Fish reveals aquatic ancestor’s journey from sea to land

By Mark Waghorn via SWNS

A blinking fish has revealed how our aquatic ancestors first left the water to begin exploring land - 375 million years ago.

They evolved into tetrapods – today's amphibians, reptiles, birds, and mammals. Their survival in a terrestrial environment has baffled scientists for decades.

Now a study has shown that the answer lay in the humble mudskipper - a species of fish that uses its pectoral fins like legs to walk and even climb trees.

It lives in and out of water and forages for food on the ground. It also blinks - just like us.

Humans blink around 13,500 times a day to keep our eyeballs clean - and boost communication. Rates increase during conversation.

The mudskipper's blink lasts about the same length of time. Its eyes bulge out of the top of its head - like a frog's.

The fish momentarily retracts them into sockets where they are covered by a stretchy membrane called a 'dermal cup.'

Study lead author Dr. Thomas Stewart, of Penn State University, said: "Animals blink for many reasons.

"It helps us keep our eyes wet and clean, it helps us protect our eyes from injury, and we even use blinking for communication.

"Studying how this behavior first evolved has been challenging because the anatomical changes that allow blinking are mostly in soft tissues, which don't preserve well in the fossil record.

"The mudskipper, which evolved its blinking behavior independently, gives us the opportunity to test how and why blinking might have evolved in a living fish that regularly leaves the water to spend time on land."

The U.S. team analyzed the action using high-speed videos. They also compared the anatomy of mudskippers with a closely related water-bound fish doesn't blink.

Co-author Dr. Brett Aiello, of Seton Hill University in Pennsylvania, said: "Blinking in mudskippers appears to have evolved through a rearrangement of existing muscles that changed their line of action and also by the evolution of a novel tissue - the dermal cup.

"This is a very interesting result because it shows that a very rudimentary, or basic, system can be used to conduct a complex behavior.

"You don't need to evolve a lot of new stuff to evolve this new behavior - mudskippers just started using what they already had in a different way."

In humans, tears are critical to keep cells in the eye healthy and oxygenated.

Dr. Aiello said: "We found that, just like humans, mudskippers blink more frequently when confronted with dry eyes.

"What's incredible is they can use their blinks to wet the eyes, even though these fish haven't evolved any tear glands or ducts.

"Whereas our tears are made by glands around our eyes and on our eyelids, mudskippers seem to be mixing mucus from the skin with water from their environment to produce a tear film."

Further tests also showed blinking protects the mudskipper's eyes from injury - clearing out dust or debris.

Dr. Aiello said: "Our study, which considered the behavior and anatomy of a living fish that underwent a transition to life on land, similar to the earliest tetrapods, helps us to reimagine how and why these early tetrapods might have been blinking.

"Having the opportunity to study how and why this behavior first evolved provides an amazing opportunity to learn more about the way humans came to be as they are and gives us insight into changes associated with major transitions in the history of animals - like inhabiting land."

Blinking is something humans and other tetrapods are constantly doing throughout the day - often without even noticing.

The subtle action is complex - and critical to the health and safety of the vertebrate eye.

Dr. Stewart added: "The transition to life on land required many anatomical changes, including changes for feeding, locomotion and breathing air.

"Based on the fact that mudskipper blinking, which evolved completely independently from our own fishy ancestors, serves many of the same functions as blinking in our own lineage, we think that it was likely part of the suite of traits that evolved when tetrapods were adapting to live on land."

The study was published in the journal Proceedings of the National Academy of Sciences.