From Fish to Four Legs
The Emergence of Tetrapods and the Evolutionary Journey that Followed
The evolutionary history of life on Earth is an impressive tale of constant change and adaptation, with stops, and starts, and organisms diversifying into the multitude of species we observe today. Among the most significant transitions in this vast history is the emergence of Tetrapods, the four-legged vertebrates that came to inhabit both land and water, and would begin an evolutionary path eventually leading to ourselves.
In this post, we will explore the fascinating journey of the Tetrapods, from their aquatic origins as fish-like ancestors to their remarkable diversification into mammals, birds, reptiles, amphibians, and some aquatic species, like whales!
Contents of this post
- What are Tetrapods?
- Pre-Vertebrates
- The first Vertebrates
- The Evolution of Jaws
- The Evolution of Fins
- The Osteichthyes
- Fish in the Devonian
- Tiktaliik
- The transition to land
- Diversification of the Tetrapods
- Evolution of Amniotes
- Conclusion
What are Tetrapods?
Tetrapods are a diverse and complex group of vertebrates distinguished by the presence of four limbs, each featuring digits (fingers and toes), with this group encompassing terrestrial animals such as mammals, birds, reptiles, and amphibians, as well as some aquatic species like whales, which have evolved from land-dwelling ancestors.
The emergence of tetrapods can be traced back to around 390 million years ago during the Devonian period when they evolved from lobe-finned fish-like ancestors, with this critical transition marking a significant milestone in the evolution of life on Earth, allowing for the colonization of terrestrial habitats and subsequently, the diversification of various life forms. This shift from aquatic to terrestrial environments necessitated several adaptations, such as the development of robust skeletal structures and more efficient respiratory systems, which enabled tetrapods to thrive in their new surroundings, and while this did not occur overnight, the process took place over in a relatively short period of time.
The evolution of tetrapods is a testament to the process of natural selection, as various lineages adapted to a wide range of habitats and ecological niches, with one of the most noteworthy examples of this being the emergence of amniotes, a group of tetrapods that includes reptiles, birds, and mammals. These early animals developed the amniotic egg, which allowed for reproduction in dry environments, further expanding the range of habitats that the tetrapods could inhabit.
Modern Tetrapods
Modern tetrapods are the contemporary descendants of the ancient four-limbed vertebrates that first evolved during the Devonian period, and they encompass a wide range of species found across the globe, having adapted to various environments and ecological niches.
Modern tetrapods can be classified into four primary groups:
Mammals: This group includes a diverse array of warm-blooded, hair-covered vertebrates that typically give birth to live young and nurse them with milk. Mammals themselves are further divided into three subclasses: monotremes (egg-laying mammals, such as the platypus), marsupials (pouched mammals, such as kangaroos), and eutherians (placental mammals, which include humans and most other familiar mammal species).
Birds: Birds are warm-blooded, feathered vertebrates that lay hard-shelled eggs. They are characterized by a unique skeletal structure that includes hollow bones, enabling them to be lightweight and capable of powered flight. Birds are descendants of theropod dinosaurs and are considered the only living dinosaurs today
Reptiles: This group includes cold-blooded, scaly-skinned vertebrates such as snakes, lizards, turtles, and crocodiles. Reptiles are amniotes, meaning they lay eggs with a protective membrane or give birth to live young. They typically have lungs for respiration and are ectothermic, relying on external sources of heat to regulate their body temperature.
Amphibians: Amphibians are cold-blooded, moist-skinned vertebrates that generally have a biphasic life cycle, transitioning from an aquatic larval stage to a terrestrial adult stage. This group includes frogs, toads, salamanders, and caecilians, and the Amphibians typically rely on their moist skin for respiration and are closely tied to aquatic environments for reproduction.
Pre-Vertebrates
Prior to the appearance of the first vertebrates, there were simple marine animals called protochordates, which possessed some of the essential characteristics of the chordates, including a notochord, a dorsal hollow nerve cord, and gill slits. These animals were most likely filter feeders, similar to modern tunicates and lancelets, and the first vertebrates evolved from these protochordate ancestors through a series of very gradual changes.
