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Kingdom – Animalia
Phylum – Chordata
Subphylum – Vertebrata
Superclass – Osteichthyes (bony fish)
Class – Sarcopterygii (lobe-finned fish, distinct from ray-finned fish)
Infraclass – Tetrapodomorpha
Superorder – Osteolepiformes
Order – Elpistostegalia (prehistoric lobe-finned fishes)
Family – Elpistostegidae
Genus – Tiktaalik
If you have ever wanted to see an unmistakable transitional form in the history of life on Earth, then lay an image of Tiktaalik on your retinas. Among the first of the Vertebrata to crawl out of the water to spend significant elapses of time on land, Tiktaalik clearly bridges the gap between fish on one side and amphibians, reptiles, and mammals on the other side; between pure water breathers and pure air breathers.
It had to happen, if the theory of evolution gives us a correct picture of life on Earth. The theory tells us, quite reasonably, that life originated in water as single-celled forms and could only have evolved into multicellular forms there. Indeed, because life, at its most basic, consists of an aqueous solution of complex organic molecules, we should expect to find life existing only in water. Yet today we see a mind-boggling array of life forms, especially vertebrates, occupying the land. Those two statements necessitate that, at some time in the far past, a swimming, water-breathing creature got transformed into a walking, air-breathing creature, just as certain algae got transformed into things that can grip the soil and raise their leaves to the sun, just as certain arthropods got turned into insects.
When Charles Darwin had "On the Origin of Species" published in 1859 Humanity had only a sparse knowledge of the fossil record. Darwin understood the risk that fact posed for his hypothesis. But he also understood the opportunities inherent in "missing links": in order to become a fully scientific theory an hypothesis must predict the results of observations or experiments. The description of a missing link serves as a prediction and finding the fossil evidence of such a creature serves as the confirmation of the prediction. Certainly those missing links that displayed major transitions in the history of life would play an important role in proving and verifying Darwin’s hypothesis.
Imagine the history of life as a glass-plate hologram. The plate has shattered and the fragments now lie scattered all over the world. The fragments that we possess constitute the fossil record and when we shine the light of the theory of evolution through them we get a blurry picture of the history of life. As paleontologists recover more of the fragments, the picture becomes clearer. But we can also anticipate what some of the remaining fragments, the missing links, will contribute to the picture.
We know that at least one of those fragments must show us something that looks like a fish that could haul itself out of the water and move around on land. The oldest fossils of tetrapods, animals that live on land, found in rocks dated to around 365 million years ago, show us amphibious creatures, such as Ichthyostega, that could barely walk on land. That fact leads us to expect that we would find fossils of a barely-more-than-a-fish in rocks only slightly older. And, confirming the theory of evolution yet again, Neil Shubin and his team found Tiktaalik in rocks dated to 375 million years ago.
What conditions lead to the evolution of such creatures? What factors would make a water-dwelling animal come out onto land? What features did it have to evolve in order to have migration onto land as a possibility and how did it evolve those features?
In order to gain the land, a creature that had more in common with a fish had to evolve, at the very least, crude lungs and weight-bearing legs. Around 382 million years ago, in the Devonian Period (the Age of Fishes, 416 – 359.2 Myrs ago), just such a creature evolved in a subtropical swamp. In genus Tiktaalik we may see a reflection of what evolved from that creature.
Evidence dug out of the same rocks whence the fossils of Tiktaalik came imply that this weird fish lived in estuaries or swampy river deltas, such as those of the Mississippi River, the Ganges River, or the Orinoco River. Coastal swamps provided the environment in which Tiktaalik lived and, likely, the environment where they evolved from fishier fish. In each of these places a high-volume river runs into the effectively motionless water of the sea, bringing the water’s rush to a halt and allowing the water to drop its load of silt, thereby creating a broad, flat, soggy landscape. The opportunities that such an environment offers drew fish into its network of channels. And the challenges that such an environment offers promoted those mutations that gave some fish the ability to walk and to breathe air. Contrary to the popular image of a fish crawling onto a beach, the image that cartoonists favor, the vertebrate invasion of the land had to come through such a staging area, a place that both enabled and promoted the evolution of a means to breathe air instead of water and of a means to move efficiently in an environment that does not buoy up the body. And, in accordance with the doctrine of natural selection, that evolution had to occur through a combination of pure happenstance (mutation) and biological necessity (selection).
Imagine a river delta or estuary in the middle Devonian. Where the surging current of a great river once met the rolling waves of the sea, it unraveled into a myriad of channels meandering across a broad silty plain of its own creation. Plants, which had evolved land-dwelling forms 450 million years ago, in the Ordovician Period, grew over virtually the entire expanse of the plain, in the shallower channels as well as on the relatively dry land, their roots holding the soil in place and enabling the swamp to grow further.
