I think any step which occurred independently on multiple occasions is probably not a hard step. The origin of multicellular life from unicellular life occurred

I think any step which occurred independently on multiple occasions is probably not a hard step. The origin of multicellular life from unicellular life occurred on at least three separate occasions independently. However, I think at the very least that the origin of the first cells, as well as perhaps the transition from Archaean/Prokaryotic cells to our symbiotic Eukaryotic ancestor may qualify.