One critical development in this gradual string of evolutionary changes was the strengthening of the notochord, which provided the necessary support for a more complex body plan, additionally, the evolution of a more intricate nervous system allowed for increased sensory perception and coordination.
The First Vertebrates
The first vertebrates, characterized by the presence of a backbone or spinal column, originated around 525 million years ago during the Cambrian period. This era, commonly referred to as the “Cambrian Explosion,” was a key moment in the history of life on Earth, witnessing an accelerated diversification of various major animal groups that began to appear in the fossil record.
Although only a relatively short evolutionary event, occurring over approximately 20–25 Million years, the Cambrian explosion marked a drastic increase in the complexity of life, with the emergence of most of the major animal phyla, including the first arthropods, mollusks, and echinoderms in the period.
This rapid diversification is thought to have been driven by several factors, including increased oxygen levels in the oceans, the development of more complex ecosystems, and the evolution of novel traits such as predation and burrowing.
Essentially, many of the organisms inhabiting the planet were undertaking an “arms race”, attempting to quickly overcome the barriers to evolution, in order to seize control of their environment and food source. This, coupled with seemingly perfect conditions, led to the explosion in diversity, and the earliest development of our modern ecosystems, and its inhabitants.
The first vertebrates to emerge during the Cambrian period were small, jawless fish called agnathans, which lacked a well-defined head and paired fins but had a primitive cartilaginous skeletal structure. The agnathans can be classified into two primary groups: ostracoderms and conodonts, both of which were exclusively marine and inhabited shallow coastal waters, where resources were abundant and environmental conditions relatively stable.
Over time, these early vertebrates evolved several key adaptations, including the development of jaws and the appearance of paired fins, allowing for more efficient predation and increased maneuverability in the water, respectively, and the evolution of these features played a crucial role in setting the stage for the eventual emergence of the tetrapods.
The Evolution of Jaws
The development of jaws was a major turning point in the history of vertebrates, enabling them to exploit a wider range of food sources and paving the way for the diversification of early fish and their descendants.
Jaws likely evolved from the anterior gill arches of early jawless fish (agnathans) through a series of modifications over time, with the first jawed fish, the gnathostomes, appeared in the Late Silurian period, around 420 million years ago. Gnathostomes had two sets of paired fins and a cartilaginous jaw structure that was more robust and versatile than the mouthparts of their jawless ancestors, giving them a maassive advantage in most respects.
Over time, the jaws of gnathostomes diversified, giving rise to a wide variety of jaw shapes and sizes adapted to different feeding strategies, for example, predators with sharp, pointed teeth evolved for seizing and holding onto prey, while herbivorous species developed flatter, grinding teeth for processing plant material.
The Evolution of Fins
The evolution of fins in early vertebrates was another significant event in their diversification, providing these aquatic creatures with improved stability, maneuverability, and propulsion in their watery environments.
In early vertebrates, there were two main types of fins: unpaired (median) fins, and paired fins. Unpaired fins include the dorsal, anal, and caudal (tail) fins, which provided stability and prevented the fish from rolling while swimming, whilst paired fins consist of the pectoral and pelvic fins, which contributed to maneuverability and propulsion. Over time, these early paired fins would evolve into the limbs of tetrapods, facilitating their transition from water to the terrestrial environment.
Fins originated from simple fin folds in early jawless fish, such as the ostracoderms, and were initially supported by flexible rods known as fin rays, made of dermal bone or cartilage. As jawed fish evolved, these fin structures became more complex, with the development of an endoskeletal support system composed of fin radials (cartilaginous or bony elements) connected to the pectoral or pelvic girdle.
The genetic and developmental processes underlying fin formation are regulated by a complex interplay of signaling pathways and gene networks, aand several key genes, including members of the Hox gene family and the T-box transcription factor family, play essential roles in fin development, determining the positioning, size, and shape of fins.
