Simple sea creatures may hold the secret to the first eyes

Nature · Published 22 October 2026 Swipe more like this →

In short: How did vision first emerge in the deep history of life? By studying cnidarians—the relatives of jellyfish—scientists are uncovering a complex genetic toolkit that suggests the building blocks of sight were repurposed and recombined multiple times across the tree of life.

Cnidarians are a basal phylum that serves as a sister group to Bilateria, the group containing all animals with bilateral symmetry. Because they have independently evolved eyes at least eight times, they provide a unique laboratory for understanding the evolution of vision. Unlike vertebrates, which primarily use ciliary opsins, or arthropods, which use rhabdomeric opsins, cnidarians often possess a third type known as cnidop or cnidarian opsins. These visual opsins function as G-protein coupled receptors. They initiate the phototransduction cascade, which converts light into electrical signals for interpretation. This process is the fundamental mechanism for translating environmental information into biological responses.

A recent study of the eyeless cnidarian Hydra vulgaris revealed a complex genetic landscape for light detection. Researchers identified 45 different opsin genes in this species, many of which are clustered together, suggesting they evolved through tandem duplications. While Hydra lacks formal eyes, it still exhibits light-dependent behaviors. By using RNA-seq and phylogenetics, scientists found two specific opsins in Hydra that show expression patterns similar to known phototransduction genes. The researchers also identified two specific genes, HvOpA1 and HvOpB1. These unique genes likely function within mucous gland cells and nematocytes. This data provides a springboard for future investigations into specific gene functions.

The most striking finding is that the phototransduction cascade in Hydra vulgaris appears to be a hybrid. The researchers propose that this cascade contains components of both the ciliary and rhabdomeric systems. This suggests that the molecular machinery for converting light into electrical signals did not follow a single linear path but rather involved a mosaic of genetic components that were independently co-opted and refined as diverse visual systems emerged. The study utilized transcriptomics to characterize the molecular evolution of these genes. This approach allowed for a detailed look at the expression patterns across various tissues. It highlights how diverse visual systems emerged from a shared genetic toolkit.

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