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.