Archae and bacteria are both Prokaryotes. Why are they group into two separate domain?

  • $\begingroup$ have you tried google - there is plenty out there for you to discover about this...google "why are archaea in a different domain from bacteria" $\endgroup$ Nov 22, 2016 at 13:14
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    $\begingroup$ Homework questions and questions that show little or no prior research effort are off-topic on Biology unless you have shown your attempt at an answer. For more information see our homework policy. $\endgroup$
    – MattDMo
    Nov 22, 2016 at 18:16

1 Answer 1


Reducing your question to its fundamental misunderstanding

Similarly one could ask "Mammals and birds are vertebrates. Why are they grouped into two separate taxa?" (Bacteria and Archea include all procaryotes unlike my example). Or even better, one could ask "procaryotes and eukaryote are all alive. Why are they grouped into two separate taxa?" At the end of the day, your question boils down to why do we group lineages together?.

Why do we group lineages together?

Population tend to diverge through time. For this reason we end up with a multitude of different lineages. As humans we can enjoy grouping lineages as we like to. One might group all yellow flowers together for example. However, a grouping which is of particular interest is one which represent evolutionary relationship. Insecta for example represents a group that include all currently living insects and all of their ancestors until the most recent common ancestor of all insects (that is the individual toward which all currently existing lineages coalesce). Such groups are called monophyletic.

You should have a look at this post where I made an intro to phylogenetic.

In short, to answer your question,

[W]hy are [Bacteria and Archea] grouped into two separate domain?

Because they represent two separate lineages.

Phenotypic differences

Of course, different lineages often have distinct phenotypes which makes their grouping intuitive to a naturalist. For Bacteria and Archae, it takes some knowledge in molecular biology to tell how different they are phenotypically. This document from Colorado University does a good job in introducing the similarities and differences between Bacteria and Archae

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    $\begingroup$ I would add to this that an important element of this classification is that bacteria and archea are at least as different from each other as plants and animals; our bias as eukaryotes might be to split the eukaryotes into all sorts of classes and lump prokaryotes together as just being sufficiently different from us, but if we try to be slightly less biased it is clear there is just as much (and by most definitions more) variation within the prokaryotes. $\endgroup$
    – Bryan Krause
    Nov 22, 2016 at 16:01

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