Eocyte hypothesis

The eocyte hypothesis in evolutionary biology proposes that the eukaryotes originated from a group of prokaryotes called eocytes (later classified as Thermoproteota, a group of archaea). After his team at the University of California, Los Angeles discovered eocytes in 1984, James A. Lake formulated the hypothesis as "eocyte tree" that proposed eukaryotes as part of archaea. Lake hypothesised the tree of life as having only two primary branches: Parkaryoates that include Bacteria and Archaea, and karyotes that comprise Eukaryotes and eocytes. Parts of this early hypothesis were revived in a newer two-domain system of biological classification which named the primary domains as Archaea and Bacteria. Lake's hypothesis was based on an analysis of the structural components of ribosomes. It was largely ignored, being overshadowed by the three-domain system which relied on more precise genetic analysis. In 1990, Carl Woese and his colleagues proposed that cellular life consists of three domains – Eucarya, Bacteria, and Archaea – based on the ribosomal RNA sequences. The three-domain concept was widely accepted in genetics, and became the presumptive classification system for high-level taxonomy, and was promulgated in many textbooks. Resurgence of archaea research after the 2000s, using advanced genetic techniques, and later discoveries of new groups of archaea revived the eocyte hypothesis; consequently, the two-domain system has found wider acceptance. In 1984, James A. Lake, Michael W. Clark, Eric Henderson, and Melanie Oakes of the University of California, Los Angeles described a new group of prokaryotic organisms designated as "a group of sulfur-dependent bacteria." Based on the structure and composition of their ribosomal subunits, they found that these organisms were different from other prokaryotes, bacteria and archaea, known at the time. They named them eocytes (for "dawn cells") and proposed a new biological kingdom Eocyta. According to this discovery, the tree of life is represented by four kingdoms, Archaebacteria, Eubacteria, Eukaryote and Eocyta.