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Three-Domain System (wikipedia.org)
65 points by hhs on March 29, 2020 | hide | past | favorite | 16 comments

In elementary school I got taught about six-kingdom system, so I am surprised there's been an update in 1990. To which domain do viruses belong? Still an odd exception?

There's a lot of different ways that species can be classified:


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.

I was taught the three domain system, although this was a couple decades after the model. Viruses don’t fit in because they’re not alive.

According to Wikipedia[1]:

> 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.

[1]: https://en.wikipedia.org/wiki/Virus

> 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:


I was taught that viruses are weird little machines, similar but unrelated to the things we recognise as alive. Interesting that there's difference of opinion on the matter.

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?

It's an overstatement to say that viruses are "unrelated to the rest of life". They have RNA or even DNA, which almost certainly makes them related.

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.

Thanks for the clarification, that makes sense.

That sounds like vitalism in disguise.

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.)

Kudos for prominent feature of the dissenting view.

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.

Ignorant to even the basics of biology, I happily traversed the wikipedia rabbit hole and learned a ton thanks to this link.


The Wikipedia article is sadly quite light on details for a system that underpins modern taxonomy…

There's a more detailed article at:


Seems pretty pointless when you take horizontal gene transfer into account.

I've always thought that as more of a mutation source rather than a more direct driver for evolution and therefore our artificial classification divides.

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.

Is this classification arbitrary, or does it try to actually reflect some sort of genetic ancestry tree?

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 ?

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