Some 380 million years ago reeds, cycads, and other simple plants covered this wetland. Insects throve in their swarms. Arthropods (such as crabs and insects) and molluscs (such as mussels and snails) filled their own ecological niches. And fish, once they had evolved the ability to live in fresh, albeit often muddy, water, patrolled the channels, slurping up prey and avoiding getting slurped up. In that world swam a fish the size of a small shark, one that we might call BaltoIchthys (Greek for swamp fish).
We can see an example of such a fish in Panderichthys, a roughly meter-long fish from the Devonian period, 380 million years ago. In addition to having a large tetrapod-like head, Panderichthys exhibited transitional features between lobe-finned fishes and early tetrapods, such as Acanthostega. The evolution from fish to land dwelling tetrapods required many changes in anatomy and physiology, most importantly in the legs and their supporting structure, the pelvic and shoulder girdles. Well-preserved fossils of Panderichthys clearly show these structures in transition, revealing early stages in their evolution into larger versions of themselves.
The skeletal foundation to which the muscles and other organs attach must change in certain ways to enable a swimming creature to walk on land. One of the major changes must occur in the skeletal foundation of the appendages; in particular, a shift in the relative dominance in locomotion from the pectoral to the pelvic appendages, from what would become forelimbs to what would become hind limbs. Panderichthys constitutes evidence of this shift because the shape of its bones shows that the fin to limb transition began in the foundation of the pectoral fins and only later occurred in the foundation of the pelvic fins. Thus Panderichthys gives us a good example of a transitional state in tetrapod evolution because its pectoral girdle shows derived characteristics while its pelvic girdle retains ancestral ones. Even though Panderichthys itself does not show the actual shift in the dominance of pelvic limbs over pectoral limbs, it looks as though it had the capability of some kind of shallow water or terrestrial body flexion locomotion (i.e. slithering) and had some limited ability to prop itself up.
In addition, a CT scan of existing Panderichthys fossils at the extremities shows four very clearly differentiated distal radial bones at the end of the fin skeletal structure. These finger-like bones do not show joints and they are short, but nonetheless they show us what an intermediate form between fully fish-like fins and the legs of tetrapods looks like.
In fish like those of genus Panderichthys we see the ancestors of the first tetrapods, the air-breathing, amphibious animals from which the land vertebrates, including humans, descended. But how did Panderichthys get there? Look again at our hypothetical BaltoIchthys.
BaltoIchthys didn’t sit at the top of the food pyramid. It spent at least as much time swimming for its life as it did making other, smaller fish swim for their lives. That predation provided the selection in natural selection. Whatever traits gave a BaltoIchthys even a slight advantage in fending off or escaping from bigger predators got preferentially preserved in the fish’s genetic code, because the fish with those traits had an improved probability of surviving long enough to reproduce themselves.
Outswimming a predator seems an obvious ploy, but it necessitates the expenditure of a lot of energy. Any traits that produce a lower-energy ploy will tend to get preserved by natural selection. And in a swamp our BaltoIchthys, too large to duck into a bed of reeds or under a pile of debris, as smaller fish do, had another obvious ploy for evading predators: it could swim into a channel too shallow for the predator to follow. The success of that ploy promoted the preservation of changes in BaltoIchthys’ genetic code, changes that eventually turned BaltoIchthys into Tiktaalik. But what kind of changes would come about by pure happenstance in that situation?
If the creature had an instinct to head for shallowest water when pursued (and that instinct would have to evolve first), then over time its descendants would develop flatter bodies. To put it into proper evolutionary terms, we must say that in this particular environment a BaltoIchthys with a flatter than normal body had a greater than normal probability of surviving predation (because it could go into shallower water than the normal member of its species could) and thus had a greater than normal probability of reproducing itself and, thus, of passing on its genetic coding for a flatter body. If the appropriate mutations could occur in this creature’s genetic code and if a large enough population of them inhabited a given wetland, then over thousands, or perhaps as few as hundreds, of generations BaltoIchthys would evolve to have flatter, wider bodies than their forebears had: they would have evolved the ability to wriggle into the shallowest of their wetland’s channels.
But that would not have sufficed: it never does. As the population of BaltoIchthys evolved into a flat-bodied form, their predators would have come under evolutionary stress. The predators had only three ways in which they could respond to that stress – go after alternative prey, evolve flatter bodies and continue pursuing BaltoIchthys, or starve. Those predators that already had flatter than normal bodies, due to the usual differences found in any trait in a given population, followed the second path and thereby continued to exert evolutionary stress on BaltoIchthys.
