Breath Taking. Michael J. Stephen

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СКАЧАТЬ from the moist soil around them, letting it dissolve through their skin and into their blood. Dry a worm out and you suffocate it. Spiders and insects have a respiratory system, but it is simply a long pipe, going through their bodies, that allows oxygen to diffuse into the surrounding tissues. In all of these species, there is no muscular system to augment the utilization of oxygen, and no way to increase the supply of oxygen significantly in a time of need. These primitive systems are limited by their lack of efficiency. This limitation prevents the bodies and brains of these creatures from growing bigger. They are constrained by their lack of lungs.

As worms and spiders were creeping out of the sea, life was advancing at a much faster rate in the ocean than on land. Creatures were both growing bigger and developing more complex organs. Vertebrate animals, with an endoskeleton and a skin covering, evolved, as did fish with familiar organs like a brain, liver, heart, and alimentary canal. These sophisticated vertebrates began to colonize many different aquatic niches, from the highest river streams to the deepest ocean trenches. The Devonian Period, spanning from 420 million to 359 million years ago, is known as the age of fish for the explosion in the number of species and in the number of habitats they colonized.22

      Fish likely diversified because they developed an ability to utilize oxygen through an efficient circulatory system. A large part of that system is the gill. Most fish have a single slit on either side that allows water to pass through. As the water flows in, a vast network of capillaries embedded in the gills extracts oxygen from the water. The capillaries also expel carbon dioxide, in a gas exchange system parallel to our own. Most fish also have muscles around their gills that can cause them to flap and increase the stream of water and oxygen into the system as energy needs increase. It’s a worthy system of oxygen utilization, and it explains why fish have developed into some of the biggest creatures on Earth.

      In time, and only after they developed lungs as a way to extract oxygen from the atmosphere, fish made their way onto land. It is a single miraculous development, albeit spread out over tens of millions of years. It fascinates us because we can think of it as the moment of our birth, a symbol of when life as we know it was finally within reach. What made this transition possible was the creation of lungs, the organ that defines us as terrestrial creatures.

      The metamorphosis of fish is thought to have begun in the shallow muddy waters where the ocean and land meet. There was a clear adaptive benefit to being able to stay out of the water for extended periods of time to take advantage of a landmass full of food in the form of plant life.

      Exactly how lungs first developed in fish is a question that has long been debated. One thing that appears clear, though not intuitive, is that our modern lungs did not evolve from gills. Interestingly, the gills of some fish, most notably the walking catfish, have evolved into a partial lung. Native to Asia, but now taking over Florida, these fish have developed a very small area of gas exchange that opens only when they close their rear gills.

      Our lungs, however, likely started as an outpouching of the esophagus as fish began to breathe by simply swallowing air that then diffused into the circulation by simple osmosis. Some fish have retained this early outpouching, known as a swim bladder, which is filled with air. Modern fish use the swim bladder as a ballast mechanism for buoyancy. But the bladder in some earlier fish developed into the lungs we know today.

      One other important transformation necessary for fish to thrive on land was the development of legs to maximize maneuverability outside of water. Creatures with four appendages are referred to as tetrapods, a class that today is made up of all the mammals, reptiles, birds (wings count), and amphibians. Most likely, during the Devonian Period, about four hundred million years ago, the first type of tetrapod emerged from the ocean with newly, and simultaneously, evolved lungs and legs.

      The fossil record from that time reveals clear signs that some fish were making attempts at coming onto land. These early colonizers had a more defined bony structure in their fins, and the beginnings of a lung along with their gills. One such fish was the coelacanth, which was thought to have gone extinct millions of years ago. This belief changed by chance on a sunny day in 1938, when a young woman in South Africa spotted something unusual on a fishing vessel, spawning an extraordinary fish story and an international sensation.

Marjorie Courtenay-Latimer was a museum curator from East London, South Africa, which lies between Cape Town and Durban on South Africa’s eastern coastline. As part of her job, Marjorie received calls from fishermen coming in from the local waters with an interesting catch. The call that would change her life came on December 22, 1938, from Captain Hendrick Goosen, who had been fishing the waters of the Indian Ocean at the mouth of the Chalumna River. Marjorie came down to inspect the catch for any standout specimens and noticed a blue fin protruding beneath a pile of rays and sharks on the deck. Pushing the other fish aside, she came upon, as she would later write, “the most beautiful fish I had ever seen, five feet long, and a pale mauve blue with iridescent silver markings. It was covered in hard scales, and it had four limb-like fins and a strange puppy dog tail.”23

      Marjorie had never seen a fish like this before, and she sent a telegram with a crude drawing to Dr. James Smith, a local chemistry professor with a reputation as an amateur ichthyologist. Dr. Smith immediately saw the importance of this find and cabled back: “MOST IMPORTANT: PRESERVE SKELETON AND GILLS [OF] FISH DESCRIBED.” Because of his excitement, he cut two days off his vacation and went to East London, where he immediately identified the fish as a coelacanth, a ghost from the evolutionary past believed to have been extinct for sixty-six million years. It was named Latimeria chalumnae (from Marjorie’s last name and the name of the river it was caught in), and from studying it, along with another one caught a few years later, scientists clearly saw from its anatomy that the fish represented an early transition from the ocean to land. First, it had a structure in the thorax that could be described as a lung, only in the coelacanth it was filled with fat. Second, its four fins had cartilage in them, unlike the simple fins of modern fish, making them clear forerunners of our modern limbs. Being a bottom dweller, the coelacanth used its fins in sequence for crude locomotion on the ocean floor.

      The coelacanth caused an international sensation when it was “discovered” in 1938, but other species live among us that illuminate even more clearly the early development of lungs and legs. While the coelacanth has the beginnings of a lung, some fish have actual lungs. The most recognizable of these creatures are the mudskippers, three-and-a-half-inch fishlike creatures whose natural habitat is the muddy flats in the eastern part of Madagascar, as well as in parts of southern China and northern Australia. The beauty of the mudskipper lies not in its looks; in fact, its bulbous, puffy face and bulging eyes are naturally repulsive, its slimy body is off-putting, and its two fins, strangely placed on its back, appear pasted on in random fashion. But there is existential redemption for the mudskipper, because it has the remarkable ability to breathe in the water and on land. One minute it is happily swimming under water, and the next it’s jumping onto the land, aggressively defending its territory with mouth gaping open and fins aggressively flared out. To be able to do this, the mudskipper has retained its gills, but has also adapted to absorb oxygen through its skin, its mouth, and the lining of its pharynx (the area below the mouth but above the esophagus and trachea). It can stay above water for days, sequestering its gills and keeping them moist under a flap of retractable skin. It has also developed rudimentary forelimbs—small arms that can push its slimy body around over its muddy habitat.

      The mudskipper is not the only species to survive from this period of water-to-land transition four hundred million years ago. Amphibians, notably frogs, toads, and newts, can breathe using cutaneous respiration, in which blood passing past the skin picks up oxygen and releases carbon dioxide. Amphibians utilize this system both under water and on land. The Australian lungfish is another whisper from our evolutionary past. It is one of six remaining lungfish species, and the one that most effectively still straddles the worlds of the ocean and the air. Its demeanor is nonthreatening, and it has a long, olive-green, heavy snakelike body, small eyes, and four fins that help with propulsion both in water and on land. Size-wise it is not diminutive, averaging a healthy twenty pounds and measuring СКАЧАТЬ