ATMOSPHERE × LIFE

The Age of Giant Insects

~330 to 260 million years ago

Phanerozoic · Paleozoic · Carboniferous

Coal swamps buried carbon faster than decay could hand it back, and the oxygen left over built up to the highest level in Earth's history, somewhere near a third of the air instead of today's fifth. Dragonfly relatives with wingspans reaching seventy centimetres flew through those forests. With so much oxygen, wildfires may have been more frequent, and at times catastrophic.

Why it matters

Life set the composition of the air, and the air then set a ceiling on how large a body could get. Measured across more than ten thousand fossil wings, the size of the largest insects tracks oxygen for a hundred and fifty million years and then stops following it: from the Early Cretaceous the biggest wings shrink while oxygen rises, in step with the spread of early birds. After that, the limit on an insect is not the air. It is what eats it.

Dating & uncertainty

Models built from the carbon and sulfur record put atmospheric oxygen near 30 to 35 percent in the late Carboniferous, well above today's 21. The link to giant insects is plausible but not settled: an analysis of insect wing lengths finds that maximum size tracks oxygen only until the Cretaceous, after which the radiation of birds accounts for it better.

Sources

1. Berner et al. (2003), Annual Review of Earth and Planetary Sciences · Phanerozoic atmospheric oxygen
2. Dudley (1998), Journal of Experimental Biology · Atmospheric oxygen, giant Paleozoic insects and the evolution of aerial locomotor performance
3. Clapham & Karr (2012), PNAS · Environmental and biotic controls on the evolutionary history of insect body size

Earth history as one day

22:15:191 h 45 min before midnight

if Earth’s 4.54 billion years were compressed into a single day