One other factor comes into play when a one- to two-meter fish evolves to swim in the shallow channels in a swamp – fins. The little organic submarines that evolved in the sea have a feature that their bigger, clankier artificial analogues lack: they have active control surfaces. For their primary propulsion they slither through the water with an assist from their broad tail fins. But for small movements and movement in reverse they flutter their pectoral and pelvic fins.
The muscles that move those fins and the bones to which they attach grow from the expression of part of the fish’s genetic code, so mutations in the fish’s DNA can change the size and shape of those muscles and bones in subsequent generations of fish. Thus, natural selection can promote the evolution of those muscles and bones into fleshy lobes that stick out from the side of the fish’s body, as in Coelacanth or Eusthenopteron, a fish that lived in the middle Devonian 385 million years ago. Of course, natural selection only promotes mutations that give the animal some advantage in its particular environment and thus enables the animal to reproduce itself more prolifically and thereby preserve the mutation.
Lobe-based fins generate more thrust than do lobeless fins, making a lobe-finned fish more maneuverable than a lobeless fish of the same size. In a swamp small fish would not likely evolve lobed fins, because they can avoid predators by ducking into a bed of reeds or a pile of debris. But a mid-sized fish, like our BaltoIchthys, doesn’t have that option available to it. It offers too big a target to predators, so it must depend on speed and maneuverability, all the more so because it preys on smaller fish itself.
Under pursuit and guided by an evolved instinct, a BaltoIchthys heads for the shallows. Goaded by panic, the creature goes too far and gets itself into water too shallow for swimming. It continues to wriggle from side to side, as it does in swimming, and now something new happens: its lobed fins press against irregularities in the channel, such as the root clumps of plants, the burrows of crustaceans, even ripple patterns in the mud, and it continues to move forward. Like a soldier crawling under the barbed wire on an obstacle course, using his elbows and knees to brace himself against the ground as he wriggles forward, BaltoIchthys would have half slithered, half crawled into even shallower water, effectively beaching itself. In the modern era we see that process occurring in certain species of catfish.
In Southeast Asia, especially in the basins of the Mekong and Chao Phraya Rivers, we find the walking catfish (Clarias batrachus). It doesn’t truly walk, but, rather, it uses its pectoral fins (the analogue of arms) to brace itself as it wriggles across the land. As a fish, it must still remain moist as it walks, so it does not go far from water. Its natural environment constitutes an estuary-like waterscape, so it easily meets that criterion. And over time we could expect that those catfish with fins slightly larger and more muscular than those of other catfish would gain a reproductive advantage over other catfish and thus begin evolving toward something with legs. BaltoIchthys went through the same evolutionary process.
But something else, something essential to a land animal, must evolve: the fish must evolve the ability to breathe air. We use the term "fish out of water" to designate a creature that finds itself in deep trouble. But, as every fisherman knows, fish can survive out of water for a significant elapse of time. With no water flowing over its gills, the fish, in essence, holds its breath. And it can do this for a relatively long time because a cold-blooded creature needs significantly less oxygen than does a warm-blooded animal of comparable size. That fact reveals the kind of opportunity that our BaltoIchthys had that enabled it to evolve the means to breathe air.
At this stage another organ comes into play and began its own evolution. The evolution of BaltoIchthys’s swim bladder gives us an excellent example of how an organ that evolved to fulfill one purpose can re-evolve to fulfill a completely different purpose. In this case an organ that evolved to enable fish to control their buoyancy re-evolved to enable animals descended from fish to breathe air: they became lungs.
The swim bladder normally consists of two gas-filled sacs located in the dorsal portion of the fish, under the spine, though in a few primitive species the swim bladder consists of only a single sac. The swim bladder has flexible walls that contract or expand according to the ambient pressure, thereby giving the fish ready autonomic control over its buoyancy. The walls of the bladder contain very few blood vessels and a lining of guanine crystals makes those walls impermeable to gases, a feature that prevents the gas in the bladder from diffusing into the fish’s other tissues, thereby diminishing its buoyancy. By adjusting the gas pressure in its swim bladder through the gas gland or the oval window, which occupies a small part of the bladder wall, the fish can obtain neutral buoyancy and ascend and descend easily over a large range of depths. Due to its position under the spine, at the top of the fish, the swim bladder also gives the fish lateral stability.
