What Was the First Animal to Walk on Land?

If we turn back the evolutionary clock around 420 million years, nearly all life on Earth was confined to the water. There weren’t even any leafy plants or insects on land, let alone animals with backbones, legs and lungs. But over the next 100 million years, there was an explosion of new life that evolved to include the first land-dwelling “tetrapods,” four-legged, salamander-like creatures that were the earliest common ancestors of all reptiles, amphibians, birds and mammals—including humans.  

That momentous evolutionary leap from water to land is known as the “fish-to-tetrapod transition,” and while scientists have a big-picture understanding of how it happened, the details remain elusive.  

Paleontologists have unearthed remarkable fossils from the Devonian Period (420 to 359 million years ago) of creatures that are somewhere on the evolutionary spectrum between fish and four-legged animals. They’ve also found much later fossils from the Carboniferous Period (358 to 299 million years ago) of animals that were clearly land dwellers. But so far, no one has found the “missing link”—an animal that was definitively the first to live and walk exclusively on land.   

The process of how and when animals made the transition to land remains one of the great unsolved puzzles of evolution. But scientists have more tantalizing clues than ever. 

To understand how the fish-to-tetrapod transition happened, it’s important to shed some outdated assumptions, says Robert Gess, a paleontologist at the Albany Museum in South Africa.  

“People tend to think that this is a story about how fish climbed out on land,” Gess says, “but it’s quite likely that the first tetrapods evolved with no thoughts of going on land.” 

According to Gess and other paleontologists, the key evolutionary leaps that occurred in fish during the Devonian period—namely the development of fins that functioned like limbs, and lungs that could breathe air—were adaptations to survive in a watery environment. Specifically, shallow water.  

During the Devonian Period, shallow water offered some real advantages to fish that could thrive there, says Ted Daeschler, a vertebrate zoologist who specializes in the “fin-to-limb” transition. First, shallow water was the ideal place for smaller fish to hide from large predators.  

“It was a pretty serious ‘fish-eat-fish’ world out there in the Devonian, and shallow water offered some refuge,” Daeschler says. 

Then there was food! Without water birds like herons to compete with—birds wouldn’t exist for another 100 million years—fish that could navigate the shallow edges of Devonian lakes, rivers and estuaries could feast on schools of baby fish.  

Scientists believe the survival advantages of living and hunting in shallow water provided the evolutionary pressures that slowly—over tens of millions of years—selected for remarkable adaptations in Devonian fish. They developed flatter, longer bodies to stay submerged in shallow water. The fins on their backs disappeared while others migrated downward to function as proto limbs for scooting along the bottom (though they could still swim with their tails). And because shallow water is often low in oxygen, they adapted to breathe air through special paired openings behind their eyes, in addition to water through gills.  

In the early 2000s, Daeschler and a colleague, paleontologist Neil Shubin, found the fossilized partial skeleton of a 375-million-year-old fish in the Canadian Arctic, which was a tropical swamp back in the Devonian. The creature, named Tiktaalik, had unique anatomical features that placed it smack in the middle of the fish-to-tetrapod transition. It had the gills and scales of a fish, but also a flattened, triangular head and muscular front fins that clearly functioned like limbs.  

“Maybe it needed [limb-like fins] to get into very shallow water and propel itself along using these appendages,” Daeschler says. “Maybe it’s a ‘sit and wait’ kind of predator that lays on the bottom and then shoots up after prey. Maybe it walked over sandbars and into ponds where there were no other big predators to compete with. [The fins were] definitely doing something more than what we usually think of as a fish fin doing.” 

Tiktaalik is sometimes called a “fishapod,” a catchy name for several Devonian fish species that featured hybrid body types somewhere on the fish-tetrapod spectrum. Elpistostege is another 375-million-year-old fishapod whose fins not only contain arm and wrist bones like Tiktaalik, but also primitive digits (finger bones) still embedded in the fins. Elpistostege might have used its muscular forefins to push its head out of the water so it could breathe through a pair of holes on the back of its skull called “spiracles.” 

The question is, did fishapods like Tiktaalik and Elpistostege live on land? Daeschler thinks that Tiktaalik could probably “squirm over a sandbar” like a mudskipper but that it was still very much tied to the water for food, reproduction and respiration. Elpistostege, with its more developed fins and breathing holes, might have spent more time at the water’s edge and beyond.  

“By this point, the land has become invaded by plants, and those plants have creepy crawlies in the undergrowth,” Gess says. “So theoretically, once your eyes are capable of seeing out of the water, maybe you start noticing that there’s food out there, too.” 

Millions of years after the emergence of fishapods like Tiktaalik and Elpistostege, a wave of even more land-friendly animals arrived. Paleontologists digging in Greenland found complete skeletons of two of these early four-limbed fellows, called Acanthostega and Ichthyostega.  

While Acanthostega was small (2 feet long) with underdeveloped hind limbs, Ichthyostega was a full 5 feet long with four well-developed limbs, each ending in seven discernible digits. Both animals breathed air in addition to water and lived in the shallows. Living 365 million years ago, their world was populated with larger and more varied plants that dropped their dead leaves at the water’s edge. Interestingly, the smaller Acanthostega was equipped with two rows of sharp teeth seemingly designed for eating above the water, not below.  

But does that mean these early four-limbed animals lived on land? Not yet, says Daeschler.  

“Acanthostega was clearly aquatic for most of its life, and how much and exactly what it could do with its appendages are still open questions,” he says. “For tens of millions of years, all of these animals were still tied to the water. They’re essentially ‘amphibian-like’ aquatic animals, although true amphibians didn’t exist yet.”  

One of the main things that kept these early tetrapods in the water was reproduction. All these creatures laid soft, unprotected eggs that could only survive in the water. It wasn’t until the late Carboniferous Period (around 330 million years ago) that the first hard-shelled eggs were definitely laid on land. That signaled the arrival of the “amniotes”—the first true tetrapods that could live most of their adult life cycle on land.  

By that measure, an amniote tetrapod like the Cacops is a candidate for being among the earliest known animals to live its entire adult life on land. Cacops literally means “ugly look,” and this small (16 inches long), strange-looking tetrapod lived in the early Permian period, around 80 million years after Acanthostega and Ichthyostega. That’s an incredibly long period of time during which countless species emerged and disappeared, any one of which might have been the “first” true land animal long before the Cacops.  

Paleontologists have also found two sets of footprints (fin prints?) in Poland and Ireland dating back 390 million years ago, making them millions of years older than the earliest fishapod or tetrapod fossils. So there’s a good chance that there was some kind of fish walking the Earth long before the Carboniferous period—we just haven’t found it yet. 

“Even if we had every organism that ever lived, if they were somehow preserved and fossilized—and we’d be drowning in skeletons and fossils if we did—we weren’t there at the time,” Daeschler says. “The best we can do is hypothesize about how these skeletons functioned and just how free of water [the creatures] might be.” 

What’s remarkable is that one of these undiscovered tetrapods that walked on land hundreds of millions of years ago was the ancestor of nearly every living creature on Earth, with the exception of fish and invertebrates.  

“Everything with backbones and legs descended from Devonian tetrapods,” Gess says. “They are the ancestors of all mammals, which includes whales and dolphins, funny creatures like that. They’re the ancestors of all reptiles, including snakes, legless skinks and things. Birds, which are a specialized group of reptiles, are descended from Devonian tetrapods, too, and all amphibians, of course.”  

As mammals, humans are also direct descendants of flat-headed, shallow-water fish that unintentionally developed legs and lungs that allowed them to take tentative steps into a new world.