Before the development of sequence-based methods, it was impossible to know the evolutionary relationships connecting all of life and thereby to draw a universal evolutionary tree.
Whittaker, in 1969, just as the molecular methods began to develop, summarized evolutionary thought in the context of the "Five Kingdoms" of life: animals, plants, fungi, protists ("protozoa"), and monera (bacteria). There also was recognized a higher, seemingly more fundamental taxonomic distinction between eukaryotes, organisms that contain nuclear membranes, and prokaryotes, predecessors of eukaryotes that lack nuclear membranes .
These two categories of organisms were considered independent and coherent relatedness groups. The main evolutionary diversity of life on Earth, four of the five traditional taxonomic kingdoms, was thought to lie among the eukaryotes, particularly the multicellular forms.
The breakthrough that called to question many previous beliefs and brought order to microbial, indeed biological, diversity emerged with the determination of molecular sequences and the concept that sequences could be used to relate organisms .
The breakthrough that called to question many previous beliefs and brought order to microbial, indeed biological, diversity emerged with the determination of molecular sequences and the concept that sequences could be used to relate organisms .
The incisive formulation was reached by Carl Woese who, by comparison of ribosomal RNA (rRNA) sequences, established a molecular sequence-based phylogenetic tree that could be used to relate all organisms and reconstruct the history of life .
Woese articulated the now-recognized three primary lines of evolutionary descent, termed "urkingdoms" or "domains": Eucarya (eukaryotes), Bacteria (initially called eubacteria), and Archaea (initially called archaebacteria)
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