Modern fish have one of two kinds of swim bladder. Physostomous swim bladders, as opposed to physoclistous swim bladders, retain a connection, the pneumatic duct, between the swim bladder and the gut. That feature allows the fish to fill up the gas bladder by "gulping" air and forcing it into the gas bladder. Reversing that process, "burping" out gas, allows the fish to expel excess gas. That feature enables the mutations that would turn the swim bladder into lungs. We assume, of course, that BaltoIchthys had this feature.
On first impression we might think that the transformation of a simple bladder into an efficient biological gas exchanger would require a magic mutation. But the swim bladders of fish all have the ability to conduct gas exchange between the bladder and the fish’s bloodstream. As mechanical submarines do, fish need to control their buoyancy. The benefits of controllable buoyancy led primordial fish to evolve the swim bladder with a small part called the oval window, where the fish’s blood comes into contact with a thin membrane that enables the blood to exude gas into the bladder or absorb gas from it. As the swim bladder evolved into lungs, mutations had to make the oval window expand to occupy all of the bladder’s wall and then make the wall branch off miniature sacs to expand its surface area.
The existence of the swim bladder reflected the genetic foundation on which BaltoIchthys could evolve another necessary feature of living on land. As a feature under control of the fish’s nervous system, the swim bladder came with a small repertoire of instincts, one of which gave the fish the possibility of evolving into a land animal. The instinct to increase buoyance when it feels something pressing it against the bottom of a channel would have become the instinct to breathe air.
Chased into the shallows and coming partly out of water, BaltoIchthys clamped its operculi closed to prevent its gills from drying out. It could then partly empty its swim bladder and let carbon dioxide diffuse into the empty volume as it started to build up in the creature’s blood. That process works most efficiently when a gas for which the blood has a greater affinity displaces the carbon dioxide and hemoglobin-based blood has a greater affinity for oxygen than it has for carbon dioxide. Even in the middle of the Devonian period Earth’s atmosphere contained a high enough fraction of oxygen that alternately collapsing and expanding its swim bladder would prevent BaltoIchthys from suffocating due to carbon dioxide buildup in its blood. The absorption of extra oxygen into the blood would have come as an added benefit.
The continuous activity of prey and predator engaging each other in the dance of mutation and selection promoted changes in BaltoIchthys. As we have seen, the creature became flatter, splaying out its pectoral and pelvic fins. Those fins’ lobes developed larger bones and stronger muscles, enabling them to exert more force. Other bones evolved, such as those of the shoulders, to connect the creature’s proto-legs to its spine. The swim bladder, while retaining its original function in the water, had evolved into a primitive lung, originally more for eliminating carbon dioxide than for obtaining oxygen. At the same time the ribs became thicker, wider, and overlapping in places to provide extra support for the creature lest its weight collapse it and prevent it from breathing. Those things evolved from mutations of pre-existing structures, mutations promoted by preferential reproduction driven by predation.
If we go away from the swamp for a few hundred or a few thousand generations and then come back, about 375 million years ago, we will see creatures little different from BaltoIchthys in appearance and behavior. But these creatures would not be able to interbreed with BaltoIchthys if any still existed: so many mutations have accumulated in the creature’s genetic code that we now see a new species, indeed the founding species of a new genus – Tiktaalik.
Tiktaalik (a name that comes from the Inuktitut language of Canada’s Nunavut Territory) denotes a genus of extinct sarcopterygian (lobe-finned) fish from the late Devonian period. In its features Tiktaalik shows us the changes that led to the evolution of amphibians, features that make it an exquisite transitional creature standing between fish and tetrapods.
If we take a closer look we can see the subtle but significant differences between Tiktaalik and BaltoIchthys. Tiktaalik had a wide, flat head shaped much like a bellows, which gave the fish an improved ability to swallow air into its primitive lungs. That fact implies that Tiktaalik made more use of oxygen from air than BaltoIchthys did. A related fact comes to us when we look inside the lobes that support Tiktaalik’s fins: in addition to having the ability to slither through mud and reeds as BaltoIchthys did, this fish could partly lift itself up and crawl, because Tiktaalik had in its fin lobes bones analogous to the limb bones, including hand/foot and wrist/ankle, the shoulder and the elbow, of modern land vertebrates, such as the crocodile. Tiktaalik could flex its lobes and fins at what would later evolve into the tetrapod shoulder and elbow, hip and knee. It could also flex its fins almost through a right angle as we do with our hands when we push on something. This fish could come into the shallows of its swamp, get up on its fins, much as a seal does, and walk, however poorly, out of the water. BaltoIchthys could do that even more poorly, if at all.
