About 3.5 - 3 billion the first cell formed.
Then about 100 million years later we find that photosynthesis occurs then creating large quantities of oxygen start to appear.
Then about 900 million years later lifeforms begin to utilise oxygen
Then about 600 million years B.C.E the first multicellular organism formed , it was a sponge like creature
Some time after Cnidaria formed as the first creatures to possess nerves and muscles.
The only reason you have a brain is because you need to move.
Flatworms were the first creatures to have a brain , they needed this brain to be able to move in the advanced manner that they did.
Acorn worms introduced the heart and circulatory system and a kidney with gills.
Through the advances made by the chorodates we get to tetrapods.
Panderichthys was the first tetrapod it is a 60 - 90 cm fish with arm like fins it was a vertabrae (the first vertabrae were the ostracoderms).
I guess you could call this a fish