Two-domain system

<p>The two-domain system is a <a href="/facts/Biological_classification/Qkg8wuyw">biological classification</a> by which all organisms in the <a href="/facts/Tree_of_life_(biology)/woHaYdLT">tree of life</a> are classified into two <a href="/facts/Domain_(biology)/RpP6V7n6">domains</a>, <a href="/facts/Bacteria/wC8xSCsU">Bacteria</a> and <a href="/facts/Archaea/Adxh0aHw">Archaea</a>. 
It emerged from development of knowledge of archaea diversity and challenges the widely accepted <a href="/facts/Three-domain_system/np75fPUv">three-domain system</a> that classifies life into Bacteria, Archaea, and <a href="/facts/Eukarya/SkwyPLMA">Eukarya</a>. It was preceded by the <a href="/facts/Eocyte_hypothesis/EXpryCJn">eocyte hypothesis</a> of <a href="/facts/James_A._Lake/41dDKvKE">James A. Lake</a> in the 1980s, which was largely superseded by the three-domain system, due to evidence at the time. Better understanding of archaea, especially of their roles in the origin of eukaryotes through <a href="/facts/Symbiogenesis/2gHlgj4j">symbiogenesis</a> with bacteria, led to the revival of the eocyte hypothesis in the 2000s. The two-domain system became more widely accepted after the discovery of a large <a href="/facts/Kingdom_(biology)/QGRKbHos">kingdom</a> of archaea called <a href="/facts/Asgard_(Archaea)/fjpYQQBw">Promethearchaeati</a> in 2017, which evidence suggests to be the evolutionary root of eukaryotes, thereby making eukaryotes members of the domain Archaea.
</p><p>While the features of promethearchaea do not completely rule out the three-domain system, the notion that eukaryotes originated within Archaea has been strengthened by genetic and proteomic studies. Under the three-domain system, Eukarya is mainly distinguished by the presence of "eukaryotic signature proteins" that are not found in Archaea and Bacteria. However, promethearchaea contain genes that code for multiple such proteins.
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Two-domain system open-in-new

Two-domain system