The Evolution of Eyes Began With One

Scientists now propose that the vertebrate eye traces back about 560 million years to a cyclopean invertebrate with a single patch of light-sensitive cells on the top of its head. That primitive organ likely tracked day and night and helped the animals orient themselves while they poked their heads out of the seafloor to filter food.

The researchers outline a sequence of changes as some descendants left their burrows and took to swimming. Cup-shaped depressions evolved to sense the direction of incoming light, and as the light-sensitive tissue migrated to the sides of the head it formed the proto-retinas and sharper wiring needed for navigation and obstacle avoidance.

Earlier work by Dan-E. Nilsson and Susanne Pelger showed that the shape of an image-forming eye could arise quickly, and the new scenario draws on much more extensive molecular data. Fossils add complexity: some early vertebrates show eyes on the sides of the head and a second pair on top, a configuration that may have helped small animals avoid predators.

eye evolution, vertebrate eye, cyclopean invertebrate, light-sensitive cells, photoreceptor patch, cup-shaped depressions, proto-retinas, nilsson pelger, fossil evidence, predator avoidance