How real is the Cyclops in The Odyssey?

When Odysseus and his men reach Sicily on their 10-year voyage home, they find a formidable foe: a one-eyed giant with a taste for sailors. As told in Homer’s The Odyssey, the Cyclops holds the men captive, eating them two-by-two, until he is outsmarted.

Cyclopes may be the stuff of mythology (and now a live-action movie version of Odysseus’ epic journey), but they have their roots in biology. Homer and the ancient Greeks, though, may have been more inspired by misinterpretations of fossils than by real-life one-eyed critters, which are far from towering — or terrifying.

Inspiration for Cyclopes might have come from the fossilized bones of prehistoric elephant ancestors. In the time of Homer, “nobody in Greece had ever seen a living elephant,” says Adrienne Mayor, a historian of ancient science at Stanford University. Ancient peoples thought the immense bones of massive mammals belonged to giants, gods and mythical creatures. These elephant ancestors have a big opening in the center of the skull for the trunk, which may have evoked one big eye. “The eye sockets are really not very noticeable,” she says.

For a true cyclops, look to the copepod. About a millimeter long, these tiny crustaceans make up a big portion of the base of the food chain in oceans and many bodies of water. A single, bright red eye peers from between two antennae on the heads of some species.

“They wouldn’t have high resolution [sight] like we would,” says Megan Porter, an evolutionary biologist who studies visual systems at the University of Hawaii at Manoa. These relatively simple eyes contain three cuplike structures that are sensitive to light, point in different directions and probably give the copepods a broad field of vision. It’s good enough, Porter says, to guide ocean copepods to deeper waters during the day to evade predators.

Around 560 million years ago, an ancestor of humans and all other vertebrates also had a single eye — a median eye — on the top of its head that could detect light but couldn’t resolve detailed images.

For this water-dwelling critter, going to one eye was an evolutionary innovation. Its ancestor had three eyes: one on each side of the head, plus the median eye, says George Kafetzis, an evolutionary neuroscientist at the University of Sussex in England. The pair of side eyes probably handled navigation while the median eye followed the daily rhythms of the sun.

This ancestor might have lost its side eyes because it abandoned a free-swimming lifestyle for a sedentary burrowing one, says Kafetzis, whose team described the evolution of the vertebrate eye in February in Current Biology. “When you’re partially burrowed, the steering eyes don’t have much purpose to serve anymore,” he says.

Over a few million years, lifestyles changed again, vertebrates evolved and three eyes again became better than one.

In humans, the cells that eventually become eyes begin as one cluster before splitting into two. In rare cases, genetics or environmental factors can cause cyclopia, a deadly disorder in which that cluster doesn’t split, instead developing as one large eye. “Cyclopia is sort of the extreme end of a whole range of malformations,” says Philip Beachy, a developmental molecular biologist at Stanford University.

Our “third eye” descended into the brain to become the pineal gland, where it regulates our circadian rhythm based on information it receives from our seeing eyes. For some species of fish, reptiles and amphibians, a version of this median eye remains exposed on the top of the head and can still sense light directly.

“People of antiquity were always on the lookout for marvels,” Mayor says. While today’s one-eyed wonders are a world apart from the monsters of Greek mythology, they still inspire us to explore — and marvel at — our world.