Now that you've developed into a tube, some parts of your body are a long way from some other parts. You already have nerves and sensory apparatus, but it would be useful to have some decent plumbing to get signals from one end to another.
As you develop, you are comprised of three layers of cells - the endoderm, mesoderm and ectoderm, which become, roughly, skin, muscle, and gut respectively. You need to make a new kind of tissue to become a communications hub. It can develop from either the mesoderm (muscle layer) or the endoderm (gut layer).
Muscle.
Gut.
Evolution starts with the Last Universal Ancestor.
You can find out what happened before evolution here.
You can also short-cut to Land plants, or Animals
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