Woese C R, Kandler O, Wheelis M L
Department of Microbiology, University of Illinois, Urbana 61801.
Proc Natl Acad Sci U S A. 1990 Jun;87(12):4576-9. doi: 10.1073/pnas.87.12.4576.
Molecular structures and sequences are generally more revealing of evolutionary relationships than are classical phenotypes (particularly so among microorganisms). Consequently, the basis for the definition of taxa has progressively shifted from the organismal to the cellular to the molecular level. Molecular comparisons show that life on this planet divides into three primary groupings, commonly known as the eubacteria, the archaebacteria, and the eukaryotes. The three are very dissimilar, the differences that separate them being of a more profound nature than the differences that separate typical kingdoms, such as animals and plants. Unfortunately, neither of the conventionally accepted views of the natural relationships among living systems--i.e., the five-kingdom taxonomy or the eukaryote-prokaryote dichotomy--reflects this primary tripartite division of the living world. To remedy this situation we propose that a formal system of organisms be established in which above the level of kingdom there exists a new taxon called a "domain." Life on this planet would then be seen as comprising three domains, the Bacteria, the Archaea, and the Eucarya, each containing two or more kingdoms. (The Eucarya, for example, contain Animalia, Plantae, Fungi, and a number of others yet to be defined). Although taxonomic structure within the Bacteria and Eucarya is not treated herein, Archaea is formally subdivided into the two kingdoms Euryarchaeota (encompassing the methanogens and their phenotypically diverse relatives) and Crenarchaeota (comprising the relatively tight clustering of extremely thermophilic archaebacteria, whose general phenotype appears to resemble most the ancestral phenotype of the Archaea.
与传统的表型相比，分子结构和序列通常能更清晰地揭示进化关系（在微生物中尤其如此）。因此，分类单元定义的基础已逐渐从生物体层面转向细胞层面，再转向分子层面。分子比较表明，地球上的生命分为三个主要类群，通常称为真细菌、古细菌和真核生物。这三类差异极大，将它们区分开的差异比区分典型界（如动物和植物）的差异更为深刻。不幸的是，关于生命系统自然关系的两种传统公认观点——即五界分类法或真核生物 - 原核生物二分法——都没有反映出生物世界的这种主要三分法。为了纠正这种情况，我们建议建立一个正式的生物分类系统，在界之上存在一个新的分类单元，称为“域”。地球上的生命将被视为由三个域组成，即细菌域、古菌域和真核生物域，每个域包含两个或更多的界。（例如，真核生物域包含动物界、植物界、真菌界以及一些尚未定义的界）。虽然本文未讨论细菌域和真核生物域内的分类结构，但古菌域正式细分为两个界，即广古菌界（包括产甲烷菌及其表型多样的亲缘生物）和泉古菌界（由相对紧密聚集的极端嗜热古细菌组成，其一般表型似乎最类似于古菌的祖先表型）。
