How fish came to walk

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

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

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

Our sarcopterygian ancestors developed lungs and other air-breathing mechanisms, bony limbs, and a stronger spinal column before hitting the ground. These adaptations were useful not only in aquatic environments but also 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 significant events in vertebrate evolution. It may have started as a way to escape from predators, but the landscape our ancestors found was already rich with plants like moss, sedges and ferns, as well as arthropods (millipedes) that colonized the land in millions of years earlier.

We are not alone

Walking evolved independently several times in fish, making it an example of evolutionary convergence (similar traits that evolve independently, like the wings of bats and birds). However, walking in fish is rarely successful. There are more than 30,000 species of fish as we know them today (not in the evolutionary sense), and only a handful of them 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 key to the evolution of tetrapods (amphibians, mammals, reptiles and birds) during our transition from water to land in the late Devonian period, around 375 million years ago. Many of the genes involved in forming limbs and digits 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 looked like the coelacanth, which has a rich fossil record and is a “living fossil” living today in the western Indian Ocean and Indonesia.

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

One example of a living walking fish is the mud squib (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 for food (they eat organic matter in mud), and even interact with each other on land by finding other humans.

Another example is the walking fish (Clarias batrachus), which uses its pectoral fins to travel over land, helping it escape from drying up pools and find new habitats.

How did genes involved in walking first evolve?

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

The little skate is of interest to scientists researching the evolution of locomotion because it evolved from fin-based walking independently from the sarcopterygians. However, until now, it has been difficult to study the genetics behind small feet due to a lack of quality data.

That recently changed when researchers from Seoul and New York used cutting-edge technology to make a high-quality assembly of the little skate’s genome. ​​​​The scientists found that it uses only ten muscles for walking on fins, while tetrapods usually 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 the genes that were active in the nerves that control limb muscles (motor nerves) in a mouse, a chicken and a little skate.

They found similar gene expression patterns in motor neurons that help these muscles function. So walking fish may have taken several different evolutionary paths, but this recent study suggests a common genetic mechanism.

Humans emerged as the best walkers

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

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

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

It’s a long way from the origins of walking in our fish-like ancestors who first colonized land. But walking and running are an integral part of our lives, and our evolutionary success.

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