Why Did Fish Evolve to Walk On Land?
First steps of tetrapod evolution (I mean, literally)
∘ Beyond the meme
∘ The origin of four-legged animals
∘ First steps out of the water
∘ Conquering the land
Beyond the meme
I believe that the transition of fish from water to land is one of the best-known and accepted examples of evolution by society.
Many of us have been exposed, throughout our lives, to many popular references to the first steps of tetrapods on the land.
If you put the right words in a google image search, you will find many funny memes that caricature this event (I invite you to use the words: “Tiktaalik fish meme”).
Even The Simpsons have an episode (Treehouse of Horror V) that parodies this event, where Homer accidentally kills a fish that is leaving the water and dramatically changes the future [check out this video].
All of these popular references point to the fact that society would not be the same, for better or worse, if these steps had not been taken.
But in reality, this water-land transition is even more important than that, since life on the land would be completely different from what we know if it had never occurred.
The origin of four-legged animals
The term “Tetrapod” (superclass Tetrapoda) refers to any vertebrate animal with four limbs. They are easily recognizable because they are present in practically all media.
The group encompasses all species diversity of amphibians, reptiles, mammals, and birds. It not only includes humans but also extinct animals like dinosaurs.
Tetrapods are mostly terrestrial animals that had already fully colonized the land by the Carboniferous period (between 358 and 298 million years ago [MA]).
Despite being probably the most famous and striking biological group, they do not present as much diversity as other groups of vertebrates. In fact, there are more species of fish than all the species of tetrapods combined [1, 2].
Indeed, from an evolutionary perspective, tetrapods arose from a small branch of fish that adapted to living on land.
First steps out of the water
Tetrapods evolved from a group of bony fishes (Osteichthyes), specifically from the lobe-finned fishes (Sarcopterygii).
Trace fossils suggest that the group might have originated around 390 MA in the middle Devonian (a period often known as the “Age of Fishes”) [3, 4].
Clear clues of this evolutionary step can be seen in extinct sarcopterygians. For example, the pectoral and pelvic fins of some Sarcopterygii have articulations resembling those of tetrapod limbs [check out this image].
Early tetrapods had legs and lungs as well as gills and looked like transitional forms between fish-like animals and large salamanders.
There are several families of Devonian fishes that have been proposed as the nearest ancestral relatives of tetrapods (see Tetrapodomorph), however, the elpistostegids (order Elpistostegalia) are the most notable.
The most representative members of elpistostegids are from the Tiktaalik genus; aquatic carnivorous animals that had the ability to move on land in the manner of a seal [5].
Elpistostegids were relatively large shallow-water predatory fishes that could exploit shallow, low-oxygen tidal areas [5].
They developed adaptations that allowed them to survive the environmental instability of shallow water bodies, similar to some modern living groups of lungfish (order Dipnoi) and some catfish genera.
These changes in the manner of locomotion allowed them to move from one body of water to another and a shift towards reliance on lungs rather than gills as the primary means of obtaining oxygen.
Conquering the land
Despite having transitional characteristics between terrestrial and aquatic animals, early tetrapods still had an aquatic lifestyle. It wasn’t until the early Carboniferous, that these amphibian-like tetrapods could fully colonize the land [5, 6, 7, 8].
It is not entirely clear how and why the first terrestrial tetrapods were so successful.
The most accepted hypothesis suggests that it could be related to the environmental changes that occurred during the late Devonian, which in turn also triggered one of the five largest mass extinctions in the history of life on Earth.
The planet cooled significantly and there were changes in sea level and productivity, as well as an increase in ocean anoxia.
What triggered these planetary changes is still open to debate. Various factors such as the impact of an asteroid, a reduction of atmospheric CO2 by photosynthesis, and an increase in volcanic activity have been proposed as the main causes.
These abrupt environmental changes, along with others such as aquatic predators and competitors, waterborne diseases, and parasites, could have facilitated the survival of tetrapod ancestors that were able to inhabit terrestrial habitats.
The radiation of early tetrapods (such as Ichthyostega, Acanthostega, Ventustega, Sinostega, Hynerpeton, Densignathus) took place immediately after the Late Devonian extinction (more specifically after the Kellwasser event).
Tetrapods subsequently colonized land and diversified into a wide variety of morphologies and sizes during the Carboniferous (an exponential increase from a few species to many thousands), which in turn gave rise to what is known as the “Age of amphibians” [9].
The transitional shift between aquatic and terrestrial lifestyles was one of the major events that marked a before and after in the evolution of life on earth since it involved a complete reorganization of the body plan of early vertebrates [5, 6].
It involved a wide range of changes in both anatomy and function, leading to new and unprecedented forms of locomotion, respiration, osmoregulation, sensory systems, behavior, and reproduction.
Studying the fossil record not only allows us to make useful inferences about how these changes might have occurred but also gives us clues about the evolutionary relationships among these organisms.
Tiktaalik is frequently used as a popular reference for this water-land transition because it was originally considered by the press to be a “missing link” between past and modern vertebrate groups [10].
Nevertheless, it is important to clarify that Tiktaalik has never been claimed to be the direct ancestor of tetrapods, but rather a close relative that evolved in parallel [5].
In any case, Tiktaalik has great relevance since it helps us to identify evolutionary trends and approximate the true hypothetical tetrapod ancestor, which would have been very similar to it.
You can find more information about the evolution of early tetrapods at the following links: [1, 3, 4, 5, 6, 7, 8, 9, 10].
To learn more about Tiktaalik, check out this music video (credits to Neil Shubin, music by the Indoorfins:, more information in the YouTube description):
Let me know your opinion in the comments.
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