The History of Air

Paleontologists are looking to the fossil record to decipher what the earth's atmosphere was like hundreds of millions of years ago

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    The earth’s atmosphere is made up of a lot of nitrogen (78 percent), a bit of oxygen (21 percent), a splash of argon (0.93 percent), a small amount of carbon dioxide (0.038 percent) and trace amounts of other gases. But it has not always been so. The composition of gases in the atmosphere can change (and is changing now as we burn fossil fuels), and the fossil record reveals how something as deceptively simple as air can influence the history of life.

    If you visited what is now North America 300 million years ago, near the close of the Carboniferous period, you would have been greeted by a very unfamiliar scene. The landscape was dominated by vast swamps filled with huge lycopods (relatives of club mosses that grew to the size of trees), amphibious vertebrates up to nearly 20 feet in length and enormous arthropods. The Meganeura, a relative of the dragonfly that had a wingspan more than two feet across, buzzed through the air over the giant Arthropleura, a nine-foot-long millipede. Never before or since have terrestrial invertebrates grown to such prodigious sizes.

    The trigger for this rampant gigantism was a peculiar, newly evolved characteristic of plants that drove oxygen levels to as high as 35 percent of the atmosphere during the Late Carboniferous. Lush equatorial forests produced a considerable amount of oxygen as a byproduct of photosynthesis, but that alone wasn’t enough to drive atmospheric oxygen to such high levels. The cause was the chemical compound lignin, which plants use to build themselves up. Bacteria of the time were so inefficient at breaking down lignin in dead plants that they left behind a huge amount of carbon-rich plant material to become sequestered in the swamps (and eventually to transform into the rich coal deposits that gave the Carboniferous its name). Bacteria use oxygen as they break down carbon-rich material, but lignin prevented this process until bacteria evolved the ability to decompose the compound. This biological quirk caused oxygen levels to soar.

    The surplus of oxygen allowed amphibians, which take in some of the gas through their skins, to breathe more efficiently and grow to larger sizes. Arthropods breathe in a different way: they possess a network of branching tubes called tracheae that connect small openings in an invertebrate’s exoskeleton to its cells, and oxygen seeps through the body via this system. In an oxygen-rich atmosphere, more oxygen could be diffused through this branching network, and this opened up evolutionary pathways that allowed arthropods, too, to grow to gargantuan proportions. The fact that the oxygen would have increased the air pressure as well meant that the large flying insects of the time would have gotten more lift for each beat of their wings, allowing flying arthropods to reach sizes that are structurally impossible for their present-day relatives.

    While the giant arthropods were crawling and buzzing about, the first amniotes—lizard-like vertebrates that had broken their link with the water through their ability to reproduce via shelled eggs—were also diversifying. During the next chapter of earth’s history, the Permian (about 299 million to 251 million years ago), these early relatives of dinosaurs and mammals gave rise to a variety of new forms, with the relatives of early mammals (collectively known as synapsids), especially, gaining ecological dominance. For the first time, terrestrial ecosystems supported an interconnected network of predators and herbivores of various sizes, and by about 250 million years ago there were approximately 40 different families of land-dwelling vertebrates inhabiting the globe. But at the period’s close almost all of that diversity was extinguished by the greatest natural catastrophe this planet has ever known.

    During the early days of paleontology, naturalists marked off boundaries in geological history by the abrupt, mass disappearance of some species from the fossil record followed by the appearance of a new, different fauna. They did not realize it at the time, but what they were doing was marking off mass extinctions, and the one that ended the Permian was perhaps the worst in earth’s history. Up to 95 percent of all known sea creatures were wiped out, as were 70 percent of terrestrial animals. University of Bristol paleontologist Michael Benton has called this event “when life nearly died.”

    Identifying a mass extinction event is not the same as explaining it, however, and the catastrophe at the end of the Permian is perhaps the most puzzling murder mystery of all time. Scientists have proposed a list of possible extinction triggers, including global cooling, bombardment by cosmic rays, the shifting of continents and asteroid impacts, but many paleontologists’ prime suspect now is the intense eruptions of the Siberian Traps, volcanoes that covered nearly 800,000 square miles of what is now Russia with lava.

