How fish evolved to walk

When you think of human evolution, there’s a good chance you imagine chimpanzees exploring ancient forests or early humans huddling woolly mammoths against cave walls. But we humans, along with bears, lizards, hummingbirds and Tyrannosaurus rex, are actually lobe-finned fish.

It may sound strange, but the evidence is in our genes, anatomy and fossils. We belong to a group of land-dwelling animals called sarcopterygians, but vast amounts of evolutionary change have obscured our appearance.

We think of fish as expert swimmers, but they have actually evolved the ability to “walk” at least five times. Some species pull themselves forward using well-developed front flippers, while others “walk” along the ocean floor.

Our sarcopterygian ancestor evolved lungs and other air-breathing mechanisms, bony limbs, and a stronger spine before venturing onto land. These adaptations were useful not only in aquatic environments, but allowed our ancestors to explore land—they were “pre-adaptations” for life on land.

The transition from water to land was one of the most important events in the evolution of vertebrates. It may have started as a way to escape predators, but the landscape our ancestors discovered was already rich with plants like mosses, horsetails and ferns, as well as arthropods (millipedes) that had colonized the land millions of years earlier.

We are not alone

Independent walking evolved several times in fish, making it an example of evolutionary convergence (similar features evolving independently, such as wings in bats and birds). However, the evolution of walking in fish is rare. There are more than 30,000 species of fish as we know them today (not in the evolutionary sense), of which only a handful can “walk”.

Sarcopterygians differ from other types of fish in several important ways. For example, our fins (limbs) have bony supports and muscle lobes that allow us to move on land.

This adaptation is thought to have been critical to the evolution of tetrapods (amphibians, mammals, reptiles and birds) during our transition from water to land in the Late Devonian period, about 375 million years ago. Many of the genes involved in limb and toe formation in tetrapods are also found in water-bound sarcopterygians such as lungfish, indicating that these traits evolved in our ancient common ancestor.

We don’t know what species this ancestor was, but it probably resembled the coelacanth, which has a rich fossil record and is a “living fossil” that now inhabits the Western Indian Ocean and Indonesia.

Walking sarcopterygian fish are either extinct, like Tiktaalik, or are so evolved that we no longer recognize them as fish (quadrupeds).

An example of a living, walking fish is the mudfish (family Oxudercidae). These fish live in mangrove swamps and tidal flats and use their pectoral fins to walk on land. These fins help them escape from aquatic predators, forage (they consume organic matter in the mud), and even interact with each other on land to find mates.

Another example is the walking catfish (Clarias batrachus), which uses its pectoral fins to travel on land, helping it escape drying lakes and find new habitats.

How did the genes involved in walking first evolve?

The little skate (Leucoraja erinacea) is a cartilaginous fish related to rays and sharks (as opposed to bony fish, including sarcopterygians). It is another fish that “walks” underwater on fins like legs, mimicking the movements of land animals.

The little skate is of great interest to scientists investigating the evolution of locomotion because it evolved flipper-based walking independently of sarcoptera. However, until now, the genetics behind little skate walking have been difficult to study due to a lack of quality data.

That recently changed when researchers from Seoul and New York used cutting-edge technology to construct a high-quality assembly of the little skate’s genome. Scientists have discovered that it uses only ten muscles to walk on its wings, while quadrupeds normally use 50 muscles to move their limbs.

A big question about vertebrate evolution is: which genes are important for the development of the muscles that enable walking? To find out, the team looked at which genes were active in the nerves that control limb muscles (motor nerves) in a mouse, a chicken and a little skate.

They discovered similar patterns of gene expression in the motor nerves that help these muscles work. So walking fish may have followed many different evolutionary paths, but this recent study suggests a common genetic mechanism.

Humans evolved to be the best walkers

By the end of the Triassic period about 201 million years ago, both dinosaurs and mammals had developed exceptional running abilities. Humans have refined these locomotor powers, evolving numerous adaptations that make us one of the most efficient and capable running species on the planet.

These adaptations include a spring-like Achilles tendon that helps store energy, a long stride and balanced center of gravity, and sweating to cool down. These adaptations allow us to run long distances with great endurance, albeit at slow speeds.

Our ancestors used running to hunt, to escape from predators and to search for food. It has shaped our anatomy, physiology and culture. And many studies show that walking and running are vital to our well-being and physical health.

It’s been a long way from the beginnings of walking to our fish-like ancestors who first colonized the earth. But walking and running remain a central part of our lives and our evolutionary success.

This article is republished from The Conversation under a Creative Commons license. Read the original article.