How fish evolved to walk and became humans

When one thinks of human evolution, one most likely envisions chimpanzees exploring ancient forests or early humans painting woolly mammoths on cave walls. But humans, along with bears, lizards, hummingbirds, and the tyrannosaurus rexwe are actually lobe-finned fish.

It may seem strange, but the proof is in our genes, our anatomy, and in the fossils. We belong to a group of animals called terrestrial sarcopterygians, but enormous evolutionary changes have darkened what we look like.

We think that fish are expert swimmers, but in reality they have developed the ability to “walk” at least five times. Some species pull themselves forward using well-developed front flippers, while others “walk” across the ocean floor.

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

The transition from water to land was one of the most significant 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 (centipedes) that had colonized the earth millions of years before.

We are not alone

Walking evolved in fish several times independently, making it an example of evolutionary convergence (similar traits evolving independently, such as wings in bats and in birds). However, the evolution of gait in fish is rare. There are more than 30,000 species of fish as we know it 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 flippers (limbs) have bony supports and muscle lobes that allow us to move on land.

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

The coelacanth fish still exists in tropical seas.
Catmando/Shutterstock

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 today inhabits the western Indian Ocean and Indonesia.

The walking sarcopterygian fish have become extinct, as tiktaalikor have evolved so much that we no longer recognize them as fish (tetrapods).

An example of a living fish that walks is the mudskipper (of the family Oxudercidae). These fish live in mangroves and salt marshes and use their pectoral fins to walk on land. These fins help them escape from aquatic predators, search for food (they consume organic matter in the mud) and even interact on land looking for a mate.

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

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The walking catfish is a species of air-breathing freshwater catfish.
bajaphotos/Shutterstock

How did gait-related genes first evolve?

The small line (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 with fins as legs, imitating the movements of land animals.

The little skate is of great interest to scientists investigating the evolution of locomotion because it evolved walking with fins independently of sarcopterygians. However, until now, the genetics underlying the little skate’s gait have been difficult to study due to a lack of quality data.

That changed recently, when researchers in Seoul and New York used state-of-the-art technology to build a high-quality assembly of the little skate’s genome. The scientists found that it only uses 10 muscles to walk with its flippers, while tetrapods typically use 50 muscles to move their limbs.

A big question about vertebrate evolution is: which genes are important for developing the muscles that allow us to walk? To find out, the team looked at which genes were active in the nerves that control the muscles of the limbs (motor nerves) in a mouse, a chicken, and a small ray.

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

The fish propels itself forward with its fins through the mud.
There are 32 living species of mudhoppers.
Polbkt/Shutterstock

Humans evolved to be the best walkers

By the end of the Triassic, about 201 million years ago, both dinosaurs and mammals had developed excellent running abilities. Humans honed these locomotor abilities, developing 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 low speeds.

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

It has been a long way from the origin of the gait in our fish-like ancestors who first colonized the land. But walking and running remain a fundamental part of our lives and our evolutionary success.

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