergent re-evolution of CNS structures: focus on the fovea.

The fovea is a specialized retinal region that supports high-acuity vision through extreme photoreceptor and ganglion cell densities and near one-to-one circuit connectivity. Although present only in a subset of vertebrates, foveae share core functional principles while differing widely in morphology, cellular composition, number, and retinal location, indicating multiple independent evolutionary origins. Here, I review comparative, developmental, and genomic evidence that the fovea represents a case of convergent re-evolution built on conserved central nervous system patterning mechanisms. I highlight retinoic acid signaling as the earliest known molecular pathway demarcating foveal progenitors across distantly related species and discuss how evolutionary modulation of this positional system may repeatedly generate high-acuity retinal specializations.