I'm working on a pseudorandom generation procedure to eat stellar catalogue data and spit out descriptions of planets suitable for human settlement and planetary adventure. I've come to the point where I determine what the biosphere is like, i.e. whether there are biofilms, photosynthesising scums, vast turfs of ~ mosses, great forests, whether there are little crawling/wriggling animals, large quick animals, animals with complex adaptive behaviour, social co-operation, economic co-operation, agriculture and domestication, cities, tools, machines, powered vehicles, spaceflight, ecological catastrophe….
I'm very conscious that some of the developments that are of profound importance to palaeontologists, such as the development of eukaryotic cells by endosymbiosis, might not be the way that alien life does things, might not matter to settlers, or might not matter to PC adventurers. Taking that into account, what are the developments that matter to the biosphere, settlers, and PCs, in the most abstract terms?