Giant dragonflies used to rule the skies

The Carboniferous period was a nightmare for anyone with entomophobia. Dragonflies called Meganeura had wingspans reaching nearly thirty inches. Scorpions grew to the size of dogs, and millipede-like creatures called Arthropleura stretched up to eight feet long. This wasn't because they were higher on the food chain, but because the Earth was essentially on an oxygen bender.

Insects don't have lungs. They breathe through tiny tubes called tracheae that passively diffuse oxygen into their tissues. This system is efficient for small creatures, but it fails at larger scales because oxygen can only travel so far into a body before it's used up. Today, our air is about 21 percent oxygen, which keeps bugs small. But back then, a massive explosion of plant life pushed oxygen levels to a dizzying 35 percent.

With that much fuel in the air, the diffusion limit expanded. Insects could grow to terrifying proportions while still getting enough oxygen to power their muscles. It turned the entire planet into a high-pressure nursery for giants. Once oxygen levels dropped, these behemoths literally couldn't breathe anymore and shrunk or vanished.

It's a stark reminder that the size of life isn't just determined by genetics; it's dictated by the chemical limits of the room we're standing in. If oxygen levels remained high for thousands of years, the descendants of the spiders in your basement could eventually evolve to be much larger.