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Only six kingdoms of life.

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US National Library of Medicine
National Institutes of Health Proc Biol Sci. 2004 Jun 22; 271(1545): 1251–1262. doi:  [10.1098/rspb.2004.2705] PMCID: PMC1691724 PMID: 15306349

Only six kingdoms of life.

Thomas Cavalier-Smith Author information Copyright and License information Disclaimer Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK. Thomas Cavalier-Smith: [email protected] Copyright notice This article has been cited by other articles in PMC.


There are many more phyla of microbes than of macro-organisms, but microbial biodiversity is poorly understood because most microbes are uncultured. Phylogenetic analysis of rDNA sequences cloned after PCR amplification of DNA extracted directly from environmental samples is a powerful way of exploring our degree of ignorance of major groups. As there are only five eukaryotic kingdoms, two claims using such methods for numerous novel 'kingdom-level' lineages among anaerobic eukaryotes would be remarkable, if true. By reanalysing those data with 167 known species (not merely 8-37), I identified relatives for all 8-10 'mysterious' lineages. All probably belong to one of five already recognized phyla (Amoebozoa, Cercozoa, Apusozoa, Myzozoa, Loukozoa) within the basal kingdom Protozoa, mostly in known classes, sometimes even in known orders, families or genera. This strengthens the idea that the ancestral eukaryote was a mitochondrial aerobe. Analogous claims of novel bacterial divisions or kingdoms may reflect the weak resolution and grossly non-clock-like evolution of ribosomal rRNA, not genuine phylum-level biological disparity. Critical interpretation of environmental DNA sequences suggests that our overall picture of microbial biodiversity at phylum or division level is already rather good and comprehensive and that there are no uncharacterized kingdoms of life. However, immense lower-level diversity remains to be mapped, as does the root of the tree of life.

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Selected References

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