How Life Made The Extraordinary Transition From Water To Land
The remarkable evolutionary journey of life on our planet, from its early origins to the moment our ancestors began walking on earth.
The Origin of Life
Nearly 4 billion years ago, life was forged on planet earth through the forces of chemistry and natural selection. Against all odds, on a large rock floating in space, the very first single-celled organisms began to emerge in our small pocket of the cosmos. Over the course of a few billion years, life would slowly begin to take shape. And one day, it would evolve and diversify into the many forms of life present on earth today.
The process that allows life to begin is an ongoing mystery. One that scientists are very eager to resolve. But we do know that life began during this time. And we know that it began in water.
Water is the most critical ingredient for the development of life on our planet. Without it, chemical reactions would not have the liquid solution needed to produce the organic building blocks of life. Fortunately, our planet held plenty of water, supplied by vast oceans, largely seeded by asteroids during our solar system's early formation.
These primordial oceans provided the perfect setup for organic chemistry to take place. Chemicals could easily move around and interact with one another, sunlight was readily available near the surface, food was in ample supply, and organisms would not have needed to support their weight against the forces of gravity. The stage was set. But life was not guaranteed.
For life to succeed, there were many obstacles it needed to overcome. First, dead chemistry needed to come together to form complex molecules like lipids for cell membranes, carbohydrates for various sugars, and amino acids for protein synthesis. But most importantly, it needed a way to manufacture self-replicating molecules that could change and evolve over time: it needed to produce DNA.
How the forces of chemistry and physics pulled this off has yet to be discovered. However, once it did, single-celled organisms began to flourish, forming mats of microbes that covered the primordial oceans. But for these microbes to grow and diversify into the many forms of life we see today, they needed to become considerably more complex.
For starters, they needed to build extremely intricate structures. And to make that happen, they needed to make far more efficient use of the energy from their surroundings. This would require an extraordinary feat in biological evolution. One that would shape the trajectory of life on planet earth. And by roughly 2 billion years ago, it happened.
A single cell floating around in the primordial sea absorbed another cell, establishing an ancient bond that exists in every single plant, animal, and fungi alive today. That cell that was absorbed would become known as the powerhouse of the cell (the mitochondria), and it would produce the vast amounts of energy needed for its host (the eukaryote) to build complex structures like organelles, cytoskeletons, and nuclei for its DNA.
In this remarkable moment of earth’s history, a new lineage of complex life was formed, paving the way for biological evolution to take the next major step in emerging complexity.
That step meant finding a way for small single-celled organisms to work closely together to form the first multicellular life on earth. How this happened is also not entirely well understood. But once it did, there was no going back. And by roughly 540 million years ago, multicellular life began rapidly diversifying into a wide variety of complex life forms, forming nearly every animal phyla present on earth today.
The Cambrian Explosion
This moment of rapid evolutionary diversification is known as the Cambrian Explosion, representing a profound transformation in earth’s history. Before this moment, the majority of life was relatively small, unicellular, and simple. But afterward, a large variety of organisms with animal body plans covered the planet, filling our oceans with animals of many shapes and sizes.
The earth’s aquatic ecosystems were far more vibrant than they ever were before. Vast shallow seas surrounded the earth’s continents, and marine life thrived in these rich environments, slowly filling every niche available to them. Over time, these aquatic creatures would continue to expand and diversify, and some would eventually evolve into the first true fish, who would one day dominate our oceans.
On land, there didn’t appear to be much going on. The earth’s dry surfaces were seemingly void of life, while the oceans were teeming with complex multicellular organisms.
But life was not completely absent on land. Microbes were living there, potentially forming the earliest terrestrial ecosystems as far back as 2.7 billion years ago. There were also lichens, mosses, and very simple photosynthetic plants on land. There may have even been primitive forms of fungi living there as well.
Comparatively, the earth seemed barren. But in reality, it contained microbial ecosystems that covered the planet. Over time, these microbes generated large supplies of carbon-rich soil and promoted a significant rise of oxygen in the earth’s atmosphere. And eventually, the stage was set for a new wave of terrestrial evolution.
By 500 million years ago, plants were well on their way to colonizing the planet. And shortly thereafter, invertebrate arthropods, including insects, millipedes, scorpions, and spiders, were on the move, gradually making their way on land by roughly 420 million years ago.
Finally, by 375 million years ago, land was looking pretty good. It had vascular shrub-like plants and arthropods, many of which were fully adapted to terrestrial life on earth. Trees were abundant, forming extensive forests and canopies that covered the continents. And then something extraordinary happened. The very first vertebrates began dragging themselves on land.
