How a 4-legged fish 375 million years ago made us what we are today.
The incredible Tiktaalik.
In 2004, scientists in Nunavut discovered one of Forbes magazine’s “5 undersung signposts of Earth’s evolution.”
Specifically, they found fossils of a seemingly 4-legged fish — like something straight out of a cartoon. They called it the Tiktaalik roseae, Inuit for “large freshwater fish.”
Humans, and all land-dwelling vertebrates, evolved from fish. However, the transition between water and land was a bit of a blur. We had evidence of similar creatures existing at the time, but nothing truly in between water and land — there was a 15 million year gap and no middle-man to fill it.
Morphologically, the Tiktaalik is a massive transition point. Its placement in the timeline and unique physical features let us know a lot more about how tetrapods — amphibians, reptiles, and mammals — came to be. For this reason, the Tiktaalik was deemed to be a fishapod, a seemingly half-fish, half-tetrapod creature.
While this is by no means a new discovery, it’s so influential that articles and research on the Tiktaalik are still being done to this day. The wealth of knowledge about evolution and more specifically, human evolution, that have arisen and can arise from this initial discovery is astonishing.
Its Fascinating Hybrid Features
On the surface, its scales, gills, and fins make the Tiktaalik look like a mutant fish, but its fossils actually show a lot of connections to the beginnings of terrestrial life.
1. Skull
Similar to fish, its skull has some moveable joints to be able to breathe and feed underwater, but like tetrapods, it also has some rigidity and stiffness to it. These hybrid characteristics mean the Tiktaalik had evolved to rely less on pumping water across its gills to get oxygen.
As well, the flat shape of its skull and the placement of eyes on top of its head closely resemble crocodiles. This means they most likely lived in shallow waters, as having eyes on top of your head allows you to remain hidden below the water surface while still being able to see above.
This diagram indicates how the Tiktaalik’s skull and eyes follow a clear transition between its evolutionary siblings. These are the same 5 species shown in the timeline above. This shows that the fossils themselves align right what we already knew, helping put these findings into perspective.
2. Spiracle
The most important clue about its breathing, however, comes in the location of its spiracular tract. This is the hole you see in fish that allows them to breathe while eating. In fish that breathe air, like dolphins, you find the spiracle on top (blowholes!). In fish that only breathe water, spiracles are found towards the side of their face, like that hole you see next to sharks’ eyes (look it up!). In Tiktaalik, the spiracles are at an angle between the side and the top, suggesting the presence of both lungs and gills, which is awesome. This shows that fish were developing the ability to breathe air along with water. Over time, tetrapods lost their spiracular tracts and began sporting nostrils to allow them to breathe air while eating. Wild.
3. Mouth
Aquatic creatures use suction to ingest their food, like a vacuum. As you may know from experience, suction to lure prey doesn’t really work on land. Instead, tetrapods have to put food in our mouths and then chew to break it down — you know how it goes. Well, a recent study analyzing its skull shows that the Tiktaalik had abilities to do both. Much like a crocodile, it could chomp down on its prey, while also having the ability to suck them in when in water. This was no doubt a huge evolutionary advantage.
4. Limbs
Being coined a “4-legged fish”, how could we not discuss its fins? These powerful leg-like fins had to be able to support its weight on land and push off the ocean floor when in shallow waters.
To accomplish this, the Tiktaalik’s fins had many bones that actually directly correspond to human upper limb bones. There is a functional ‘wrist’ joint that helped it to prop up and move about, and as can be seen, there are very early remnants of arm and forearm bones that exist within us today. It also had thin ray-bones for padding, which was characteristic of fish at the time.
With this information, we know the fins of these aquatic creatures were already well developed. Structures like the radius, ulna, and digits are seen everywhere these days, from birds to dogs to us. All of these structures originated somewhere, so it’s with findings like these that we are able to gain a better understanding of when our features began to appear.
5. Pelvis & Ribs
Lastly, its large and robust pelvis allowed it to maintain stability while moving on land — a huge advantage for movement that is seen more in tetrapods than in fish. They also had ribs found for support and breathing on land. There’s actually a whole paper on the pelvic girdle if that’s what floats your boat. The main takeaway here again, though, is that it indicated a hugely transitionary state.
Did these features actually work?
Okay yeah bones woop woop, that’s great and all, but how did all this actually play out? In order for humanity to be where it is, the Tiktaalik’s half-fish half-tetrapod (“fishapod”) traits had to have been advantageous to living in its environment, right?
Well, there’s quite a few differences between living on land and living in water: respiration, reproduction, feeding, locomotion, and much more. To be able to have played a role in passing traits and genes forward, the Tiktaalik’s hybrid characteristics would’ve had to allow them to survive long enough in their environment to sufficiently reproduce.
1. Locomotion (Moving)
As discussed before, the Tiktaalik’s fins allowed it to swim in shallow waters and also push off the ocean floor, while supporting itself sufficiently on land. So movement in the two mediums to hunt or escape was covered.
2. Respiration
During the Devonian period where these creatures existed, atmospheric oxygen levels were much lower than the levels they’re at now. Ocean oxygen stores were even lower, as water holds much less oxygen than air. On top of this, terrestrial plants were dying and getting washed up into the ocean, using up even more of the ocean’s limited oxygen stores.
By inhabiting shallow waters and bearing both lungs and gills, the Tiktaalik was seemingly optimized to be able to get oxygen from either water or land, depending on the availability of resources. This was no doubt a huge advantage it had over many other creatures in the same period.
3. Feeding
As predators, inhabiting shallow waters allowed Tiktaalik to prey on the smaller fish hiding in shallows along with insect-like creatures found in the leaves along water banks. As prey, staying in shallow waters was safer as they avoided the massive predators found in deep waters and there weren’t as many threats coming from land. On top of this, having both suction and chewing capabilities allowed them to efficiently get energy from food without having to put themselves at high risk.
4. Reproduction
So they were able to move, breathe, and feed just fine. The last step in allowing life to propagate is being able to have sufficient offspring.
The Tiktaalik actually reproduced the way many amphibians do today — females laid a very large amount of eggs and left them for male insemination. Of the few that would hatch, even less would survive, but the immense number of eggs laid ensured enough were being born. This was sufficient in allowing their genes and traits to propagate and evolve over time.
Why this is so important.
Those inspired by life, evolution, and animals will already be captivated by how interesting the Tiktaalik was as a standalone species — its primitive structures and innovative characteristics are really fascinating.
As for its implications, I think Neil Shubin, major contributor to the original 2004 expedition, put it best:
“every innovation, every invention used by tetrapods on land, originally appeared in some form in fish, including lungs, appendages, and now, feeding.”
Discoveries like these help piece together the puzzle of how species evolved over time, and how we got to where we are today.
Information like this also allows us to ask more insightful questions and push scientific research and knowledge forward. In fact, in March 2020, fossils of one of the Tiktaalik’s Devonian siblings was discovered, further helping put our origin in perspective.
TL;DR
- The Tiktaalik roseae was a species that existed 375 million years ago, during a period in time where the vast majority of species were aquatic.
- This “4-legged fish” had several traits that were hybrids of aquatic and terrestrial species, representing the transition of life from water to land — the origin of amphibians, reptiles, and mammals.
- It also bore primitive versions of human structures like the skull, nostrils, shoulders, neck, and hands.
- This is important because it gave us a better understanding of not only how organisms began to shift from water to land, but when the major transitions happened, as well as where certain human structures are derived from.