Simulation granularity

I had to decide on what part of evolution I wanted to simulate. On what level of granularity.
Start at Big Bang? Base it on cellular automata, and build from there? Something else?
Both of those would be fun exercises, but I decided to start with something that would be fun to interact with in a game, where players could emphasize with the playable characters. Something different from what we know, but still close to home.