We also note the spiracles, primitive nostrils, on the top of Tiktaalik’s head. That feature suggests that the creature had primitive lungs as well as gills. That development led to the evolution of a more robust ribcage, a key evolutionary trait of creatures living on land. The thicker bones in Tiktaalik’s ribcage helped support the animal’s body any time it ventured outside full immersion in water. Tiktaalik also lacked a characteristic that most fishes have – bony plates in the gill area that restrict lateral head movement. That feature makes Tiktaalik the earliest known vertebrate to have a neck, something that gave the creature more freedom to turn its head, which gave it more flexibility in hunting prey either on land or in the shallows of its swampy habitat.
As if to remind us of the blurry nature of the fossil record and the tentative nature of the inferences that we draw from it, some paleontologists have found evidence that land animals may have evolved before the date that we put on the evolution of that feature based on the discovery of Tiktaalik. They discovered tetrapod footprints in Poland and, as they reported in the January 2010 issue of Nature, securely dated them at 10 million years older than evidence of the oldest known elpistostegids (of which Tiktaalik is one). That discovery implies that animals like Tiktaalik represent late-surviving relics that possess features that actually evolved around 400 million years ago.
We have already identified the primary reason for the evolution of the ability of certain fish to come out of the water in the negative benefit of not getting eaten. But once these no-longer-entirely-fish came out of the water they found a positive benefit waiting for them – food.
Long before Tiktaalik prowled the coastal swamps of Euramerica certain seaweeds had evolved forms that could grow and thrive on land. With these plants came molluscs (e.g. snails) and arthropods, some of which began evolving into insects. When Tiktaalik came out of the water it could easily have encountered something like a crab or a wingless insect. Though Tiktaalik’s teeth and jaws indicate that it ate fish, on land it would follow one of the oldest instincts of Kingdom Animalia – if it moves, flee it or eat it. With the size of a small alligator (a seven-foot length), Tiktaalik would not likely have fled insects, even though they grew bigger than the ones that we see. The ability to grab a snack after some bigger predator chased it out of its usual hunting canals would have enhanced its survivability.
But Tiktaalik did not come out of the water alone. Any of its predators that could follow it also gained a slight reproductive advantage over those of its species that couldn’t. Thus Tiktaalik’s predators also evolved lungs, legs, and a load-bearing skeleton. The slow interplay between populations of predator and prey ensured that Tiktaalik would continue to change.
By 365 million years ago the species of Tiktaalik had evolved into new genera, typified by Acanthostega. Still breathing through gills and still swimming with a long, broad fish-like tail, Acanthostega had improved abilities to live on land. The robust lobe fins of Tiktaalik had evolved into true legs with bones supporting the eight digits of the creature’s webbed feet. Unlike fish, whose skulls have smooth sutures connecting the bones that comprise them, Acanthostega’s skull had jagged, interlocking sutures, a feature found in creatures that grab and bite their prey instead of slurping them up, as fish do. That latter feature implies that Acanthostega did a significant amount of its eating in shallow water or on land.
Fossils extracted from rocks of about the same age as those in which the remains of Acanthostega occur show us even more land-worthy amphibians, such as Hynerpeton and Ichthyostega. Where Acanthostega, still very much a fish, had the bones of eight digits on each foot, Ichthyostega, one of the first amphibians, had the bones of seven digits, three of which appear nested side by side as if in the process of fusing into a single digit. In these creatures we see the evolution of the neck, legs, and lungs as the major adaptations to living on land. Thus we see how Class Osteichthyes (bony fish) spun off Class Amphibia, from which Classes Reptilia and Mammalia would then evolve. Well along on the road to evolving five-toed feet, Ichthyostega and other amphibians stood ready to take evolutionary advantage of the next big change in Earth’s biosphere.
As the Devonian period segued into the Carboniferous period, about 360 million years ago, plants got big. Having evolved from seaweed, evolving roots to anchor them to the ground and provide nutrients from the soil, and evolving a rigid stem to hold their leaves up to the sun, some plants grew them bigger and thicker: they became trees. They covered the swamps first, then they became fully terrestrial and spread over the land in vast forests. Over millions of years they increased the oxygen content of Earth’s atmosphere from fifteen percent to thirty-two and a half percent.
The increased oxygen enabled insects to evolve into bigger forms that prowled the forests. For something like Ichthyostega, insects went from light snack to full meal. In taking advantage of that bounty the descendants of Tiktaalik evolved into Class Amphibia. Those creatures could not go far from the swamps where they evolved, but eventually some of them evolved skins that would offer them some protection from dessication. Some evolved cleidoic eggs and became the first members of Class Reptilia or evolved the practice of giving live birth and became the first members of Class Mammalia. In time those creatures took over the land completely. And all because a fish formed the habit of heading for shallow water when pursued.
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