Eyes are not just the windows to the soul. They’ve been key to the evolution of life as we know it.

Predators developed binocular vision, to give them better depth perception when hunting prey. As a response, prey developed eyes on the sides or tops of their heads, for a wide field of view to detect predators. A raptor can spot a rabbit from more than a mile away. Butterflies see ultraviolet light, to find flowers and mates. But there was a time long ago when life on Earth was blind.

Then, about 500 million years ago, trilobites evolved. Much like today’s horseshoe crabs, they scuttled around the ocean floor looking for marine worms or carrion to scavenge. And to help find food, they began to develop eyes.

This amazing new capability allowed them to spread across the globe, adapt to different environments, and diversify into as many as 20,000 species, ranging in size from a fingertip to trash-can lid.

Trilobite eyes forced other organisms to evolve defense mechanisms like locomotion and camouflage—and eyes—to compete with them for food and to avoid being eaten. Some scientists think that trilobite eyes encouraged the explosion of diverse life forms that led to the dinosaurs.

Vision enabled trilobites to dominate Earth for more than 100 million years. And although they eventually went extinct, vision has survived across the millennia.

So next time you use your eyes, thank the lowly trilobite for getting the party started.