Читать книгу Breath Taking - Michael J. Stephen - Страница 9

Chapter 1

Oxygen, Then Existence

The story of our need for breath goes back many millions of years. As each person’s biological life has a conception, gestation, and early, middle, and late stage, the same is true of Earth itself. Just as an infant coming into the world can flourish only when it has mastered breathing, Earth started flourishing only when some kind of breathing and oxygen use began.

The Earth has not always had oxygen in its atmosphere. Its early gases would have been toxic to most of the species alive today. But when oxygen first appeared, it changed the world radically. And, remarkably, we didn’t know how oxygen first came to envelop our planet until the 1970s.

The universe, namely all matter that we see before us in the form of stars and planets and everything else contained within their apparent space, is thought to have emerged some fourteen billion years ago. Almost certainly, in the single instant of the Big Bang’s explosion, the entire past and present matter of the universe burst into space and spread over the cosmos. Over time, various parts of the universe have expanded and cooled, with different solar systems springing up as stars exploded into violent supernovas and the leftover nebulae of gases condensed into solid matter.17

Our own solar system formed about 4.5 billion years ago. The other planets near us are basically rocky masses, but Earth is obviously different. Pictured from outer space, it appears as the aptly named Blue Planet, a cool, serene mixture of deep aqua oceans and swirling white atmosphere. It stands in stark contrast to the harshness of neighboring Mars, the Red Planet, or our own moon, white and barren.

But Earth came into being devoid of its beautiful oceans, lush green landscapes, and the give-and-take of evolution, life, and death. For the first four billion years of its existence, Earth fluctuated between extremes of heat and cold, its atmosphere a toxic mixture of nitrogen and carbon dioxide. And for the first two billion years of its existence, it had absolutely no oxygen in its atmosphere.

Oxygen is so important because of its ability to generate energy efficiently. Organisms derive energy from molecules called adenosine triphosphate (ATP), which are formed through cellular respiration. Without oxygen, cells, through a process called anaerobic fermentation, can still produce ATP—but only a measly two units from each molecule of sugar. This is highly inefficient compared to metabolism with oxygen, through which cells can produce thirty-six ATP units from each sugar molecule. Equipped with these extra units of energy, organisms are able to grow bigger, run faster, and jump higher. Without oxygen, the only living mobile organisms would be anaerobes, tiny creatures that are no match for this world’s oxygen consumers.

Thus, for the first few billion years of its existence, Earth contained no plants and no animals. Oceans formed shortly after Earth came into existence, as the planet cooled and atmospheric water vapor condensed, but the only life that they could sustain were small, single-celled anaerobic microorganisms. Then, about 2.5 billion years ago, oxygen slowly began to be deposited in the atmosphere. It took a long time to reach a level of significance, but finally, about a billion years ago, the oxygen sinks of the Earth, mostly iron deposited in rock, became saturated. Oxygen then began to build up in the atmosphere and in the oceans. Termed the “Great Oxygenation Event,” or GOE, this watershed precipitated an explosion of life, with ocean plants arriving about six hundred million years ago, and then later sponges, mollusks, fish, and finally terrestrial plants and advanced life.18

Figure 1: The Great Oxygenation Event: the natural history, over time, of atmospheric gases.

A single question remained for a long time, however: Where had all this oxygen come from? Something substantial must have happened for a whole new gas to transform the planet in such a unique way. The story of how we began to understand where oxygen came from, and how it changed the world, is an extraordinary tale of hard work, keen observation, and luck (a combination that likely describes many, if not most, scientific discoveries). It’s also a story that simply is not well known—but should be.

John Waterbury grew up in the Hudson Valley of New York but spent his summers in the coastal Cape Cod town of Wellfleet, Massachusetts. There, in the early 1960s, Waterbury wandered around the expanse of dunes that stretched into long beaches and looked out into the blue-green ocean water of the Atlantic. Not satisfied to remain on the shore, he took to the ocean in his Lightning dinghy racer. Surrounded by salt water and rolling waves, he was filled with a sense of wonder as his boat glided over the waters off Cape Cod.19

