Three Domains of Life

As molecular sequencing techniques matured through the twentieth century, biologists realised that the traditional plant/animal split — and even the more refined five-kingdom system — did not capture the single deepest branch point in the tree of life: the split between two profoundly different kinds of prokaryotic organisms. In 1977, the microbiologist Carl Woese and his co-workers proposed the Three Domain system, based on comparing differences in 16S ribosomal RNA gene sequences across organisms, a molecular rather than purely structural basis for classification. …