The Osteichthyes
Osteichthyes, or bony fish, are a diverse and abundant group of fish that first appeared around 480 million years ago during the Ordovician period, and are characterized by their endoskeleton made primarily of bone, as opposed to cartilage found in Chondrichthyes (cartilaginous fish, such as sharks and rays).
The Osteichthyes include two major lineages: Actinopterygii (ray-finned fish) and Sarcopterygii (lobe-finned fish), with the divergence between these two groups occurring some 420 million years ago, during the Early Devonian. The reasons for this divergence are not entirely clear, but several factors likely contributed to the separation between the ray-finned and lobe-finned fish, including ecological pressures and competition for resources.
In addition to this, the Early Devonian period saw the formation of new aquatic environments, such as shallow coastal waters, deep ocean trenches, and freshwater habitats, and the colonization of these new habitats may have provided opportunities for fish to adapt and evolve, ultimately leading to this divergence between ray-finned and lobe-finned fish.
When it occured, and for whatever reason, the divergence between Actinopterygii and Sarcopterygii led to the incredible diversification of both groups, and while the Actinopterygii diversified into numerous lineages, and now comprise over 95% of all fish species, the Sarcopterygii, although less diverse, played a crucial role in the transition from water to land and the evolution of terrestrial vertebrates, including amphibians, reptiles, birds, and mammals!
Fish in the Devonian
The Devonian Period, also known as the “Age of Fishes,” lasted from approximately 419 to 359 million years ago, and witnessed a dramatic increase in the diversity and complexity of fish, as well as significant ecological changes in marine and freshwater environments.
The period saw the continued diversification of jawed fish, with both placoderms and acanthodians reaching their peak in abundance and diversity during the time. The Placoderms, heavily armored fish with powerful jaws, became the dominant predators in many marine ecosystems, while the Acanthodians, small, spiny fish, who also diversified during the Devonian, eventually giving rise to the first true sharks (Chondrichthyes).
The Devonian was also a critical period for the evolution of Sarcopterygii, or lobe-finned fish, which diversified into two main lineages during this time: the Coelacanths, which are still extant today, and the Rhipidistians, who are of particular importance because they include the ancestors of tetrapods, the first four-limbed vertebrates.
While the exact details of the divergence remain unclear, several factors likely played a role in the separation between these two groups, and the evolution of different fin structures allowed the two groups to exploit different ecological niches and adapt to various aquatic environments.
Tiktaalik
Tiktaalik is an extinct lobe-finned fish that lived around 375 million years ago during the Late Devonian period. Discovered in 2004 on Ellesmere Island in Nunavut, Canada, Tiktaalik has become a significant fossil find because it represents a transitional form between fish and early tetrapods (four-limbed vertebrates), for this reason it is often referred to as a “fishapod” due to its unique combination of features.
During the Late Devonian period, the environment in which Tiktaalik lived was characterized by shallow, slow-moving waters, swamps, and marshlands, and this habitat likely played a crucial role in the evolution of its adaptations, as the ability to navigate both water and land would have provided access to new food sources, escape from predators, and potential nesting sites.
Tiktaalik, as a “fishapod”, exhibits a fascinating mix of characteristics that bridge the gap between fish and early tetrapods, including:
Fish-like features: Tiktaalik possessed scales, gills, and fin rays similar to those found in fish, and its overall body shape was elongated and flattened, resembling that of a crocodile.
Tetrapod-like features: Tiktaalik had a robust skeletal structure with a flat, crocodile-like head and a neck, which are not seen in most fish, and it also had large, strong pectoral fins with a well-developed internal skeletal structure, suggesting that these fins were capable of supporting the animal’s weight and allowing it to “walk”, or navigate shallow waters and land.
The transition to land
The transition from water to land was, for obvious reasons, a significant event in the history of life on Earth, marking the origin of terrestrial vertebrates and their subsequent diversification in Kingdom Animalia.
The first tetrapods evolved from lobe-finned fish (Sarcopterygii) during the Devonian period, around 390 million years ago, and as we have mentioned, these early tetrapods, such as Acanthostega and Ichthyostega, possessed a combination of fish-like and tetrapod-like features.