Waterbury’s first academic stop was at the University of Vermont, where he earned a degree in zoology in 1965. After graduation, his options were narrowed down to two. There was a research position at the Woods Hole Oceanographic Institution in Massachusetts, a mere forty miles up the Cape from his Wellfleet summer home. If he didn’t stay in academics, the draft awaited him, with a possible tour of duty in Vietnam. Not surprisingly, Waterbury chose Woods Hole. He spent four years there, studying nitrifying bacteria, little organisms that digest nitrogen-containing matter. Afterward, he enrolled in a doctoral program at the University of California, Berkeley, and spent a few years in Paris. He returned to the Oceanographic Institution in Woods Hole in 1975, this time to stay. At Woods Hole, Waterbury discovered how Earth had changed from a planet without oxygen, inhabited only by microscopic organisms, to one with oxygen, teeming with all sizes of life.20

During his doctoral studies at Berkeley, Waterbury found his passion in cyanobacteria, microorganisms that were known to colonize fresh water. More commonly known as blue-green algae, these organisms have properties more like those of plants than of bacteria. Foremost among these unusual properties is the ability to photosynthesize—to turn carbon dioxide and water into oxygen and carbohydrates. But in the 1970s, cyanobacteria were mostly known to colonize only small freshwater areas and were thought to have had a limited role in the Earth’s process of oxygen production. They were not talked about outside of a small academic circle, and no mention of them appeared in major oceanography textbooks.

After his doctoral studies, Waterbury settled into his job as a research scientist at the Oceanographic Institution. A primary mission in the field at the time was to study ocean bacteria, of which not much was known. Field trips were a regular part of the investigation, and in August 1977 Waterbury headed out on the research vessel Atlantis II to the Arabian Sea, the mass of ocean between India and Saudi Arabia known for having very high levels of inorganic nutrients and a rich marine life. His team’s mission was to analyze samples from the ocean using a new technology: epifluorescence microscopy. The goal was to establish typical levels of known bacteria in the ocean with this new technique.

The basics of epifluorescence microscopy are straightforward. Tags, made up of the building blocks of DNA, are added to a sample of water, where they attach themselves to corresponding parts of the DNA of bacteria, like puzzle pieces fitting together. Under the blue light of the microscope, these bacteria then fluoresce green from their newly attached tags. If no matching bacteria are present, the tags won’t be activated, and the view in the microscope will remain blank.

Before adding his DNA tags to the Arabian Sea water, Waterbury did one thing that all students are taught in science class, a mandatory step for every experiment at every level of science, from middle school classrooms to Nobel Prize–winning labs: he set up a rigorous control to ensure his results would be valid. Scientists know that controls are the backbone of all discovery. To find something abnormal, one needs to be able to see, and prove, the existence of what one thinks is normal. So, prior to adding the DNA tags, Waterbury analyzed an unaltered specimen of water from the Arabian Sea under the new epifluorescence microscope so that he would have a baseline for comparison.

Waterbury assumed he would see nothing unusual in the Arabian Sea water, but instead he was stunned. The blue light of the epifluorescence microscope went through the water, and a bright-orange fluorescence came shooting back out of the eyepiece. Because of his background in cyanobacteria, Waterbury recognized the orange light as the natural fluorescence of phycoerythrin, a photosynthetic pigment that works with chlorophyll to drive the all-important carbon-dioxide-to-oxygen-and-carbon reaction, making life on this planet possible. Cyanobacteria had never been reported to exist in deep-sea salt water, so this was a monumental new finding.

The initial discovery of cyanobacteria in the Arabian Sea was an introduction, but in order to be able to study saltwater cyanobacteria in depth, Waterbury knew he would have to grow them in culture. He tried for months, each time using a new medium and different nutrition to coax the cyanobacteria to replicate. But each time the same thing happened—within twenty-four hours the cells were all dead. Culturing these bacteria was a must if the study of saltwater cyanobacteria was going to advance. In order to succeed, Waterbury had to go back to basic environmental biology.

Ocean organisms and freshwater organisms behave very differently. Normally we think of ocean creatures as hardy and adaptable, and the ocean as a rough and wild place. Freshwater bodies, by comparison, seem quiet and tranquil, without sharks and stingrays and deadly jelly­fish. This is the human perspective. From the perspective of bacteria, the reverse is true.

