Humans may exist because a fish learned how to blink 375 million years ago

The mudskipper, an intriguing amphibious fish with a penchant for blinking, is offering scientists insight into how and why blinking might have evolved as our ancestors made the transition from aquatic to terrestrial life. 

Researchers at Penn State and Seton Hill University, led by Thomas Stewart and Brett Aiello, have been studying this unusual fish and have found that its blinking behavior serves many of the same purposes as our own.

Published in the Proceedings of the National Academy of Sciences, the study suggests that blinking may be one of the key traits that allowed for the evolution of tetrapods – animals including mammals, birds, reptiles, and amphibians – around 375 million years ago. 

“Animals blink for many reasons,” said Stewart. “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.”

How the study was done

To better understand how mudskippers evolved the ability to blink, the team analyzed their behavior using high-speed videos and compared the anatomy of mudskippers to that of a closely related fish that does not blink.

Like a frog’s eyes, the mudskipper’s eyes protrude from the top of its head. To blink, the fish retracts its eyes momentarily into sockets, where they are covered by a stretchy membrane known as a “dermal cup.” Interestingly, the mudskipper’s blink lasts about as long as a human blink.

“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,” said Aiello. “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.”

The blinking behavior of mudskippers

The researchers investigated the reasons behind the mudskipper’s blinking behavior by considering the roles that blinking plays in humans and other tetrapods. In humans, tears keep eye cells healthy and oxygenated, so the team analyzed whether mudskippers also blink to keep their eyes wet.

“We found that, just like humans, mudskippers blink more frequently when confronted with dry eyes,” said Aiello. “What’s incredible is that 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.”

Additionally, the researchers tested if blinking in mudskippers could be triggered to protect the eye from injury and if blinking cleaned the fish’s eyes of dust or debris. In both instances, the answer was affirmative. 

As a result, it appears that blinking in mudskippers serves three primary functions – protecting, cleaning, and maintaining moisture – similar to humans and other tetrapods.

“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,” said Aiello. 

“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 may be the earliest trigger behind the evolution of humanity

Brett Aiello explained that humans and other tetrapods blink constantly throughout the day, often without even realizing it. Despite being a subtle action, blinking is a complex and fascinating behavior that can perform multiple functions, all of which are vital to the health and safety of the vertebrate eye.

“The transition to life on land required many anatomical changes, including changes for feeding, locomotion and breathing air,” said Stewart. “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 conducted by a multidisciplinary team that included researchers from the Georgia Institute of Technology, Westmead Hospital in Sydney, Australia, and the University of Chicago. Funding for this research was provided by the U.S. National Science Foundation, the Open Philosophy Project, the U.S. National Institutes of Health, the Biological Sciences Division of The University of Chicago, and the Brinson Foundation.

More about animal evolution from water to land

The transition from aquatic to terrestrial life is a significant event in the evolutionary history of animals. This process, which took place around 375-360 million years ago during the late Devonian period, involved a group of fish called sarcopterygians (lobe-finned fish) evolving into tetrapods. These are four-limbed vertebrates that include mammals, birds, reptiles, and amphibians.

Several factors and adaptations contributed to this transition:

Lobe-finned fish

Unlike the ray-finned fish, which are the ancestors of the majority of modern fish species, lobe-finned fish had muscular fins with a robust bone structure. This unique fin anatomy provided the foundation for the development of limbs with digits, which allowed early tetrapods to move and support their body weight on land.

Lungs

Some early fish had simple lungs in addition to gills, which enabled them to extract oxygen from the air. This adaptation was crucial for survival in oxygen-poor or stagnant waters and later became essential for life on land.

Skeletal and muscular adaptations

Early tetrapods underwent several skeletal and muscular adaptations to facilitate walking on land. These changes included the development of a stronger spine and ribcage to support their body weight, as well as modifications to the limb bones and joints to allow for efficient movement on land.

Sensory adaptations

As life on land presented new sensory challenges, early tetrapods evolved several adaptations to navigate their new environment. For example, they developed a more advanced hearing system, with modifications to the skull and jawbones, to detect sounds in the air. Additionally, their eyes became positioned higher on the head, allowing for a broader field of vision.

Reproduction

Another important aspect of the transition to land was the evolution of reproductive strategies that did not depend on water. Early amphibians, for instance, still laid their eggs in water, but as evolution progressed, amniotic eggs with a protective shell developed in reptiles. This allowed for reproduction on land without the risk of desiccation.

Fossil evidence, such as the discovery of Tiktaalik, a “fishapod” with features of both fish and tetrapods, has significantly contributed to our understanding of this evolutionary transition. While we continue to learn more about the exact sequence of events and the intermediate species involved, the transition from water to land remains a fascinating area of study in the history of life on Earth.

More about mudskippers

Mudskippers are unique amphibious fish belonging to the family Gobiidae, with around 32 known species. They are primarily found in the intertidal zones of tropical and subtropical regions, such as the shores of the Indo-Pacific, West Africa, and the Atlantic coast of South America. 

Mudskippers exhibit a range of fascinating adaptations and behaviors that allow them to thrive in their challenging habitats.

Some notable aspects of mudskippers

Locomotion

Mudskippers are known for their ability to “walk” on land using their pectoral fins. They can also perform a unique skipping or hopping movement, which gives them their name. These movements allow them to navigate the muddy, slippery terrain of their habitat with ease.

Respiration

Mudskippers are able to breathe both in water and on land. They have gills for underwater respiration, and when they are on land, they can extract oxygen through their moist skin and the lining of their mouth and throat. To prevent their gills from drying out, mudskippers retain water in specialized gill chambers.

Vision

Mudskippers have excellent vision due to their large, protruding eyes, which are positioned on the top of their heads. This elevated position allows them to maintain a wide field of view both above and below the water surface. As mentioned in the previous answer, mudskippers also have a unique blinking mechanism that helps keep their eyes moist, clean, and protected while they are on land.

Feeding

Mudskippers primarily feed on small invertebrates such as insects, crustaceans, and worms. They can also consume plant matter and algae. When hunting on land, mudskippers use their specialized mouthparts to create a vacuum, allowing them to quickly suck in their prey.

Burrowing and territorial behavior

Many mudskipper species dig burrows in the soft mud, which serve as shelter and nesting sites. They are known for their territorial behavior and can be aggressive in defending their burrows from rivals. Males often engage in elaborate displays, such as raising their dorsal fins and performing push-ups, to establish dominance and attract females.

Reproduction

Mudskippers reproduce in burrows, where they lay their eggs. They exhibit unique reproductive behaviors, such as the male aerating the eggs by gulping air and releasing it in the burrow, which helps maintain a suitable oxygen supply for the developing embryos.

Mudskippers are fascinating creatures that have evolved remarkable adaptations to thrive in their unique intertidal environments, and their study can provide valuable insights into the evolutionary history of vertebrates.

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