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Tetrapods

EVOLUTION

Lungs Before Land

Lungs Before Land

The lung/swim bladder originated as an outgrowth of the gut, forming a gas-filled bladder above the digestive system. In its primitive form, the air bladder was open to the alimentary canal, a condition called physostome and still found in many fish.[10] The primary function is not entirely certain. One consideration is buoyancy. The heavy scale armour of the early bony fishes would certainly weigh the animals down. In cartilaginous fishes, lacking a swim bladder, the open sea sharks need to swim constantly to avoid sinking into the depths, the pectoral fins providing lift.Another factor is oxygen consumption. Ambient oxygen was relatively low in the early Devonian, possibly about half of modern values.[12] Per unit volume, there is much more oxygen in air than in water, and vertebrates are active animals with a high energy requirement compared to invertebrates of similar sizes.[13][14] The Devonian saw increasing oxygen levels which opened up new ecological niches by allowing groups able to exploit the additional oxygen to develop into active, large-bodied animals.[12] Particularly in tropical swampland habitats, atmospheric oxygen is much more stable, and may have prompted a reliance of lungs rather than gills for primary oxygen uptake.[15][16] In the end, both buoyancy and breathing may have been important, and some modern physostome fishes do indeed use their bladders for both.

To function in gas exchange, lungs require a blood supply. In cartilaginous fishes and teleosts, the heart lies low in the body and pumps blood forward through the ventral aorta, which splits up in a series of paired aortic arches, each corresponding to a gill arch.[17] The aortic arches then merge above the gills to form a dorsal aorta supplying the body with oxygenated blood. In lungfishes, bowfin and bichirs, the swim bladder is supplied with blood by paired pulmonary arteries branching off from the hindmost (6th) aortic arch.[18] The same basic pattern is found in the lungfish Protopterus and in terrestrial salamanders, and was probably the pattern found in the tetrapods' immediate ancestors as well as the first tetrapods.[19] In most other bony fishes the swim bladder is supplied with blood by the dorsal aorta.[18]

Breathing

Breathing

In order for the lungs to allow gas exchange, the lungs first need to have gas in them. In modern tetrapods, three important breathing mechanisms were inherited from early ancestors, the first being a CO2/H+ detection system. In modern tetrapod breathing, the impulse to take a breath is triggered by a buildup of CO2 in the bloodstream and not a lack of O2. Interestingly, a similar CO2/H+ detection system is found in all Osteichthyes, which implies that the LCA of all Osteichthyes had a need of this sort of detection system.The second mechanism for a breath is a surfactant system in the lungs to facilitate gas exchange. This is also found in all Osteichthyes, even those that are almost entirely aquatic. The highly conserved nature of this system suggests that even aquatic Osteichthyes have some need to breath, which is strange because they live underwater. The third mechanism for a breath is the actual motion of the breath.. In Lampreys, this mechanism takes the form of a "cough", where the lamprey shakes its body to allow water flow across its gills. When CO2 levels in the lamprey's blood climb too high, a signal is sent to a central pattern generator that causes the lamprey to "cough" and allow CO2 to leave its body. This linkage between the CO2 detection system and the central pattern generator is extremely similar to the linkage between these two systems in tetrapods.

Nasal Openings

Nasal Openings

The nostrils in most bony fish differ from those of tetrapods. Normally, bony fish have four nostrils, one nostril behind the other on each side. As the fish swims, water flows into the forward pair, across the olfactory tissue, and out through the posterior openings. This is true not only of ray-finned fish but also of the coelacanth, a fish included in the Sarcopterygii, the group that also includes the tetrapods. In contrast, the tetrapods have only one pair of nostrils externally but also sport a pair of internal nasal openings, called choanae, allowing them to draw air through the nose. Lungfish are also sarcopterygians with internal nostrils, but these are sufficiently different from tetrapod choanae that they have long been recognized as an independent development.

The evolution of the tetrapods' internal nares was debated in the 20th century. The internal nares could be one set of the external ones that have migrated into the mouth, or the internal pair could be a newly evolved structure. To make way for a migration, however, the two tooth-bearing bones of the upper jaw, the maxilla and the premaxilla, would have to separate to let the nostril through and then rejoin; until recently, there was no evidence for a transitional stage, with the two bones disconnected. Such evidence is now available: a small lobe-finned fish called Kenichthys, found in China and dated at around 395 million years old, represents evolution "caught in mid-act", with the maxilla and premaxilla separated and an aperture—the incipient choana—on the lip in between the two bones. Kenichthys is more closely related to tetrapods than is the coelacanth,which has only external nares; it thus represents an intermediate stage in the evolution of the tetrapod condition.

Shallow Waters

Shallow Waters

As lineages moved into shallower water and onto land, the vertebral column gradually evolved as well. Fishes have no necks. Their heads are simply connected to their shoulders, and their individual vertebrae look quite similar to one another, all the way down the body. Mobile necks allow land animals to look down to see the things on the ground that they might want to eat. In shallow water dwellers and land dwellers, the first neck vertebra evolved different shapes, which allowed the animals to move their heads up and down. Eventually, the second neck vertebra evolved as well, allowing them to move their heads left and right. Later tetrapods evolved necks with seven or more vertebrae, some long and some short, permitting even more mobility.

Human veterbrae consists of a spool-like centrum, which connects in front and back with other centra. On top of the centra are vertebral spines and arches to which muscle segments attach, and lateral to the centra are the ribs; these anchor muscles that flex as the animals move. Fishes swim with simple lateral motions, so their arches are relatively straight and needle-like, and so are their ribs Because fishes live in the water, gravity is not a big problem for them. But on land, a quadruped with a backbone between forelimbs and hindlimbs faces the problem of sag. As the fleshy-finned organisms began to venture onto land, they evolved a series of interlocking articulations on each vertebra, which helped them overcome sag and hold the backbone straight with minimal muscular effort.

Fins to feet

Fins to feet

Once, the only organisms that existed were fish. It was a fish-eat-fish world. Some fish evolved to live by the outskirts of water and ultimately on land to avoid being eaten.

Society

When human beings think about evolution, they usually think about apes. Inner Fish allows humans to think about evolution in a wider context, down to our bones and digits. The realization that all organisms evolved from a similar blueprint is significant for manipulation of our genome. Also, it could be a reality that billions of years from now, the Earth will be submerged by water. This would force the hands of human beings to evolve webbing in order to better adapt. While this is an incredibly distant reality that none of us will live to see, it is important to understand that humans are still evolving, and so are the smaller ogranisms around us.

Intro to tetrapods

Intro to tetrapods

The word "tetrapod" means "four feet" and includes all species alive today that have four feet — but this group also includes many animals that don't have four feet. That's because the group includes all the organisms (living and extinct) that descended from the last common ancestor of amphibians, reptiles, and mammals. So, for example, the ichthyosaur, an extinct swimming reptile, is a tetrapod even though it did not use its limbs to walk on land. So is the snake, even though it has no limbs. And birds and humans are tetrapods even though they only walk on two legs. All these animals are tetrapods because they descend from the tetrapod ancestor described above, even if they have secondarily lost their "four feet."

DIAGRAM

DIAGRAM

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