But adapting to life on land was not an easy feat for our vertebrate ancestors. They needed limbs to carry themselves against gravity, gas exchange systems to breathe oxygen from the earth’s atmosphere, adaptations to avoid drying out, and reproductive systems that did not depend on water for fertilization.
This was a very tall order. To make the jump from swimming around in the ocean to walking around on land appeared to be virtually impossible. So how did they pull it off? How did bony fish living in the ocean make the transition to living life on land? And what did this fish-like ancestor look like?
To answer those questions, we have to look backward in time at the organisms that preceded this transition. There were several prerequisites that our ancestors needed to acquire before they could make that jump. To start, they needed to acquire lungs.
Acquiring Lungs
From what we see in the fossil record, lungs did not actually arise when fish evolved to walk on land. They arose much earlier during the early Devonian when vertebrate fish began dominating the sea. During this time, it was very common for fish to have both lungs and gills. They would use their gills when they were in the water and their lungs when they needed oxygen from the air. But why would fish have needed two sources of oxygen?
The short answer is that oxygen concentrations in the water varied during the Devonian. Sometimes it was very high. Other times it was much lower. These oxygen level variations meant that when the oxygen content in water was high, fish could use their gills. When it was low, they could rely on their lungs as an accessory organ to gulp air from the surface.
The coexistence of lungs and gills is also seen in many fish alive today. For example, fish like the coelacanth and the lungfish have true lungs like ours that are lobed, possess alveoli, and have an intricate microstructure. And the genes that build them are very similar to the genes that build our own lungs. But what is most fascinating is that they have gills as well that are entirely separate organs.
Breathing oxygen from the air was an essential adaptation for our earliest ancestors to live on land one day. But it’s important to understand that they didn’t acquire lungs in some grand preparation for this shift to terrestrial life. Lungs arose independently to help fish breathe oxygen in these early oxygen depleted environments. They were only later repurposed for terrestrial life when vertebrates began moving to land.
From Fins To Limbs
And this brings us to the next part of our story. In one moment, vertebrate bony fish covered with fins were dominating the oceans and lived exclusively in the water. Then within a relatively short period of time, four-legged tetrapods were walking around the earth with two arms and two legs. How did this happen?
To begin, when the first true fish appeared in our oceans shortly after the Cambrian Explosion, they immediately started developing appendages to help navigate through the water. The first of these appendages were the dorsal fins, unpaired single fins that sat on top of the fish like a sail. These fins were primarily used to help stabilize marine animals against rolling and assist when making sudden turns.
The dorsal fins were very effective at helping fish move in the water. So effective that the genetic machinery used to build them were later co-opted to start building paired pectoral and pelvic fins, which gave fish even greater control over their environment. They were also co-opted to build other unpaired fins, like the tail fin and the adipose fin.
When looking at the fields of molecular biology and genetics, we find that many of the genes that build the dorsal fin in ray-finned fish are also building the paired fins. Essentially, this means that instead of evolving sets of paired fins from scratch, the genetic code used to make dorsal fins were just copied and pasted to produce them.
This is a very common process in evolution. When a gene or set of genes arise to make an organ, like the dorsal fin, they are often repurposed to make new structures, like paired fins. Essentially, once natural selection produces a recipe for building something successfully, that recipe can then be redeployed elsewhere as needed—no need to reinvent the wheel.
Eventually, some bony fish began spending more of their time in shallow freshwater ecosystems full of roots and lush vegetation. Paired fins were very suitable in helping them navigate these environments. But with water levels being so low, they needed a way to move more easily through these shallow habitats. And over time, the paired fins were gradually modified to assist with movement along the seafloor.
By the mid-Devonian, these shallow-dwelling fish had successfully adapted to their new surroundings, developing robust and fleshy lobelike fins that made them well-adapted at moving through shallow rivers and streams. Their lobed fins gave them a much-needed advantage over their ray-finned relatives. And they were so successful that lobe-finned fish radiated into an extremely diverse group of bony fish within these freshwater ecosystems.
Notably, while the paired fins adapted to help aquatic creatures navigate the shallows, the dorsal fin became virtually obsolete. Fluid modeling seems to suggest that having a dorsal fin makes it extremely difficult to move side to side in shallow freshwater environments, so the dorsal fin probably got in the way. It’s no surprise that the dorsal fin was cut from the gene pool and lost entirely within our evolutionary history. And we can see that clearly today, as no tetrapod on earth has a dorsal fin.
