Scientists find missing link in eye evolution

A primitive fish that oozes reams of slime when it is unsettled could be the missing link in the evolutionary sequence of the vertebrate eye, according to a team led by a vision expert from The Australian National University.

The scientists compared the eyes of the eel-like hagfish and its cousin, the lamprey, to show that the eye gradually evolved over millions of years — something that had even Charles Darwin stumped.

“Darwin knew that his theory of natural selection would have difficulty in explaining the existence of an organ as specialised as the eye — unless a series of gradual changes could be proved,” said Prof Trevor Lamb, head of the ARC Centre of Excellence in Vision Science at ANU.

“We think we've found the ‘missing link’ that shows how the eyes of vertebrates came to be.”

“The primitive hagfish diverged from our own evolutionary line somewhere around 530 million years ago. Hagfish are jawless and ugly animals that continue to inhabit the oceans at great depth, and are renowned for the revolting ‘slime’ they exude when they’re disturbed.

“They behave as if blind, though they have a primitive eye-like structure beneath a clear patch on either side of the head. It was previously thought that this hagfish ‘eye’ had degenerated from a lamprey-like precursor.”

With his colleagues, Prof Shaun Collin from the University of Queensland and Prof Ed Pugh from the University of Pennsylvania, Lamb discovered that the hagfish eye has all the signs of being an evolutionary missing link. They argue that hagfish did not degenerate from lamprey-like ancestors, but are instead the remnants of an earlier sister group.

“The hagfish eye has no lens, no muscles to aim it, the simplest of retinas, and primitive photoreceptors,” Lamb said.

“In the relatively brief interval of about 30 million years, between when we think hagfish split off and when lampreys split off, the vertebrate eye appears to have evolved most of its modern characteristics.”

The findings are published in the latest edition of Nature Reviews Neuroscience.