12 electron micrographs collectively show a glancing section through the retinal body of an eye-like structure from a single-celled organism. Gavelis et al./Nature

By definition, a single-celled organism can't have a multicellular organ. However, that hasn't stopped warnowiids, a class of marine plankton, from evolving an eye that looks remarkably like that of humans and other animals.

Advocates of intelligent design often use eyes as an example of an organ so complex that it could, they claim, never have evolved without intervention. The claim has been thoroughly debunked, and in fact the eye is known to have evolved several times. Moreover, if an intelligent designer was involved, it is puzzling why they chose to inflict humans and other mammals with an arguably inferior version compared to that of cephalopods, which don't have a blind spot.

What no one expected, however, was a simulacrum of such a complex organ to appear in warnowiids, a type of dinoflagellate.

"It's an amazingly complex structure for a single-celled organism to have evolved," said University of British Columbia's Greg Gavelis, whose examination of the structure led to the rare honor of being first author of a Nature paper while still a Ph.D. student.

“Multicellularity is often considered a prerequisite for morphological complexity,” the paper notes. “A notable exception exists in single-celled eukaryotes called dinoflagellates, some of which have an eye-like ocelloid consisting of subcellular analogues to a cornea, lens, iris and retina.”

So surprising was the find that when ocelloids were found inside the predatory plankton, they were thought to be from a tiny animal devoured by the warnowiid.

Warnowiids spear their victims with harpoon-like appendages, and it is thought the ocelloid helps them find food. Many of their prey are largely transparent, but it's possible that they interfere with light just enough to be spotted by the ocelloid, which may send chemical signals to the rest of the cell to orient hunting.

"The internal organization of the retinal component of the ocelloid is reminiscent of the polarizing filters on the lenses of cameras and sunglasses," said senior author Professor Brian Leander, head of Gavelis' lab. “It has hundreds of closely packed membranes lined up in parallel.”

The ocelloids sit at the center of a network of photosynthesizing plastids, whose genetics indicate they come from red algae that were once symbiotic with ancestral dinoflagellates, but have now become incorporated. At some point, it seems a plastid changed from converting light into useable energy to detecting it and alerting the rest of the lifeform. Far more simple forms that allow cells to detect the direction of light are found in other dinoflagellate species.

The warnowiids, collected off Japan and Canada's west coast, were studied using microscopes that allow a three-dimensional reconstruction of subcellular features.

"When we see such similar structural complexity at fundamentally different levels of organization in lineages that are very distantly related, then you get a much deeper understanding of convergence," said Leander.

Previous studies have been hindered by the fact that warnowiids are rare in the wild and have not yet been grown in the lab.