Every piece you add or take away from your creature changes the way you play. In the first phase of the game, adding fins gives you a sharper turning radius. Spikes allow you to harm other creatures or pop bubbles that might otherwise get in your way. During our test game we discovered a pretty high-level upgrade early on, electricity, so we decided to go for it. Any creature that tried to eat us got a shock. (Some stronger predators, we discovered, took the pain and ate us anyways.)
Happy with our new creature, we hatched from an egg and began the cycle anew. It doesn't matter how many times you evolve -- your ultimate goal in the first phase of the game is just to eat enough food, as depicted on a progress bar at the bottom of the screen. You gradually grow bigger as you eat food, and periodically the camera pans back as you pass a new threshold and find yourself able to eat bigger food. Creatures that were once a threat are now too small to hurt you... but now even bigger creatures decide you're big enough to be a tasty treat. There's a great ebb and flow to the early game.
We stuck out the challenge of being a pure herbivore, and -- after a few evolutions -- we'd proudly created a zippy little plant-eater that could flee any predator. Fins gave us superior handling and a water jet on our butt allowed us to zoom through the seas faster than any predator. We were literally scooping food out of other creatures' mouths. It was pretty clear that there were other strategies, but this was the one that worked for us. Before we knew it, we'd filled up the bar at the bottom of the screen, and had the option of evolving into an amphibian and leaving the seas behind.
Clicking the button brought up a whole new creature editor, this one a fully-realized 3D editor much like the one released as the Creature Creator. All of our creature's current parts were there, but now we could add legs. We gleefully did so.
So long, soup! We were ready for the big time.
