Dragonflies were once the size of hawks because the air was thick

Around 300 million years ago, the Earth was crawling with monsters. We aren't talking about dinosaurs, but griffinflies—insects that looked like dragonflies but had wingspans reaching nearly 28 inches. Parallel to them, millipedes the size of kayaks skittered across the forest floor.

Insects don't have lungs; they breathe through tiny holes in their sides called spiracles. This system relies on passive diffusion, which limits how far oxygen can travel into a body before it peters out. In today's air, that physical limit keeps bugs small. But during the Carboniferous period, oxygen levels were about 35 percent, compared to our measly 21 percent today.

This oxygen-rich atmosphere allowed insects to grow to terrifying proportions while still getting enough fuel to their internal tissues. It essentially lowered the barrier for physical growth, turning the entire planet into a high-pressure oxygen tank. As oxygen levels eventually dropped, these giants literally suffocated under their own weight, leaving only their smaller cousins behind.

It is a reminder that the very chemistry of our air dictates the architecture of life. If our oxygen levels spiked again, the evolutionary ceiling for creepy-crawlies would vanish, and the pests in your garden could theoretically reclaim their throne as apex predators.