When 16/18S ribosomal RNA sequences became available in the mid-to-late-1980s, Woese and Olsen (1986) based their claims for an archaebacterial urkingdom on their results of sequence analysis. In contrast, using identical 16/18S sequences but different alignments and tree reconstruction programs, we found a tree which placed eocytes as the sister group of eukaryotes. Since the results of sequence analysis produce unrooted trees (click on box) we have arbitrarily rooted the tree of life in the branch leading to the eubacteria in accord with the results of others (Gogarten, Kibak, Dittrich, Taiz, Bowman, Manolson, Poole, Date, Oshima, et. al., 1989; Iwabe, Kuma, Hasegawa, Osawa & Miyata,1989).

    The most likely reason for the differences in the trees are related to artifacts of sequence alignment and tree reconstruction known as long branch attraction. Such attraction occurs when taxa are evolving at different rates. The effect of this artifact is to cause rapidly evolving taxa to join with other rapidly evolving taxa in an evolutionary tree, and slowly evolving taxa to join with other slowly evolving ones in the tree, whether or not the taxa are phylogenetically related. Some authors, ourselves included, have concluded that it is the attraction of the long branches of the eubacteria and eukaryotes that cause them to be placed together in the archael tree, and that this tree is therefore simply an artifact (see Volters & Erdmann, 1988). For an explaination and examples of long branch attraction click on the adjacent box.