    The earth was much warmer at the end of the Permian than it is today. The atmosphere was relatively rich in carbon dioxide, which fueled a hothouse world in which there were almost no glaciers. The eruption of the Siberian Traps would have added vast amounts of greenhouse gases into the atmosphere, causing further global warming, increasing ocean acidity and lowering atmospheric oxygen levels. These drastic changes to the atmosphere and resulting environmental effects would have caused many organisms to asphyxiate from the lack of oxygen, while others would have died from an excess of carbon dioxide in the blood or otherwise perished because they were physiologically unable to cope with these new conditions. Where rich, diverse communities of organisms once thrived, the extinction left only “crisis” communities of a few species that proliferated in the vacant habitats.


    The earth’s atmosphere is made up of a lot of nitrogen (78 percent), a bit of oxygen (21 percent), a splash of argon (0.93 percent), a small amount of carbon dioxide (0.038 percent) and trace amounts of other gases. But it has not always been so. The composition of gases in the atmosphere can change (and is changing now as we burn fossil fuels), and the fossil record reveals how something as deceptively simple as air can influence the history of life.

    If you visited what is now North America 300 million years ago, near the close of the Carboniferous period, you would have been greeted by a very unfamiliar scene. The landscape was dominated by vast swamps filled with huge lycopods (relatives of club mosses that grew to the size of trees), amphibious vertebrates up to nearly 20 feet in length and enormous arthropods. The Meganeura, a relative of the dragonfly that had a wingspan more than two feet across, buzzed through the air over the giant Arthropleura, a nine-foot-long millipede. Never before or since have terrestrial invertebrates grown to such prodigious sizes.

    The trigger for this rampant gigantism was a peculiar, newly evolved characteristic of plants that drove oxygen levels to as high as 35 percent of the atmosphere during the Late Carboniferous. Lush equatorial forests produced a considerable amount of oxygen as a byproduct of photosynthesis, but that alone wasn’t enough to drive atmospheric oxygen to such high levels. The cause was the chemical compound lignin, which plants use to build themselves up. Bacteria of the time were so inefficient at breaking down lignin in dead plants that they left behind a huge amount of carbon-rich plant material to become sequestered in the swamps (and eventually to transform into the rich coal deposits that gave the Carboniferous its name). Bacteria use oxygen as they break down carbon-rich material, but lignin prevented this process until bacteria evolved the ability to decompose the compound. This biological quirk caused oxygen levels to soar.

    The surplus of oxygen allowed amphibians, which take in some of the gas through their skins, to breathe more efficiently and grow to larger sizes. Arthropods breathe in a different way: they possess a network of branching tubes called tracheae that connect small openings in an invertebrate’s exoskeleton to its cells, and oxygen seeps through the body via this system. In an oxygen-rich atmosphere, more oxygen could be diffused through this branching network, and this opened up evolutionary pathways that allowed arthropods, too, to grow to gargantuan proportions. The fact that the oxygen would have increased the air pressure as well meant that the large flying insects of the time would have gotten more lift for each beat of their wings, allowing flying arthropods to reach sizes that are structurally impossible for their present-day relatives.

    While the giant arthropods were crawling and buzzing about, the first amniotes—lizard-like vertebrates that had broken their link with the water through their ability to reproduce via shelled eggs—were also diversifying. During the next chapter of earth’s history, the Permian (about 299 million to 251 million years ago), these early relatives of dinosaurs and mammals gave rise to a variety of new forms, with the relatives of early mammals (collectively known as synapsids), especially, gaining ecological dominance. For the first time, terrestrial ecosystems supported an interconnected network of predators and herbivores of various sizes, and by about 250 million years ago there were approximately 40 different families of land-dwelling vertebrates inhabiting the globe. But at the period’s close almost all of that diversity was extinguished by the greatest natural catastrophe this planet has ever known.

