You know that creepy one-eyed monster from Greek mythology? It turns out we're all related to it - at least evolutionarily. Researchers from Lund University and the University of Sussex have proposed that the earliest vertebrate ancestor was a tiny, worm-like creature with a single eye perched on top of its head, living in the ocean nearly 600 million years ago.

According to the study, this cyclops-like stage happened after the creature abandoned its active lifestyle for a stationary one, filter-feeding like a couch potato. Because it no longer needed to chase dinner, its paired eyes gradually vanished, leaving only a central light-sensitive spot. That spot eventually became a primitive median eye, which probably just helped distinguish day from night and up from down - handy when you're stuck in one place.

But then its descendants got back on the move, and evolution had to MacGyver a new visual system. The researchers argue that parts of that median eye were repurposed to form the paired eyes vertebrates have today. This explains why our eyes are wired so differently from those of insects or squid: our retinas developed from brain tissue, not skin. "Now we finally understand why the eyes of vertebrates differ so radically," says Dan-E Nilsson, professor emeritus at Lund University, in what may be the understatement of the Cambrian period.

And the most mind-boggling part? That ancient median eye never fully disappeared. Its evolutionary leftovers are still with us as the pineal gland, a pea-sized organ in your brain that produces melatonin and regulates your sleep cycle. So next time you're lying awake at 3 a.m., you can blame your distant cyclops ancestor. "It's mind-boggling that our pineal gland's ability to regulate our sleep according to light stems from the cyclopean median eye of a distant ancestor 600 million years ago," Nilsson adds. Indeed.