Eventually, a group of these lobe-finned fish began dragging themselves on land. Their fins had articulations that closely resembled those of modern tetrapod limbs, and they were well-suited for moving through the mudflats. They spent some of their time in the water and some of their time on land. And over time, they gradually expanded their territory and starting filling the many niches land had to offer.
Finally, by the Late Devonian, tetrapods had successfully transitioned to terrestrial life and were walking around on four limbs. In a relatively short period of time (on a geological time scale), vertebrates went from living their lives exclusively in the water to living their lives on land. And while most vertebrates would continue to dominate the oceans, those that walked the earth would eventually diversify and evolve into every amphibian, reptile, bird, and mammal alive today.
Why Move To Land?
The transition towards life outside the boundaries of aquatic ecosystems is truly a remarkable feat in biological evolution. Life is extremely dependent on water, and it can be hard to imagine why any marine animal living on earth would have favored living in a foreign terrestrial environment when they were so well-adapted to life in the water.
There are a few reasons why early lobe-finned fish may have been driven out of the water. Let’s remember that the lobe-finned ancestor of the tetrapods initially left the ocean to fill the niches of shallow freshwater ecosystems. They radiated through ancient rivers and streams and became well adapted to these environments. But they were not alone in these shallow habitats.
Rivers and streams were full of predators. Some of them were up to 15 feet in length with giant teeth. It was a real predator-rich world, and surviving would have been a very challenging endeavor. In a predator-intense environment like this, there really isn’t much you can do other than adapt and allow natural selection to do its work.
There are only a few strategies in evolution to adapt to a fish eat fish world successfully. One option is to get much bigger to avoid the threat of being eaten by another fish. Another option is to develop lots of armor for protection against large predators. This is the strategy that the Placoderms used out in the ocean, and it was so successful that they became the most diverse and abundant vertebrate fish during the Devonian Period.
The other option is developing strategies to avoid predators entirely. If you can successfully remove yourself from harm’s way, there’s no need to develop tons of armor. And this was the subsistent strategy of choice for our early ancestors. In the water, there was a risk of being eaten every day. But on land, there were no predators to worry about. There were no competitors. And there were lots of food sources and unfilled niches to spread to.
With lungs and fleshy lobed fins that evolved for life in the shallows, our ancestors had all of the tools needed to drag themselves to shore. Early on, they likely spent most of their time in the water while taking trips onto land to escape predators and seek out food sources. And over time, they would continue to adapt and fill new terrestrial niches provided by the vascular plants and arthropods that colonized the land before them.
It is within the nature of life on planet earth to reach out and fill every niche available to it. And since planets have a limited carrying capacity, if a species wants to thrive and prosper, it will need to look for more places to spread to. Moving to land might seem like an odd thing for a fish to do, but if it has the means to do so, then that is exactly what we should expect to happen. And we should be thankful that it did; otherwise, we wouldn’t be here today.
The Missing Link
How life made the transition from water to land was once a great mystery. In fact, prior to 2004, we had no idea what that intermediate fish-like animal looked like. We knew from the fossil record that there were many limbed animals on land roughly 365 million years ago. And we knew that there were lots of fish that were beginning to look like limbed animals around 385 million years ago. But we had no idea what existed in between.
Through deductive reasoning, evolutionary biologists had some pretty good guesses, though, as to what this missing link might have looked like. Based on the fossil record, they assumed a few things. It was a lobe-finned fish. It had a fleshy pair of fins with bones inside. And it had both lungs and gills.
This was a great start. It was backed by the fossil record. And there were many prime examples like the coelacanth and the lungfish swimming around that shared these features. All that was missing was concrete evidence in the fossil record that such a creature existed. Fortunately, one man was up for the job.
Neil Shubin, a paleontologist, evolutionary biologist, and professor at the University of Chicago, decided that he would not rest until he found this great missing link. But doing so would require a great deal of grit and dedication, for finding a fossil like this would not be an easy task.
To find a fossil, paleontologists need to look for a few things. First, they need to look for places on earth with rocks that are just the right age. In this case, the sweet spot would be rocks from the Late Devonian, roughly 375 million years ago. That puts us right before tetrapods began walking on land, and shortly after lobe-finned fish were swimming around in the shallows.
To determine the age of rocks on earth, scientists have a very powerful tool at their disposal: the predictive decay of radioactive isotopes. In short, a radioactive isotope is essentially a very unstable version of an atom that carries more neutrons in its core than it should. Over time, these isotopes slowly decay into more stable forms, and they do so at a very predictable rate.
