Celebrating the Three Domain Hypothesis

 

This press release from the University of Illinois at Urbana-Champaign (USA) says it all.

Thirty years ago this month, researchers at the University of Illinois published a discovery that challenged basic assumptions about the broadest classifications of life. Their discovery – which was based on an analysis of ribosomal RNA, an ancient molecule essential to the replication of all cells – opened up a new field of study, and established a first draft of the evolutionary “tree of life.”

To mark the anniversary of this discovery, the university is holding a symposium Nov. 3-4 (Saturday-Sunday), with a public lecture at the Spurlock Museum on the evening of Nov. 2. “Hidden Before Our Eyes: 30 Years of Molecular Phylogeny, Archaea and Evolution” will detail the exacting work that led to the discovery of a “third domain” of life, the microbes now known as the archaea. The event will revisit the program of research that led to the discovery, explore its impact on the study of evolution, and describe the way in which genetic analysis continues to revolutionize biology, in particular microbial ecology.

There's nothing in the press release to suggest that the third domain is still controversial. Looking at the list of speakers, it's not clear whether this point will come out in the symposium although I note that Carl Woese is on the program and he's recently been lukewarm about his own hypothesis.

The best hope for the journalists in attendance is Jan Sapp, a biologist at York University here in Toronto (Canada) who has studied the history of this "discovery" over the past three decades. As I reported last year, Sapp has documented the rise and fall of the Three Domain Hypothesis and he has taken note of the fact that former supporters of the hypothesis have recently become more skeptical [The Three Domain Hypothesis (part 2)]. Hopefully, Sapp will say things like the following from his book Microbial Phylogeny and Evolution. On the other hand it may be difficult to rain on the parade so the symposium may end up ignoring the controversy and pretending that the domain of Archaea is established fact.
Theme

The Three Domain Hypothesis

Defection grew from within the ranks of molecular evolutionists during the late 1990s. Several leading microbial phylogeneticists saw in Mayr's critique much that they considered to be true, as central features of the Archaeal story of the 1980s were challenged. First, analysis of whole genomes (more than 70 had been sequenced by 2003) had shown that Archaebacteria and Eubacteria possessed numerous genes in common; they shared a rich biochemical complexity. These data did seem to contradict the hypothesis that the Archaea were so very different from Bacteria because the two groups diverged when life was quite new. Second, comparisons of genes for other functions seemed to contradict the the phylogenetic lineages deduced from rRNA sequences....

There was a third fundamental issue. Not only did the phylogenies from the new genomic studies disagree with the traditional rRNA-based phylogenies but the new genome data also conflicted among themselves. Comparisions of individual gene phylogenies (other than those concerned with the translation machinery) often indicated different organismic genealogies. Phylogeneticists suspected that the mix-up was caused by evolutionary mechanisms whose scope and significance they may have severely underestimated: gene transfer between groups.

I have argued elsewhere that the current consensus among those who are concerned with early evolution is that the early stages were characterized by rampant gene exchanges so that it is simply not possible to say what the phylogeny of bacteria lineages was before the main lines formed. It is not possible to say with any certainty that archaebacteria are one of the earliest branching lineages and in the absence of this certainty it is certainly not possible to say that archaebacteria form a disctinct domain of life.

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