ARCHIVE.

Inside many of your cells are tiny structures that carry their own DNA, separate from the DNA in the cell's nucleus. They are the descendants of bacteria that were swallowed more than a billion years ago, never got digested, and never left.

Take sea otters away from a stretch of coast and the kelp forest can vanish, leaving bare rock carpeted in spiny urchins. The odd thing is that the animal holding the forest up never eats the kelp at all.

Run your own fingers along your ribs and almost nothing happens. Let someone else do exactly the same thing and you squirm. The difference isn't in your skin. It's in a quiet forecast your brain makes every time you move.