Archaea are much more closely related to the eukaryotes than bacteria, see e.g. Figure 1. "Coral of life by János Podani (2019)" which shows how archaea diverged from eukarya almost 2 By after LUCA!
It therefore appears that the mitochondrial endosymbioses happened when a bacteria like cell joined up with an archaea.
Some notable points in which archaea look more like eukaryotes than bacteria:
- although they don't have a cell nucleus, they do have histones! Mentioned at: