Carl Woese

Carl Woese (1928 to 2012) was an American microbiologist who discovered the third domain of life, the archaea. Using the sequence of ribosomal RNA as a molecular clock, he redrew the tree of life and showed that microbes are more diverse and ancient than had been realized.

Woese worked at the University of Illinois. In the 1970s he realized that the ribosomal RNA of the small subunit, which is found in all cells and changes very slowly, could be compared to measure relationships among any organisms. He and his team cut the RNA into fragments and catalogued their sequences, a laborious process, before direct sequencing became available. With the ribosome as a universal marker, bacteria could at last be classified by ancestry and not by shape or lifestyle.

In 1977, Woese and George Fox reported that methane-producing microbes were as different from ordinary bacteria as from eukaryotes. They proposed that life should be divided into three primary lines, which in 1990 were named the domains Bacteria, Archaea and Eukarya. The announcement was resisted at first, but later genome sequencing confirmed it. The archaea include many extremophiles and methanogens.

Woese argued that early evolution was dominated by horizontal gene transfer, so that the base of the tree is a network. He studied the origin of the genetic code and suggested in 1967 that early life had used RNA for both heredity and catalysis, an idea now called the RNA world hypothesis. His work frames the question of the last universal common ancestor.

• Archaea — the domain he discovered
• Tree of Life — the diagram he redrew
• rRNA — the molecule he used