Bacteria were now the only form of life on Earth, but it couldn't be like this forever. Many new organisms appeared, but only one could beat the others with its intellect. Those creatures were...
Small ones like these: Big ones like these:
All marine beings, big and small had the same level of inteligence. They just needed a brain that could tell them to eat, hide, breed etc.
Here is a picture of the Brain of the fish:
It was a brain which could control the basic functions of the organism. It wasn't too evolved, but it was the most advanced brain back then.