Archaean

... chromosomes consist of multiple sets of linear DNA molecules. I believe eukaryotic chromosomes also have histones, which are absent in (most) non-archaean prokaryotes. Of course, eukaryotic chromosomes are housed in the nucleus and interact with the nuclear lamina.

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Raven's Biology briefly mentions that the nucleus may have originated from an archaean. Some archaea have histones and similar DNA-processing enzymes, which supports this hypothesis. I haven't seen too much research on this, though.

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... or whether it could give rise to Darwinian natural selection. The ring-of-life hypothesis proposes that eukaryotes arose from a fusion of archaeans and bacteria. However, this hypothesis cannot explain where eukaryotes got their unique genes, and how fusion could have occurred in the two ...

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... the 5 kingdom system, but some do as far back as the 70's. The point is that they are genetically worlds apart. Cyanobacteria (found in the Archaean age--supposedly when life first appeared) are found to be the oldest known fossils, found in archaean age rocks from Australia dated at 3.5 ...

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