Fortunately, radioactive isotopes are embedded inside many of the rocks within the earth’s crust, giving paleontologists the perfect tool to determine their age, and subsequently, the age of any fossil that exists within them.
Once a list of potential candidates has been selected dating back to 375 million years ago, the next step is finding rocks that are likely to hold the fossils we are looking for. In other words, we need to select rocks formed in the early environments that the organism likely lived in during this time.
In this case, Neil Shubin directed his attention to places on earth that once had ancient rivers and streams, specifically looking for rocks that formed in river delta systems. This was the perfect place to look because we knew from the fossil record that many lobe-finned fish were swimming around in these environments during this time.
The only thing left to do was find rocks that were easily accessible for Neil and his team to investigate. If the rocks were too deep, it would be virtually impossible to find what they were looking for. Ideally, the rocks of interest need to be near the surface, where they could easily be studied. And as it turns out, the best place to look was in the Canadian Arctic.
The Canadian Arctic had everything that Neil’s team was looking for. It had rocks that dated back to roughly 375 million years ago. And those rocks were really well exposed at the earth’s surface, so they didn’t have to dig very deep to find a fossil. Most importantly, these rocks were once part of large tropical rainforests with ancient river deltas, exactly where our missing link would likely have been at the time.
It might seem odd to think that tropical rainforests and the life that once lived there would be embedded in rocks from the Canadian Arctic, a place today that is very cold and full of glaciers. But these rocks were in a very different place 375 million years ago. Back then, they were very close to the equator and were full of lush tropical ecosystems. They are only where they are today due to the continental movement of the earth’s crust, slowly moving north over hundreds of millions of years.
To know where to look, Neil’s team used satellite aerial photos to search for good locations. They started in the Western part of the Arctic in 1999 but didn’t find anything. Then they moved further east to Ellesmere Island, where ancient rivers and streams were abundant, geologically speaking. Here, they started finding all kinds of fossilized lobe-finned fishes, but only bits and pieces, not whole skeletons. They were on the right track.
After finding lots of teeth, partial jaws, bits of skull, and other incomplete fossils, they decided to move again to a location where the rivers and streams were more gentle, where fossils would not have been torn apart over long stretches of time. This brought them to a nice valley that held tons of fossils. And that’s when they found it: a flat head sticking out of the rocks, attached to our long lost, missing link.
After 6 long years of searching, Neil Shubin and his team found the ultimate missing link that marked the evolutionary transition from fish to tetrapod. The fossil they found was an entire skeleton that stretched 4 feet in length and remarkably possessed all of the features they were looking for.
It was a fish with a flat head shaped like the head of an early limbed animal. It had a fish-like jaw and bones along with shoulder and elbow joints. It had an arm, a wrist, fingers, and toes. The wrist enabled the fin to reach the ground with a palm-like area to help it move across the earth's surface. The arm was also inside of a fin, which allowed it to swim in the water. And it had a neck, which no other fish has. And of course, it had both lungs and gills, so it could live in water and walk on land.
The discovery was almost too good to be true. With all of the information available from the fossil record, Neil was able to accurately predict what this intermediate species probably looked like and where, in both space and time, its fossilized remains would be hidden within the earth’s crust. To otherwise find a fossil like this would be virtually impossible, illustrating the incredibly predictive power of both science and evolution.
The only thing left to do was give this beautiful specimen a deserving name. For that, Neil decided to consult with the locals that lived near its discovery site on Ellesmere Island. And they decided to give it the name Tiktaalik, which means “large, freshwater fish” in the Nunavut people's language.
A Common Ancestor
Evolution is truly one of the most awe-inspiring and beautiful spectacles in science. The connections we share with the rest of life on this planet tell a story about our past that enables us to understand our relationship with every living thing that has ever lived on planet earth.
Through the predictive power of science, we have the remarkable ability to comprehend our story dating back to the birth of our solar system. And that story tells us that every living thing on earth shares a common ancestor. It tells us that we are all here today because of stars that seeded the universe with atoms that gave life to the cosmos. It tells us that life began under extraordinary conditions in the early oceans of our planet. And it tells us that all life is related and evolved here through natural selection.
The discovery of Tiktaalik and the many discoveries that preceded it gave us an exciting look into what that moment looked like when our ancestors first began to walk on land. But there is still a lot we do not know. Life has been here for nearly 4 billion years, and with every new discovery, we get a little bit closer to understanding how we all came to be on a rock floating in space.
