ZME Science reports:

Deep inside the human brain, a small gland in the middle of the forehead helps keep time. As darkness arrives, the pineal gland helps set the body’s nightly rhythm. But some creatures use the pineal gland as a sort of “third eye”, helping them detect light and shadow, though the organ doesn’t form pictures. Our human lineage lost this ancestral ability millions of years ago.

But a new evolutionary hypothesis suggests that this unassuming organ is the surviving central remnant of the eye from which our two image-forming eyes ultimately arose.

In a review published in Current Biology, researchers from Lund University and the University of Sussex argue that the lineage leading to vertebrates passed through a cyclops-like stage roughly 560 million years ago. The animal was not a one-eyed giant, but probably a tiny, wormlike marine filter feeder with a light-sensitive organ on top of its head. As its descendants returned to active swimming, parts of that central organ expanded sideways and were repurposed into paired retinas.

The researchers assembled a historical reconstruction from animal anatomy, eye-development genes, light-sensing proteins, neural wiring and single-cell gene-expression data. Their model may help explain why the vertebrate retina is built so differently from the eyes of insects, worms and squid. [Continue reading…]