When animals evolved, they first looked like big bag of cells. The thing that made them an animal was all those cells working together to do different things, so tissues like skin were the first thing to appear.

As you get closer and closer to humans, you evolve symmetry (why your right side looks like your left side) a head, arms and legs (the early animals had lots!), and then later a spine. We are related to all animals with spines, and we are really only different from things like horses by having different sized bones and brains!