Sometimes in nature, when evolution gets something just right there’s little to be done to improve certain structures. After all: if ain’t broke, don’t fix it. In the case of trilobites, which scuttled across the seafloor from around 520 to 250 million years ago, it seems compound eyes were one of these great adaptational feats. New research published in the journal Scientific Reports describes how the eye structure of these ancient animals is near-identical to those seen in modern insects and crustaceans. The discovery tells us that compound eyes evolved very early on in the evolution of animals and were so fit for purpose they’ve persevered in certain species to this day.

Using digital microscopy, Brigitte Schoenemann and colleagues examined a trilobite fossil retrieved from the Czech Republic in 1846 whose left eye was still intact. The eye, belonging to the trilobite species Aulacopleura koninckii, contained several internal structures associated with the compound eyes of modern insect and crustacean species.

One of these structures is called an ommatidia, which is a collection of light-detecting cells clustered around a transparent tube called a rhabdom. These light-detecting cells are separated by dark rings of pigment and topped with a thick lens and a crystalline comb, which focused light onto the rhabdom within the cell.

Given how tiny these light-detecting units were, the researchers suggest that A. koninckii probably inhabited bright shallow waters and was active in the daytime. The pigmented rings that separated each of the light-detecting cells likely gave the trilobites a mosaiced outlook, similar to cartoon renditions of what the world might look like through the eyes of a fly. The only difference between this ancient eye and modern compound eyes is that there were fewer visual units, indicating they couldn’t see in as much detail as bees can today.

A larger image made up of the contributions of many small images is what characterizes compound eyes, which are still seen in crustacean and insect species to this day such as crabs and bees. This study shows that the structure and function of this eye type has persevered virtually unchanged since the Paleozoic era, around 542-251 million years ago, and tells us more about how the world must have looked through the eyes of an ancient trilobite.