Archaebacteria and Cellular Origins: An Overview

Archaebacteria are a group of organisms distinct from all others at the highest level. They are no more related to other bacteria, i.e., the true bacteria (eubacteria) than they are to eucaryotic cells. Archaebacteria, eubacteria and (some aspect of) the eucaryotic cell each represent separate primary lines of descent.

The archaebacteria comprise a small but diverse collection of phenotypes. While they have certain unique common phenotypic characteristics, it would have been difficult to group them convincingly on the basis of these. What was needed (and what is needed in all reliable and convincing phylogenetic measurement) is a genotypic neutral measure of genealogical relationships. Macromolecules are chronometric, in that other things being constant, changes in their sequences mark time (in a stochastic, not a metronomic way). Comparative sequence analysis is therefore a powerful measure of genealogical relationships. Using such (neutral) genotypic measures, phylogenetic ordering at the higher levels can be reliably determined — and archaebacteria can be recognized for the primary kingdom that they are.

Although it is too early to generalize with certainty, it seems that the archaebacteria differ from the other two major groups in significant details of most, if not all, molecular processes. [This is the same way in which “procaryotes” were known to differ from eucaryotes at the mloecular level.] It does not seem reasonable intuitively that this extent of difference among the three primary kingdoms can be accounted for by one of the phenotypes undergoing an extensive evolution to become the others. Rather, it seems that all lines have shared a common ancestor that possessed a more rudimentary, less detailed, phenotype, and so each has evolved separately the details in which they differ. The universal common ancestor should be called a progenote; it is an organism still in the throes of evolving the link between genotype and phenotype. The important question is why there is a universal, unique ancestor.

The existence of archaebacteria provides a new and powerful perspective on the origin of the eucaryotic cell. As a result, our concept of eucaryotic origins will undergo revision. The conventional endosymbiotic model for eucaryotic origins is no longer a sufficient explanation. The main characteristics of the eucaryotic cell — those that distinguish it at the molecular level — were evolved long before the endosymbioses that led to he mitochon- drion and the chloroplast. The nature of the eucaryotic cell is perhaps related to the nature of the ancestral progenote.

The evolutionary questions and answers offered by archaebacteria should go far to rekindle the biologist's flagging interest in evolutionary matters, and hopefully divert biology to some extent from its present course of technological adventurism.