Just delurking with an idea: use
Microscope as a guide. For those that don't have it, Microscope has this broad brush approach - you know the beginning and end, you're just - fractally - drilling down through history. Every round someone chooses a lens, something to focus on, and everyone gets a chance to create something that relates to that lens.
In this game, this can be curated in turn or just going through each area of the planet in turn to do biology and the beginnings of society, then once it get to society, you can choose a specific lens ('the city of Urkash' or 'the Scepter of the True Kings'). As long as you keep the first rule of improv, 'yes and...', in mind, there shouldn't be too many arguments. The person who chooses the lens lasts either a set amount of time, or until everyone has contributed something to the lens.
In Microscope you start the game listing things you don't want to see and what you'd like to see (but might not come up), which you've started doing already to a certain extent.
