In the swampy forests of the Carboniferous Period, 360 to 286 million years ago, dragonflies with two-and-a-half-foot wingspans darted among the giant ferns. Mayflies grew to canary size. Cockroaches appeared suddenly (as cockroaches do) for the first time. The number of insect families increased from 1 or 2 to more than 100 during the Carboniferous, and many of the insects were huge, and no one has been able to say exactly why. Jeffrey Graham, a researcher at the Scripps Institution of Oceanography, has been pondering the problem since childhood. I remember seeing models of giant dragonflies as a child and wondering how they could fly, Graham recalls. Now Graham, zoologist Carl Gans of the University of Michigan, and their colleagues think they have an answer. The flight of the giant dragonfly, they say, along with the whole Carboniferous insect explosion, may have been made possible by an oxygen-rich atmosphere.
Graham and Gans’s idea is a hypothesis based on a hypothesis. Six years ago, Yale geochemist Robert Berner first suggested that the atmosphere in the Carboniferous was more oxygen rich than at any time before or since--it was 35 percent oxygen, Berner estimated, compared with 21 percent today. Berner attributed this to the rise of land plants in general and in particular to the vast and verdant swamps that characterized the Carboniferous. All those swamp plants spit oxygen into the atmosphere, and when they died, they escaped the open-air decomposition by bacteria that would have drawn oxygen back out of the atmosphere. Instead they sank into the swamps, ultimately forming the coal deposits that gave the Carboniferous its name.
There is no direct evidence of atmospheric oxygen levels 300 million years ago; Berner’s hypothesis is based on a computer model. But if there was extra oxygen around, says Graham, it was like a vitamin. It was an ecological and evolutionary resource that animals utilized to enable them to do more. The most spectacular beneficiaries, he thinks, were the insects. For one thing, the oxygen-rich atmosphere was a denser atmosphere that provided more lift and thus made it easier for them to fly. The real explosion of flight occurs in the Carboniferous, Graham says. Wings were becoming better, more proficient, and flight was being perfected.
More important, the excess oxygen made it easier for insects to breathe. Unlike humans and other vertebrates, insects do not have a circulatory system that actively transports oxygen to cells. Instead oxygen diffuses passively into their tissues through branching tracheae that connect each and every cell to pores in their skin. This limits an insect’s size, because the oxygen can only diffuse so far in a given amount of time. But in an atmosphere that was 35 percent oxygen, the gas would have diffused faster--thus enabling Carboniferous insects to grow larger. Meganeura monyi, the dragonfly with a wingspan of two and a half feet, had a body that was over an inch thick. The largest dragonfly today has a wingspan of only six inches and a body skinnier than a pencil.
The oxygeniferous Carboniferous was a golden age for other life- forms as well, says Graham. Ferns and other pre-trees grew enormous, he says, because plentiful oxygen made it easier for them to manufacture lignin, their main structural material (and later the main component of coal). And the beginning of the Carboniferous was also when our earliest four-footed ancestors hauled themselves out of the swamps and onto dry land. As they learned to carry their full weight without the help of water and to breathe with feeble lungs instead of gills, drawing in 35 percent oxygen with each gulp would have lightened their burden considerably. By reducing the number of times they had to exhale, it would also have helped them avoid dehydration.
The evolutionary explosion that began in the Carboniferous ended in the ensuing Permian Period. At the close of the Permian, around 250 million years ago, an estimated 95 percent of all species on Earth went extinct, including the giant flying insects. Various causes have been suggested for the mass extinction, from climate change to asteroid impacts. But it’s also true that the atmospheric oxygen level began to decline at the start of the Permian; by the time of the extinction, it had fallen to around 15 percent. That may have been a tough change to adapt to. It may be, Gans speculates, that only the things that developed when the going was easy because of the high oxygen concentration got into trouble later on.