I'd say that the big milestones would be photosynthesis, eukaryotic life, and then multi-cellularity. I trust the first and last are obvious but I think people don't appreciate mitochondria enough, for all the memes about "the powerhouse of the cell". Bacteria respirate over their membranes which, through the square-cube law, limits how big they can be. Our cells have many mitochondria scattered though them and can get pretty big. And with size comes the possibility of much larger genomes. Plus in life on Earth at least mitochondria are responsible for apoptosis and its hard to image multi-cellar life without that.