The transition from water to land required numerous adaptations to navigate new environments, find food, avoid predators, and reproduce, which led to a series of morphological and physiological changes that would set the stage for the diverse array of tetrapods that now inhabit our planet.
Changes in Limbs
One of the most significant adaptations that occurred during the transition to land was the development of limbs with digits (fingers and toes). Early tetrapods like Acanthostega had limbs with multiple digits, allowing them to move more effectively on land or in shallow water. Over time, these limbs became more specialized for different modes of locomotion, such as walking, running, climbing, and swimming.
As tetrapods continued to evolve, their limbs underwent further modifications:
- The development of a robust skeletal structure capable of supporting the animal’s weight on land.
- The emergence of joints, allowing for greater flexibility and range of motion.
- The differentiation of forelimbs and hindlimbs, providing specialized functions such as grasping or propulsion.
Changes in Respiration
Another crucial adaptation during the transition to land was the development of new respiratory systems. Early tetrapods, like their fish ancestors, possessed gills for extracting oxygen from water, however, as they began to spend more time on land, they needed to develop new ways to obtain oxygen from the air.
- Development of lungs: Many early tetrapods evolved primitive lungs, allowing them to extract oxygen from the air. Lungs became more efficient over time, with the development of structures like alveoli, which increase the surface area for gas exchange.
- Cutaneous respiration: Some early tetrapods, particularly amphibians, developed the ability to exchange gases through their skin (cutaneous respiration), allowing them to supplement their oxygen intake and eliminate carbon dioxide without relying solely on lungs.
- Buccal pumping: Many early tetrapods, especially amphibians, developed a mechanism called buccal pumping to move air in and out of their lungs. This involves the use of throat muscles to create a pressure difference, enabling air to be drawn into the lungs and expelled.
These changes in respiration were essential for tetrapods to survive and thrive in terrestrial environments, as they allowed these animals to access these new food sources, avoid potential predators, and reproduce on land.
Diversification of the Tetrapods
Once the first tetrapods had successfully transitioned to life on land, they continued to evolve and diversify, giving rise to the vast array of species that inhabit our planet today. This diversification was driven by a range of factors, including environmental changes, competition, and the colonization of new ecological niches.
Amphibians: Descendants of the earliest tetrapods, amphibians are characterized by their reliance on moist environments and their unique life cycle, which typically includes an aquatic larval stage. Examples of modern amphibians include frogs, salamanders, and caecilians.
Reptiles, birds, and mammals: As tetrapods continued to evolve, some lineages developed new adaptations that allowed them to exploit a wider range of habitats and resources. This led to the emergence of reptiles, birds, and mammals, each with their distinct characteristics and ecological roles. Though first, came the Amniotes…
Evolution of Amniotes
One of the key innovations that facilitated the further diversification of tetrapods was the evolution of amniotes, which are a group of advanced tetrapods that lay eggs with a protective membrane called an amnion, allowing them to develop in a relatively stable and protected environment.
The amniotic egg was a critical adaptation for life on land, as it reduced the reliance on water for reproduction and enabled amniotes to colonize a wider range of terrestrial habitats. This evolution of the amniotic egg is thought to have occurred around 340 million years ago, during the Carboniferous period, facilitating their eventual domination.
The amniotes diversified into two major lineages: the synapsids, which include mammals and their extinct relatives, and the sauropsids, which include reptiles and birds.
It is thought that these lineages diverged from a common ancestor around 320 million years ago during the late Carboniferous period.
Synapsids
(Synapsida: “Fused Arch”)
Synapsids first appeared around 320 million years ago during the late Carboniferous period, and the earliest synapsids, such as Archaeothyris and Clepsydrops, were small, lizard-like creatures with a single temporal fenestra, an opening in the skull behind the eye socket for jaw muscle attachment.
The synapsids’ true evolutionary journey began with the emergence of pelycosaurs during the early Permian period, around 300 million years ago, which were characterized by elongated neural spines supporting sail-like structures.
