One way to guess which parts of evolution were difficult is to consider how often they occurred on Earth.
- Engulfing of organelles to create new kingdoms of eukaryotes? This seems to have happened multiple times.
- Multicellular organisms? My understanding is that multicellular plants, animals, seaweeds, and fungus were all independent developments. (I think I’m missing some kingdoms from that list.)
- The initial creation of replicating chains? There may have been AEG chains even before RNA. AEG chains are still found in some cyanobacteria.
- Ocean life becoming land life? Multiple occurrences.
This is why I think the “invention” of sex was a very key development. It seems to have happened just once. I think it may have been prerequisite to rapid evolution.