Living cells that make their living inside other living cells
Inside the membrane of most of your cells are what look like little cells, with their own membranes and DNA. These little cell like things do important things, without which your cells couldn’t function, and you couldn’t live. The thing is, once upon a time, they really were cells in their own right, cells that one day moved inside of another cell, and never left.
Here are a couple of videos on the pearl fish and how it lives inside a sea cucumber.
Here’s a video about cleaner fish.
Here’s an article about African buffalo, and the birds that help keep them clean.
Here’s a video about several birds that have partnered symbiotically with other creatures.
Here’s an article about the origin and evolution of Chloroplasts, which do photosynthesis for most modern plants.
Here’s an article comparing Chloroplast, which moved into other cells over a billion years ago, to roughly the same thing happening again, much later, only 60-millean years ago.
And here are a couple of articles about the origin and evolution of mitochondria.