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 four feet in length. It inhabits the shallow, muddy fresh waters of Queensland, in northern Australia, an isolated, quiet place of sequestered species, seemingly frozen in time. At 370 million years old, the Australian lungfish also gives off the scent of the prehistoric, as if it would be at home dodging the bite of a pterodactyl or the sweeping jaws of a crocodile.
The lungfish’s use of oxygen is impressive, as it can alternate between what a fish would do under water and what a land creature would do above. Unlike the mudskipper, the lungfish has a proper lung, with appropriate gas exchange units, not just the simple diffusion of air through a membrane. It can live several days above the water, where it feeds off plants that otherwise would be inaccessible. The lung also comes in handy when the water in the fish’s natural swamp habitat runs low.
What the coelacanth, the mudskipper, and the Australian lungfish offer us is a fascinating window into our past, one that shows us how species experimented with different modes of oxygen extraction. Without oxygen and a mode of extraction, we would not exist, nor would most of the species around us.
The intersection of our existence, oxygen, and the breath is interesting not just as a story, but as a roadmap that points us in the direction of the future. Prominent scientists have warned us that life on this planet is fragile, that at any moment an asteroid or nuclear war could wipe us all out. They warn that the fate of mankind, and indeed of all species, may someday rest on our ability to get off the planet.
In order to do so, we must of course accommodate the lungs. We are now struggling again, some four hundred million years later, with the challenge, successfully confronted by the mudskipper, coelacanth, and lungfish, of learning to survive in an inhospitable environment. Unfortunately, we can’t change our organ of energy extraction as they did, but we can try to change a toxic atmosphere to one that is more hospitable.
The first planet to be considered for colonizing is Mars, and the engineering process of making that planet’s atmosphere hospitable is called terraforming. Numerous obstacles are present, including the extreme low temperature and lack of gravity compared to the Earth. An even bigger issue is the atmosphere of Mars, which consists of 95 percent carbon dioxide, 2.7 percent nitrogen, 1.6 percent argon, and only 0.13 percent oxygen. The air is also extremely thin, about one hundred times less dense than Earth’s. So somehow we are going to have to make the atmosphere more dense, and fill it up with oxygen.
One plan NASA is developing is called