To the Editor — In their Letter describing an expanded version of the tree of life, Hug et al.1 refer to a key publication of Carl Woese and co-workers2, wherein a 16S rRNA-phylogenetic scheme of the domains Bacteria, Archaea and Eukarya is shown. However, the first three-kingdom tree of life that included microorganisms was depicted by Ernst Haeckel (pictured at around 26 years old) in his General Morphology of Organisms, a seminal book that was published one hundred and fifty years ago3.

In his Origin of Species, Charles Darwin refers to the Linnean two-kingdom system, with ‘Animalia’ and ‘Vegetabilia’ as the only two branches of the living world. Haeckel3, who was also known as the German Darwin4, introduced a three-kingdom scheme in which, as a supplement to the Linnean Animalia and Plantae (synonymous with Vegetabilia), the ‘Protista’ were added5. Haeckel's protists included all microscopic organisms known at that time (Amoebozoa, Myxomycetes etc.), and, as one of its eight divisions, the ‘Monera’.

In several chapters of his General Morphology, Haeckel3 characterized the tiny Moneren as the “most simple organisms, without structure, homogeneous pieces of Plasma”, and explicitly mentioned the bacterial genus Vibrio as an example. Hence, Haeckel's Monera included microorganisms that are today known as Bacteria (a term not yet coined in 1866).

Six years later, the German botanist Ferdinand Cohn published a monograph on the occurrence, cultivation, morphology, behaviour, reproduction, nutrition, and systematics of a group of microorganisms he called Bacteria. Cohn used the word ‘bacteria’ with reference to the genus-name Bacterium, and provided a definition of these tiny microorganisms that included their characteristic mode of propagation via binary fission6.


Despite Cohn's introduction of the term Bacteria in 1872, Haeckel's term Monera survived in the biomedical literature until 1969, when Robert H. Whittaker introduced his five-kingdom-system of classification. In this scheme, Monera refers to prokaryotic microorganisms (bacteria, cyanobacteria), that are juxtaposed to four eukaryotic kingdoms (Protista, Fungi, Animalia, Plantae)7. This taxonomic interpretation of extant biodiversity was replaced by the three-domains-system of Woese et al.2, in which Monera are equated with the prokaryotic domains Bacteria and Archaea. The four remaining kingdoms were summarized under the domain Eukarya, which includes all eukaryotic micro- and macroorganisms (from amoebae to humans).

In chapter 26 entitled ‘Phylogenetic theses’, Haeckel3 re-interpreted and supplemented Darwin's evolutionary deductions of 1859. With reference to the simplest protists (bacteria), which form the root of his ‘oak tree’ (see Fig. 6 in ref. 5), Haeckel3 concluded that “all organisms are the descendants of such autogenous Moneren”. Hence, according to this Haeckelian interpretation of the phylogeny of life, the Monera (that is, Bacteria and Archaea)2 are the progenitors of all more complex living beings on Earth.

This 150-year-old idea is consistent with recent discoveries and genomic analyses that support an archaeal origin of eukaryotes1,8. A ‘Lokiarchaea’-like host cell that had rudimentary phagocytotic capabilities for an uptake of bacteria may have been the most ancient, metabolically active, self-replicating protoplasmic unit from which all life on Earth evolved9.