In many a time-travel story, little attention is paid to the local ecology of the destination of the intrepid travelers. If you’re headed back to visit Leonardo da Vinci, this probably isn’t going to be a problem, but if you’re headed back much further, like the Carboniferous or Permian, you might run into some trouble. Indeed, nothing could kill such a “time safari” quicker (like the one in L. Sprauge de Camp’s “Crocamander Quest,” collected in The Ultimate Dinosaur) than stepping out of the time portal/capsule and not being able to breathe. Fanciful notions of time travel aside, we can tell something about the ancient atmosphere without having to actually step foot there, and it has some very interesting implications for evolution and ecology.
One of the defining features of the Carboniferous is that things got big. Even though they may have seemed small compared to titanosaurs or mammoths, a 3-foot-long millipede and giant dragonflies are nothing to belittle. Indeed, terrestrial arthropods in all sorts of groups seemed to achieve sizes unheard of today. Why should the Carboniferous be dominated by giant arthropods?
Well, let’s think about the name of the time period we’re talking about: Carboniferous. Carboniferous deposits, sandwiched between the Devonian and Permian, are well-known for containing coal. The coal was originally laid down by the thick plant growth that marked changes in terrestrial ecology, and although the first plants (likely resembling liverworts) colonized the land as early as the Ordovician, the Carboniferous marked the development of lignified bark (“brown coal” is called lignite), which may have led to the huge amounts of coal deposited as bacteria needed some time to evolve a way to break down this new material. Due to Devonian drops in sea level, plants were also had more space to grow, so there was a greater number of swamps and forests produced, filling up with plant and insect life. The changes in global fauna had a major impact on the atmosphere, not only producing more oxygen due to photosynthesis, but the burial of so much biologically produced carbon also increased the amount of oxygen in the atmosphere, perhaps being as high as 35%! (It’s at about 21% today)
Another important piece of the puzzle is that nitrogren levels did not drop, and so not only was there hyperoxic conditions due to plant evolution, but the air pressure was increased as well, and these two factors probably had a lot to do with the evolution of insects flight and insect gigantism. When there is more oxygen in the air, organisms that diffuse oxygen over their trachea are able to obtain more oxygen with less effort, and it doesn’t take as much energy to become large being oxygen is so “cheap.” Likewise, increased air pressure makes flying less costly in terms of energy, as wing size has to go up with decreasing air pressure in order to keep flying animals aloft (as is that case with hummingbirds that live at high altitudes). Not only was it easier to grow large, but it didn’t take as much energy to fly, resulting in the giant dragonflies like Meganeura. In a 1998 paper, Dudley hypothesized that early tetrapods would have taken advantage of the extra oxygen as well, being that they would have “breathed” through their skin. This hypothesis is largely refuted, however, (see the comment of johannes below) and size of early tetrapods does not seem as dependent on the atmospheric oxygen make up, although it likely could have made oxygen intake for amphibious tetrapods more efficient overall.
There are costs to be adapted to an atmosphere with high oxygen content and high atmospheric pressure, however. Part of the cost is the buildup of metabolic “wastes” as a result of taking in so much oxygen, and unless there’s also an increase in enzymes to take care of these wastes, organisms may have led shorter lives. Experiments in the lab with fruit flies, however, have shown that an increase in enzyme production to combat the harmful metabolic products can be selected, and it’s reasonable to think that Carboniferous organisms underwent such selection naturally. Still, atmospheric oxygen levels began to sharply drop at the beginning of the Permian, and by the beginning of the Triassic atmospheric oxygen levels were as low as 15%. Creatures adapted to higher oxygen content and pressure would need to be adapted quickly again if they were to survive, and size/ability to deal with lower oxygen levels probably made a big difference in the great Permian extinction.
Oxygen levels were not always to remain low, however, and it has been hypothesized that hyperoxic conditions (which means more oxygen in the air plus heightened pressure) might be correlated with the evolution of flight, or at least evolution of larger flying organisms. When it’s less expensive to obtain oxygen and less expensive to fly, it would be much easier to develop flight, and there may be correlations with such atmospheric conditions and the evolution of bats, birds, and pterosaurs. Testing still needs to be done (and the more fossils we can get, the better), but if nothing else, the fluctuations in atmospheric oxygen content show us the importance of considering ecology in terms of evolution and extinction.
Dudley. “ATMOSPHERIC OXYGEN, GIANT PALEOZOIC INSECTS AND THE EVOLUTION OF
AERIAL LOCOMOTOR PERFORMANCE.” The Journal of Experimental Biology 201, 1043–1050 (1998)