The six-kingdom system you're thinking of is probably one which divided the eukaryokes into subcategories of protists, plants, fungi, and animals. There are other systems which make even finer distinctions.
> Scientific opinions differ on whether viruses are a form of life, or organic structures that interact with living organisms.
> Viruses are now recognised as ancient and as having origins that pre-date the divergence of life into the three domains.
Here is an interesting article on this aspect:
Edit: here's a question for the room; if viruses are unrelated to the rest of "life", but are endogenous to the earth, are they alien?
If we found a living organism which didn't have RNA or DNA, that would be a good candidate for an alien. It would help if we found it on another planet though.
All cells are weird little machines. Viruses are just smaller than normal.
(There are some legitimate reasons to say viruses are not “alive”, but being little machines is not one of them.)
I thought it was the mainstream. Didn't we establish that eukaryotes are descended from archaea that engulfed rickettsia or cyanobacteria, presumably after inventing nuclei? It seems presumptuous to claim a domain just for that, successful as it may have been.
Mutations happen constantly: transcription error, radiation, and yes horizontal transfer, but many other factors decide if the mutation sticks or not. But gene transfer doesn't break a hierarchical classification system much more than solar radiation does.
Also: does it have any kind of predictive power or does it just scratch the itch of putting things in little separate boxes for its own sake ?