A few months ago, Derek Thompson wrote about one of biology’s biggest mysteries: sleep. A lot of important processes happen when we sleep, but none of these explain why sleep originated. Now, a new study reported in the Economist offers evidence that sleep exists because of volcanoes, oxygen, and the need to repair damage from electron imbalances in mitochondria.

After writing about the studies, Derek writes this beautiful take:

Achilles’ mother dipped her son in the River Styx to make him immortal, but, holding him by the foot, left his ankle dry and vulnerable. So did oxygen anoint Earth with a blessing second only to immortality — complex life, which became consciousness and intelligence. But there was a price: a kind of daily death. Rest restores our ability to burn the planet’s oxygen for energy. Why do we sleep? Because the earth once learned to breathe.

It’s a poetic thought that connects something that we take as given in our day to the origins of life on Earth creating the oxygen-rich atmosphere we breathe every day.