    During the early days of paleontology, naturalists marked off boundaries in geological history by the abrupt, mass disappearance of some species from the fossil record followed by the appearance of a new, different fauna. They did not realize it at the time, but what they were doing was marking off mass extinctions, and the one that ended the Permian was perhaps the worst in earth’s history. Up to 95 percent of all known sea creatures were wiped out, as were 70 percent of terrestrial animals. University of Bristol paleontologist Michael Benton has called this event “when life nearly died.”

    Identifying a mass extinction event is not the same as explaining it, however, and the catastrophe at the end of the Permian is perhaps the most puzzling murder mystery of all time. Scientists have proposed a list of possible extinction triggers, including global cooling, bombardment by cosmic rays, the shifting of continents and asteroid impacts, but many paleontologists’ prime suspect now is the intense eruptions of the Siberian Traps, volcanoes that covered nearly 800,000 square miles of what is now Russia with lava.

    The earth was much warmer at the end of the Permian than it is today. The atmosphere was relatively rich in carbon dioxide, which fueled a hothouse world in which there were almost no glaciers. The eruption of the Siberian Traps would have added vast amounts of greenhouse gases into the atmosphere, causing further global warming, increasing ocean acidity and lowering atmospheric oxygen levels. These drastic changes to the atmosphere and resulting environmental effects would have caused many organisms to asphyxiate from the lack of oxygen, while others would have died from an excess of carbon dioxide in the blood or otherwise perished because they were physiologically unable to cope with these new conditions. Where rich, diverse communities of organisms once thrived, the extinction left only “crisis” communities of a few species that proliferated in the vacant habitats.

    Though these changes to the atmosphere greatly pruned the evolutionary tree 251 million years ago, they did not make the planet permanently inhospitable. Life continued to evolve, and levels of oxygen, carbon dioxide and other gases continued to fluctuate, spurring the climate from “hothouse” to “icehouse” states numerous times.

    The earth may now be entering a new hothouse era, but what is unique about the present is that humans are taking an active role in shaping the air. The appetite for fossil fuels is altering the atmosphere in a way that will change the climate, adding more carbon dioxide and other greenhouse gases to the mix, and these fluctuations could have major implications for both extinction and evolution.

    The earth’s present conditions are different enough from those of the Late Permian that a similar catastrophe is unlikely, but the more we learn about ancient climates, the more clear it is that sudden changes in the atmosphere can be deadly. A recent study led by biogeochemist Natalia Shakhova, of the International Arctic Research Center, suggests that we may be approaching a tipping point that could quickly ramp up the global warming that is already altering ecosystems around the world. An immense store of methane, one of the most potent greenhouse gases, lies beneath the permafrost of the East Siberian Arctic Shelf. The permafrost acts as a frozen cap over the gas, but Shakhova found that that the cap has a leak. Scientists aren’t sure whether the methane leak is normal or a recent product of global warming, but if current projections are correct, as the global climate warms, sea level will rise and flood the East Siberian Arctic Shelf, which will melt the permafrost and release even more of the gas. As more greenhouse gases build up, the planet inches ever closer to this and other possible tipping points that could trigger rapid changes to habitats all over the world.

    Perhaps the peculiar conditions that allowed giant arthropods to fly through air composed of 35 percent oxygen will never be repeated, and we can hope that the earth does not replay the catastrophe at the end of the Permian, but in fostering a hothouse climate our species is actively changing the history of life on earth. How these changes will affect us, as well as the rest of the world’s biodiversity, will eventually be recorded in the ever-expanding fossil record.


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    Comments (6)

    Steve, all you have to do is look up and see if Oxygen is denser than the other gases in our atmosphere. Than you'll know if a higher percentage of oxygen will increase air pressure. Also the article was saying that larger Flying arthropods would benefit from higher air pressure as it would make their method of flying more effective and so capable of getting the heavier body off the ground given the structure of their bodies/wings etc. NOT that it made all arthropods buoyant and less heavy.

