What evidence led Carl Woese to identify archaea as a separate group of life?

In 1977, American microbiologist Carl Woese identified archaea as a distinct group of life primarily through the analysis of ribosomal RNA (rRNA) sequences.12

Prior to this discovery, the scientific community generally classified all life into two primary lineages: eukaryotes and prokaryotes, the latter of which were considered a single group consisting of bacteria. By comparing rRNA sequences, Woese demonstrated that prokaryotes actually comprised two fundamentally different evolutionary lineages.12

This molecular evidence revealed that the rRNA of archaea is as distinct from bacteria as it is from eukaryotes, suggesting that the archaeal and bacterial lines diverged from a common precursor long ago. Subsequent research, including the publication of the first complete archaeal genome in 1996, further supported this classification by showing that archaea are genetically more closely related to eukaryotes than to bacteria. These genetic findings are also supported by biochemical differences, such as unique cell wall compositions and ether-linked membrane lipids that distinguish archaea from bacteria.321