In reconstructing early evolutionary events, however, biologists are not limited to the geologic fossil record. The cell itself retains evidence of its past in the amino acid sequences of its proteins and in the nucleotide sequences of its nucleic acids: DNA and RNA. This living record is potentially far richer and more extensive than the fossil record, and it reaches back in time beyond the oldest fossils, to the period when the common ancestor of all life existed.
In order to read the biochemical record it was necessary to develop a technology for determining (at least in part) the sequence of a gene or of the RNA or protein product encoded by a gene. For proteins this has been possible for about 25 years, but the direct sequencing of DNA and RNA has been feasible for only the past five years or so. The new technology for sequencing nucleic acids should enable biologists to uncover in relatively short order far more about the history of life than had been thought possible. It was by applying techniques of sequencing to the century-old problem of the natural relations among bacteria that my colleagues and I recognized the archaebacteria as a third form of life.