After last week’s perusal of 4-winged dinosaurs, my labmates and I got into a bit of a competition to find the most outlandish prehistoric animal, which led us to meter-long millipedes, 2-foot mayflies, and something called Hallucigenia. What most captured our attention was that arthropods in the Carboniferous period were terrifyingly enormous. 50lb scorpions? GAAAAHH!!
But then we realized the Carboniferous (about 350-300 million years ago) was the same period that gave us 50-foot horsetails and house-sized ferns, and we paused long enough to ask ourselves: “Wait, hang on. Why was EVERYTHING so darned big back then?”
Keep in mind, we are but lowly developmental biologists, and don’t routinely think about things like Paleozoic atmospheric composition. We don’t usually even think about modern atmospheric composition unless the CO2 supply on one of our cell incubators goes bust. But this was a quandary. We all remembered that the reason 1950’s era monster movies about giant man-eating insects aren’t real is because insect trachea can’t efficiently supply atmospheric oxygen over a long distance. So giant bugs means there must have been more oxygen. No problem.
But then why were the plants so big? Wouldn’t plants need a lot of carbon dioxide instead? Didn’t the Carboniferous get named because there was so much carbon getting bound up in trees that ended up as coal? In short, how do you get conditions that favor gigantism in plants AND animals?
And that’s how I ended up digging up this 1998 paper about ancient oxygen and carbon dioxide levels (it’s a bit old, so I would welcome updates by any geologists or paleontologists out there). The story goes something like this:
Long, long ago (500 million years or so), there was a ton of carbon dioxide in the air. As much as 0.5%, which is more than 10 times as much as we have now (0.036%). At the same time, oxygen levels were lower: about 15%, versus 20.9% today. Along comes the Silurian period (roughly 400 million years ago), and plants figure out how to live on land. They go bonkers for all the carbon dioxide, and throughout the following Devonian period, giant swampy forests of mosses, ferns, and horsetails sweep across the land, gobbling up the carbon dioxide and belching forth great quantities of oxygen in its place.
By the time the Devonian gives way to the Carboniferous (~350 million years ago), carbon dioxide levels have plummeted to about 0.1% (still about 3 times as high as the modern era, at least before the Industrial Revolution), and oxygen levels have skyrocketed to nearly 35% (about 1.5 times as much as now–any higher and the atmosphere would literally have combusted).
All the dinky little arthropods that have been poking around since the Cambrian get a big, deep breath of this hyperoxic air and erupt into Sci-Fi proportions. On an interesting side note, all the extra oxygen offers a major boost to those insects starting to experiment with flight, because insect flight muscles burn up oxygen like a blowtorch. Giant mayflies and dragonflies abound.
- 2 inch dragonflies are charming. Waist-high Carboniferous dragonflies, like this Meganeura fossil, are fairly intimidating. Photo from Paleontology online, by Ben Slater.
But what about the plants? Don’t they shrivel up in the newly hyperoxic environment? Not at all. Remember, carbon dioxide levels are still high (relative to today). And just like animals, plants still need oxygen for cellular respiration–all the glucose they make with photosynthesis is no good to the cells if they can’t turn it into ATP, so extra oxygen is just dandy. Finally, around the end of the Devonian period, plants caught on how to make lignin, a major structural support molecule of wood, which together with the vasculature they developed in the mid-Devonian lets them reach greater heights than ever before. As they die, their carbon-rich remains pile up with nowhere to go, eventually becoming vast coal deposits to fuel the energy demands of some clever greedy apes far in the future.
In the intervening 300 million years, atmospheric composition continued to fluctuate. Oxygen levels plummeted in the Permian period just after the Carboniferous, and all the giant athropods disappeared for a while. In the Jurassic, conditions became hyperoxic again, and perhaps it’s no coincidence that not only did enormous mayflies make a comeback, but this is also where we begin to find the gigantic herbivorous dinosaurs like Apatosaurus and Brachiosaurus, as well as flight innovation in birds.
It’s noteworthy that the historic peaks of atmospheric oxygen correlate so tidily with animal gigantism. This seems to suggest that in the absence of other selective pressures, bigger is always better. Maybe all it would take to generate a man-eating grasshopper isn’t a pool of radioactive waste, but a few patient generations of tinkering with a large hyperbaric chamber and a hospital’s worth of oxygen tanks. Just give me a minute and I’ll get right on that…
Dudley R (1998). Atmospheric oxygen, giant Paleozoic insects and the evolution of aerial locomotor performance. The Journal of experimental biology, 201 (Pt 8), 1043-50 PMID: 9510518