New research suggests the complex eyes of vertebrates, including humans, trace their origins to a worm-like creature that inhabited the oceans around 600 million years ago. This ancestor is thought to be the common forebear of all bilateral animals, a group defined by bodies that can be split into mirror-image left and right halves.
The study, which surveyed 36 major animal groups, found that light-sensing cells consistently appear in two distinct locations: paired on either side of the face, which steer movement, and at the midline of the head, which detect day from night and up from down. The authors propose that a stationary, filter-feeding ancestor lost its paired steering eyes as a result of burrowing into the seabed, leaving only the midline light sensors to support basic needs.
After this period of inactivity, the animal returned to a swimming lifestyle, prompting the re-evolution of paired eyes from the midline structures. Over time, these eye cups separated and moved to the sides of the head, ultimately forming the camera-type eyes found in vertebrates today. This loss and regain of vision is thought to have occurred between 600 and 540 million years ago.
Remnants of the ancient midline eye survive in the pineal organ, which produces the sleep hormone melatonin. In many vertebrates, this organ still responds to light through a transparent patch on the head, but in mammals it lost this ability, likely because early mammals were active at night and relied on their more sensitive paired eyes instead.
Invertebrates such as crustaceans, insects, spiders and octopuses never adopted a static lifestyle, so they retained the original paired light-sensing cells, evolving them into compound or camera-type eyes. The vertebrate retina, however, has over 100 types of neurons, making it almost as complex as the cerebral cortex. The researchers argue that much of this complexity evolved early, likely present in the midline 'cyclops' eye before it gave rise to our own.
