Researchers studying a well-preserved trilobite fossil believe they have found evidence for the earliest eye yet discovered, reports the BBC. The 530-million-year-old fossil from Estonia shows that simple compound eyes have not changed much in half a billion years.
This particular trilobite fossil, a species called Schmidtiellus reetae, was exceptionally well-preserved—eyes and all. A small portion of its eye was worn away, exposing the cellular structure of the organ, which allowed researchers to examine the eye in detail.
The tiny organ contains 100 ommatidia, or subunits that make up compound eyes found today in many insects, surrounding a light-sensing structure called a rhabdom. The structure is very similar to compound eyes found in modern dragonflies and bees, but the ommatidia are spaced farther apart and the eye lacks a lens commonly found today. Researchers detail the eye in a study published this week in Proceedings of the National Academy of Sciences.
The research suggests that animals have been seeing—or at least sensing brightness—for a very long time. “The principle of the modern compound eye most likely goes back to before the times of our first fossil records,” lead author Brigitte Schoenemann of the University of Cologne says in the press release. “Half a billion years ago, it was in the early stage of its development, and with our work we have succeeded in uncovering the first visible steps of this extremely successful visual principle.”
While it’s likely that primitive eyes formed in earlier species, researchers have not yet located fossils well preserved enough to support the transition. “Older specimens in sediment layers below this fossil contain only traces of the original animals, which were too soft to be fossilized and have disintegrated over time,” Schoenemann tells the BBC.
This is not the only trilobite eye ever found. As the researchers note in the study, another trilobite species, Holmia kjerulfi, evolved just a few million years later and had even better eyesight, approximating the modern dragonfly.
So what could the trilobites actually see? With only 100 “pixels” of information, their vision was by no means high definition. But it was enough to recognize obstructions or obstacles and also to pick up the approach of predators. In fact, as Signe Dean at ScienceAlert reports, the researchers believe the development of eyes was something of an arms race between prey species and an emerging class of predators.
“The ‘race’ between predator and prey and the need ‘to see’ and ‘to be seen’ or ‘not to be seen’ were drivers for the origin and subsequent evolution of efficient visual systems, as well as for protective shells,” the researchers write in the study.
Just a few million years later—515 million years ago—a three-foot-long creature called Anomalocaris ruled the seas. It sported sophisticated compound eyes that were on par (or better) than those of modern insects and crustaceans, seeing its way to becoming one of the first apex predators, munching on trilobites, eyes and all.