Thrive: An open-source evolution game inspired by Spore ( revolutionarygamesstudio.com )

https://github.com/Revolutionary-Games/Thrive

This entertaining game aims to be a scientifically accurate evolution game, starting off as a single-celled organism and growing into more complex organelles, competing against other organisms in the environment.

https://store.steampowered.com/app/1779200/Thrive/

It is still in active development and later stages aren't fully fleshed out yet but the early stages can still provide a few hours of fun.