Recently I finished The Invention of Air by Steven Johnson, which is a book about the English scientist Joseph Priestley, who is best known as the discoverer of oxygen. Johnson shows how Priestley had a strong influence on both science and politics (he was a close friend Jefferson and Franklin). But Priestley also sat at a historical confluence that was conducive to, basically, Enlightenment thinking, and Johnson ties together many threads in a way reminiscent of James Burke: coffeehouses and efficient postal delivery, which fostered open and fast communication; innovations in scientific technology, which let Priestley engage in the experiments he did; the wealth of the industrial age, which indirectly provided Priestley with the time to do research; and so on.
At times the chains of connections go quite far indeed -- for example, from Priestley's simple experiment with a mint plant all the way to the field of planetary ecology. A continuing theme is energy: sunlight to feed plants, coffee to feed scientific minds, oxygen to feed animals, coal to feed the industrial revoltuion, and so on. To discuss these last two, Johnson takes a side trip way back in Earth history to the Carboniferous era, where he tells the following story.
Many of the fossils that Brongniart uncovered shared a defining characteristic: compared to their modern equivalents, they were massive. He discovered ferns the size of oak trees, and flies as big as birds. In 1880 he unearthed his most startling find: a monster dragonfly (Meganeura) with a wingspan of 63 centimeters [2 feet]. Subsequent fossils have been discovered with a wingspan of more than 75 centimeters.
Meganeura was not alone. Paleontologists worldwide soon discovered that giantism was a prevailing trend between 350 and 300 million B.C., a period now called the Carboniferous era. Like some strange Brobdingnagian natural history exhibit, the landscape of the Carboniferous was populated by foot-long spiders and millipedes, and water scorpions the size of a small boy. The plant life was even more spectacular. Club mosses growing in damp forests towered above the swampland below, reaching heights of 130 feet, five hundred times taller than their modern descendants. Horsetails and rushes that now top out at around four feet regularly reached the height of a five-story building. Early conifers sprouted leaves that were more than three feet long.
The planetary fad for giantism didn’t last. The dinosaurs evolved immense body plans in the coming ages, of course, but by 250 million B.C., the rest of the biosphere had largely retreated back to the scale we now see on earth. But that pattern was distinct enough that it presented a tantalizing mystery: just as the Cambrian explosion raised the question of why life suddenly grew so diverse, the Carboniferous age raised the question of why life suddenly grew so big and how it managed to survive with such exaggerated proportions. Meganeura shouldn’t have been able to fly, given its size. The respiratory systems of modern insects and reptiles wouldn’t be able to generate enough energy to support a body plan that was ten times their current size. And yet somehow the giants of the Carboniferous managed to thrive in that exaggerated state for a hundred million years.
The "natural" level of oxygen on Earth was less than 1 percent; the 20.7 percent levels we enjoy as respiring mammals was an artificial state, engineered by the evolutionary breakthrough that began with cyanobacteria billions of years ago. [i.e., photosynthesis] The scarcity of oxygen before the evolution of plant life suggested one follow-up question: why had oxygen levels stabilized at around 20 percent for so many millions of years? Were it to drop to 10 percent, most of aerobic life would suffocate; were it to double, the combustion reactions of oxygen would engulf the planet in a worldwide inferno. So what mechanism allowed the atmosphere to regulate itself with such precision?
[Robert Berner and Donald Canfield researched atmospheric oxygen levels going back 600 million years.]
[Their data showed that] oxygen levels had been relatively stable for the last 200 million years. But the most startling finding came before that long equilibrium. The data showed a dramatic spike in oxygen levels, reaching as high as 35 percent around 300 million B.C., followed by a plunge to the borderline asphyxia of 15 percent in the Triassic era, 100 million years later. The oxygen pulse overlapped exactly with Meganeura and other giants of the Carboniferous.
Since then, dozens of paper have explored the connection between increased oxygen content and giantism, and the growing consensus is that higher oxygen concentration would support larger body plans in reptiles and insects. And the increase in atmospheric pressure that accompanies 35 percent oxygen levels would even alter the aerodynamics enough to allow Meganeura to take flight.
Where did all that oxygen come from? From plants, of course. First, the plants invented the photosynthetic engine that created an oxygen-rich atmosphere billions of years ago. But at some point near the end of the Devonian age, the plants evolved the ability to generate a sturdy molecule called lignin that gave them newfound structural support, allowing them to grow to sizes never seen before on Earth. Larger plants alone might have led to an oxygen increase, but lignin may also have had a more indirect role in the spike. One popular but unproven theory argues that lignin confounded the microbial recyclers responsible for the decomposition of organic matters. Plants absorb carbon dioxide and produce oxygen through photosynthesis; decomposition plays that tape backward, as bacteria and other animals use up oxygen in breaking down the plant debris, releasing carbon dioxide in the process. Lignin may have disrupted that cycle, because the recyclers had not yet evolved the capacity to break down the molecule, creating what the paleoclimatologist David Beerling calls an episode of "global indigestion." With the decomposers handicapped by lignin’s novelty, immense stockpiles of undecomposed biomass filled the swamplands and the forest floor, and the oxygen levels climbed even higher. Oxygen would not return to the 21 percent plateau until the microbes cracked the lignin code, millions of years later.
But the debris accumulated during the age of Meganeura did not disappear from the geologic record. It simply went underground. When it ultimately resurfaced, it would transform human history every bit as dramatically as it transformed natural history the first time around.
Update 2 Nov 2010: Interesting post on the Wired blog about how some biologists raised larger-than-normal dragonflies by keeping them in an oxygen-rich environement.
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