Mouth-First or Butt-First: Animal Development

That first opening in your embryo? It became your butt. In the animal kingdom, there are two main

developmental blueprints. For protostomes, like insects and snails, that first indentation

becomes the mouth. But for us deuterostomes, that first hole becomes the anus, and our mouth forms

second. This group includes humans, all other vertebrates, and even starfish.

This fundamental split is one of the key ways scientists map the vast tree of animal life.

From the very beginning, it all comes down to whether you're a mouth-first or a butt-first kind

of creature.