Archaea and Eukarya.
On the basis of the information provided in the question, the ultimate branch of the phylogenetic tree starts with the line of Eukarya and then possesses the species of the Archaea division that comprises those prokaryotic species, which thrives in extremely tough environmental scenarios.
As the last branch possesses species of two domains, that is, Eukarya and Archaea and it starts with Eukarya and afterward holds the species of Archaea, which shows that both Eukarya and Archaea are associated genetically.