The environments of freshwater bacteria and ocean bacteria are strikingly different. In inland bodies of fresh water, the temperature can vary widely, as can the amounts of nutrients and minerals. The summer and winter also produce very different living conditions in freshwater environments, which often host very different species depending on the season. By contrast, the ocean environment is exceptionally stable. The temperature variations are much smaller than they are in inland bodies of water, and the microenvironment of nutrients much steadier. Bacteria that thrive in a freshwater environment are what oceanographic scientists call “eutrophs,” organisms that can handle an abundance of nutrients and highly variable temperatures. Saltwater bacteria, “oligotrophs,” require lower levels of basic nutrients. So, somewhat contrary to intuition, saltwater bacteria are more sensitive, more fragile than their freshwater cousins.

Over the course of a vexing year, Waterbury came to understand this. He fastidiously scrubbed all his culture flasks and test tubes, making sure not even a microscopic amount of calcium or other substance was left over. He then calibrated his culture medium to precisely reflect the nano-amounts of nutrients he had measured in the ocean water. Finally, after a year of meticulous work, and much to Waterbury’s delight, cyanobacterium from the ocean started growing for the first time outside of their natural habitat. The discovery of the species Synechococcus was official.

The questions then remained: How much of this stuff is out there, and what is its habitat? From the end of a wooden dock in Woods Hole, Waterbury filled a few jars with salt water, a little murky but otherwise unremarkable. He looked at the specimen under his epifluorescence microscope; it was teeming with cyanobacteria.

The study of cyanobacteria exploded over the next ten years. Hundreds of different species were identified in almost every ocean habitat on Earth. We now know that blue-green algae inhabit any body of water that is warmer than five degrees Celsius, usually in massive numbers, so massive that Waterbury refers to them as “those little beasts.”

With their sheer numbers and diverse habitats, cyanobacteria are today recognized as the creatures primarily responsible for putting oxygen in our atmosphere. They do so through photosynthesis, the process by which plants, algae, and cyanobacteria capture sunlight and turn it into energy. The primary molecule that captures the sunlight is chlorophyll, which uses the energy from photons of light to drive the reaction of carbon dioxide and water to glucose and oxygen. The photosynthetic reaction also gives off energy that helps cyanobacteria convert atmospheric carbon dioxide to edible carbon, which is consumed initially by lower life forms but then carried up the food chain. This process makes cyanobacteria the source of a great deal of the food production on Earth. They are also responsible for a majority of Earth’s oil, natural gas, and coal, all of which derived from settled matter (dead cyanobacteria) that condensed at the bottom of the ocean over millions of years. Indeed, cyanobacteria as a group are the most abundant species on Earth, and one of the most important for the purpose of life.

We tend to associate the process of photosynthesis with plants, but almost certainly cyanobacteria did this first. It is thought that, millions of years ago, ancestral cyanobacteria paired with larger cells, in a process called endosymbiosis, and evolved to become chlorophyll-containing chloroplasts, which allowed the larger cells to perform photosynthesis. These chloroplast-containing cells eventually bound together to make the forerunners of present-day plants and algae.

The mastery of photosynthesis by cyanobacteria, and later by plants, is something, despite all our technological progress, we can still only marvel at. Humans figured out early how to burn carbon, but we still have not been able to produce it ourselves from carbon dioxide and light. If photosynthesis could ever be simulated artificially, it could be the golden key to solving our energy-production problems; it would also solve the problem of global warming, by making it possible to take carbon dioxide out of the atmosphere.

Looking back, we now know that the explosion of life during the Cambrian Period, some five hundred million years ago, was significantly fueled by rising oxygen levels in the atmosphere caused by oxygen production from cyanobacteria.21 Higher animal life forms would simply not be here without these little creatures, nor would most plant life forms.

Our lungs developed to utilize oxygen and efficiently drive our metabolic reactions. We are aerobic creatures, and if the lungs are our most important organ, then oxygen is the most important gas in the atmosphere. Anaerobes exist, but they are constrained by an inefficient method of energy production. With oxygen, the possibilities of the world opened up. Almost every living creature on Earth is reliant on some method of oxygen extraction, and John Waterbury and others in the field of ocean bacteria helped show us where all this life came from.

With the existence of the new gas in Earth’s atmosphere, the last five hundred million years of this planet have been radically different from the first four billion. The first period was marked by an absence of life, the second period by an abundance of it. The timing of those two appearances, of oxygen and life, is no accident. Oxygen is the life force, the source of life’s infinite possibilities.