While it was originally thought that pelycosaurs like Dimetrodon and Edaphosaurus used these sails solely for thermoregulation, it is now assumed they played other roles associated with display, whether to find a partner for mating, or to ward off predators and rivals.
As pelycosaurs declined, therapsids appeared during the middle Permian period, around 275 million years ago, and these “mammal-like reptiles” exhibited features closer to modern mammals, such as upright limb posture and a secondary palate for simultaneous breathing and chewing.
Cynodonts, a subgroup of therapsids, evolved in the late Permian period, around 260 million years ago, with more mammalian traits like an efficient jaw joint and differentiated teeth, continuing the evolutionary process to the eventual Eutherians, and much later, ourselves.
Finally, or at least finally in the context of our post, the first true mammals would emerge during the late Triassic, around 200 million years ago. Mammals, a subgroup of synapsids, possess characteristics such as hair, mammary glands, efficient respiratory systems, and complex brain structures, and would go on to diversify further into monotremes, marsupials, and placental mammals, representing the majority of modern mammalian species.
Eventually, this evolutionary pathway would lead to ourselves, the Homo Sapiens, though the journey to this ending is as we have stated, far beyond the scope of this post.
Sauropsids
(Sauropsida: “Lizard Faces”)
The divergence of the Sauropsids from amniotes can, again, be traced back to the late Carboniferous period when sauropsids and synapsids (mammals and their relatives) split from their common ancestor. This divergence was marked by differences in skull morphology, specifically the number and arrangement of temporal fenestrae.
Similar to Synapsids, Sauropsids first appeared around 320 million years ago during the late Carboniferous period, with the earliest sauropsids, such as Hylonomus and Paleothyris, being small, lizard-like creatures with either no temporal fenestrae (anapsids) or two temporal fenestrae (diapsids) in the skull for jaw muscle attachment. This is in contrast to the Synapsids, which possessed a single temporal fenestra, as mentioned.
Reptiles are a diverse group of tetrapods that first appeared around 310 million years ago during the Carboniferous period, and are characterized by their scaly skin, helping to prevent water loss, and their ectothermic (cold-blooded) metabolism, which relies on external heat sources to regulate body temperature.
Diapsids, which possess two temporal fenestrae, became the dominant group of reptiles during the Permian period, and over time, diversified into numerous lineages, including lepidosaurs (lizards, snakes, and tuataras), archosaurs (crocodiles, pterosaurs, dinosaurs, and birds), and various marine reptiles.
The Archosaurs, a major group of diapsids, first appeared around 250 million years ago during the late Permian period, and included the ancestors of crocodiles, pterosaurs (flying reptiles), dinosaurs, and birds. Archosaurs are characterized by a range of unique features, such as an antorbital fenestra (an opening in the skull in front of the eye socket) and a modified ankle joint.
Dinosaurs, a subgroup of archosaurs, would eventually emerge around 230 million years ago, during the Triassic, and would go on to become the dominant land animals for nearly 170 million years, though again, this is beyond the scope of this post.
Conclusion
Throughout this blog post, we have traced the incredible journey of tetrapods from their fish-like ancestors to the diverse array of species that exist today, and while it has been a long journey, I hope you enjoyed this deep dive into the emergence of tetrapods and their long evolutionary path.
Thank you for reading, and I invite you to follow our blog for more fascinating content exploring the depths of deep time!
Resources
Clack, J. A. (2012). Gaining Ground: The Origin and Evolution of Tetrapods (2nd ed.). Indiana University Press.
Coates, M. I., & Ruta, M. (2007). Fins to limbs: What the fossils say. Evolution & Development, 9(5), 456–466
Benton, M. J., & Donoghue, P. C. J. (2007). Paleontological evidence to date the tree of life. Molecular Biology and Evolution, 24(1), 26–53.
Carroll, R. L. (2009). The Rise of Amphibians: 365 Million Years of Evolution. The Johns Hopkins University Press.