    Posted by Beth on February 1,2012 | 12:51 PM

    An interesting article, however, I am curious as to the basis for the statement in the fourth paragraph that: "The fact that the oxygen would have increased the air pressure". Having never seen a reference to this anywhere else, I am not convinced that the higher percentage of atmospheric oxygen would result in substantially higher air pressure. Nor does it hold that the higher air pressure would allow for large sizes due to "structural" capabilities, unless these creatures somehow became buoyant.

    Please let me know if there are other references that might clear this up for me.

    Posted by Steve Hedrick on May 11,2010 | 09:19 PM

    An excellent hypothesis on the extinction on a Grand scale of life on this planet, it pretty well rounds off all the other theories the only one missing is Alien abduction.

    Nonetheless Patricia while you are rejoicing at the positive impact of your heritage please also remember the old story as follows..."drop a frog in boiling water and it will jump for freedom", slowly heat up the water and it will boil to death without moving, no this isnt a french recipe for our Amphibian friends but rather an analogy which may easily be compared with the current state of the environment on this planet and its current inhabitants. Having said that and based on research I have read, drastic (In our eyes) climatic changes are a part of this planet, which Patricia, America didnt build and America doesnt own, so hopefully we(The human race) will have enough collective intelligence and morality not to hasten our own demise, bearing in mind who will suffer, specifically our children etcetera etcetera

    Posted by Lindsay on May 3,2010 | 12:30 AM

    We are injecting gigatons (Giga means billions!) of greenhouse gases into the atmosphere at an accelerating rate. While scientists are cautious in projecting the results of their models and climate research, it is now almost certain these gases will have an effect rivaling or even surpassing any current natural phenomena like the Eyjafjallajökull volcano. Patricia, you allude to America's can do spirit in your comment. I would like us to harness that to face these problems now, instead of coming up with increasingly ridiculous reasons for stuffing our heads in the sand. I worry that people are hardening their opinions because of some ideological rigidity and blindness. The liberals, or environmentalists, or stuck-up scientists are for it, so it must be a lie. Climate change will strike at liberal and conservative, truck driver and scientist, hippie and square alike. This a problem that can only be solved if we all get involved.

    Posted by Ben on April 26,2010 | 05:33 PM

    Excellent article, but did you know that the Joggins Fossil Cliffs are the best place in the world to see fossils from the Carboniferous period?

    Inscribed on the UNESCO World Heritage List in 2008, the cliffs are renowned for having upright lycopsid trees still embedded in them, where they once grew 300 million years ago.

    Here, you can see footprints of those early creatures, including the arthropleura and the tetrapods, see fossils of dragonflies and reptiles, including the earliest known amniote; Hylonomus Lyelli.

    The fossil cliffs of Joggins are mentioned in On the Origin of Species by Darwin and continue today to reveal their mysteries, thanks to the Bay of Fundy tides, the highest in the world.

    For more information visit www.jogginsfossilcliffs.net

    Posted by Joggins Fossil Cliffs UNESCO World Heritage Site on April 22,2010 | 02:31 PM

    The Icelandic volcanic eruptions are no doubt not as threatening to our ecosystem as the Siberian Traps, which seem to have been the extinction trigger for the Permian era ecosystem. Nevertheless, the recent drastic effects of the Icelandic eruptions should serve as a reminder that it takes far more than car exhaust, coal-fueled power plants, and fireplace gasses to even approach the ecological effects of natural forces. We can not prevent massive volcanoes, continental earthquakes, and the normal extinction of many species during every ecological era. When our own extinction trigger appears, I have no doubt it will arrive as have other triggers: suddenly, drasticly, and obviously caused by a universally agreed upon source. Until then, I will rejoice in America's historical genius in creating prosperity, leisure, and progress for its citizens.

    Posted by Patricia White on April 22,2010 | 02:08 PM

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