The archaebacteria are a group of prokaryotes which seem as distinct from the true bacteria (eubacteria) as they are from eukaryotes. The evidence on which this conclusion rests is of two types: genotypic (quantitative)--that is, comparative sequence studies, and phenotypic (qualitative)--that is, differences in various organismal characteristics. The differences between archaebacteria and true bacteria are so great, both quantitatively and qualitatively, that the two bacterial groups should be considered as representing separate primary lines of descent, each tracing directly back to the universal ancestor. Furthermore, this ancestor itself seems not to be a prokaryote; rather it was a far simpler type of organism, one properly called a progenote. If this is true, the discovery of archaebacteria marks a major advance in the biologist's attempts to understand the basis for the evolution of the cell.