Fossil discoveries link one-eyed animals to cyclops myth

Stories of one-eyed giants in Homer’s The Odyssey may trace to fossil bones and small aquatic animals rather than encounters with actual monsters. Ancient peoples in Greece had no living elephants to compare against large bones they unearthed. Adrienne Mayor, a historian of ancient science at Stanford University, said the central opening for an elephant trunk could appear as a single large eye socket.

Tiny crustaceans called copepods provide a living example of a single functional eye. Megan Porter, an evolutionary biologist at the University of Hawaii at Manoa, said these millimeter-long animals use a red eye made of three light-sensitive cups to navigate daily vertical migrations.

An ancestor of all vertebrates possessed a single median eye roughly 560 million years ago. George Kafetzis, an evolutionary neuroscientist at the University of Sussex, and colleagues reported in February that this eye helped track daily light cycles before side eyes developed.

Developmental errors can still produce a single eye in human embryos. Philip Beachy, a developmental molecular biologist at Stanford University, said cyclopia occurs when the initial eye-forming cluster fails to split. The pineal gland in humans traces to this ancient third eye.

Some fish, reptiles, and amphibians retain a functional version on top of their head.