Along with the rise in oxygen from cyanobacteria, plant life began to flourish around this time. It first occurred in the ocean, and then inexorably these plant forms made their way onto the scorched orange land mass that, at the time, was completely devoid of anything except rock. First shallow moss colonized the rock, and then slowly more advanced plant life established itself. Trees came later, which further increased oxygen levels.

In the oxygenated ocean, animal life became increasingly sophisticated. With more plants came more oxygen, and with that, worms, mollusk-like clams, and jellyfish came into being, using primitive gills or simple diffusion to extract oxygen from the ocean. Eventually, over the course of tens of millions of years, creatures made their way onto the land that had been colonized by plants. Insects, spiders, and worms were the first to take advantage of a nascent verdant landscape. But they couldn’t have made this remarkable transition without some kind of ability to utilize oxygen.

Worms have no functioning respiratory system. They derive oxygen 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 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 the Mars Oxygen In-Situ Resource Utilization Experiment, or MOXIE for short. The idea is to produce oxygen from carbon dioxide, much as a tree does, by using electricity to drive a reaction of carbon dioxide into oxygen. The design is already in place to put a small version of a MOXIE machine onto a land rover and send it to Mars, where it will be tracked to make sure it functions properly. The hope would then be to build a much bigger version, which would help create oxygen for fuel as well as for the atmosphere.24

Another idea for putting oxygen into the environment of Mars is to set up biodomes throughout the planet, and then bring microbes from Earth to do what they have been doing here for millions of years. The best candidate would likely be a species of cyanobacteria, and one that already lives under extreme conditions here. There is plenty of nitrogen, their natural fuel, on Mars for them to utilize. The biodomes would then be monitored for oxygen production, and if the experiment is successful, many more of the structures could be built.25

In order for this manufactured oxygen to remain close to the planet, a denser atmosphere will have to be created. Scientists think creating a magnetic sphere around the planet, a protective coat of electromagnetic waves much like the one that surrounds the Earth, will keep destructive radiation from the sun away and minimize the effect of solar wind. A physical shield emitting protective magnetic waves will have to be strategically placed between the sun and Mars. If successful, it will allow existing carbon dioxide and newly created oxygen to build up, both warming the planet and allowing air pressure to increase. The hope is that this will help melt the ice currently trapped in the polar caps of Mars, unleashing water onto the planet once again.

This may all sound like science fiction, but there is a reasonable expectation that terraforming will be successful, and within a few hundred years we may be able to live permanently on Mars. The problems are the atmosphere, lungs, and breathing, problems that have existed since the beginning of terrestrial life. The first time around, these issues were resolved by evolution; this time, engineering is needed.

17. G. Brent Dalrymple, Ancient Earth, Ancient Skies: The Age of Earth and Its Cosmic Surroundings (Stanford, CA: Stanford University Press, 2004).

18. Bettina E. Schirrmeister, Muriel Gugger, and Philip C. J. Donoghue, “Cyanobacteria and the Great Oxidation Event: Evidence from Genes and Fossils,” Palaeontology 58, no. 5 (September 2015): 769–785.

19. John Waterbury, in discussion with the author, July 2015.

20. John Waterbury, “Little Things Matter a Lot,” Oceanus Magazine, March 11, 2005, https://www.whoi.edu/oceanus/feature/little-things-matter-a-lot/.

21. Christopher T. Reinhard, Noah J. Planavsky, Stephanie L. Olson, et al., “Earth’s Oxygen Cycle and the Evolution of Animal Life,” PNAS 113, no. 32 (August 9, 2016): 8933–8938.

22. Michael Melford, “Devonian Period,” National Geographic website, accessed July 31, 2019, https://www.nationalgeographic.com/science/prehistoric-world/devonian/.

23. Keith S. Thomson, Living Fossil: The Story of the Coelacanth (New York: W. W. Norton, 1991), 19–49.

24. National Aeronautics and Space Administration, “Mars Oxygen In-Situ Resource Utilization Experiment (MOXIE),” NASA TechPort, accessed July 31, 2019, https://techport.nasa.gov/view/33080.

25. National Aeronautics and Space Administration, “Planting an Ecosystem on Mars,” NASA website, May 6, 2015, https://www.nasa.gov/feature/planting-an-ecosystem-on-mars.