Evolution of eukaryotes as deduced from small ribosomal subunit RNA sequences

Abstract

Evolutionary tress based on small ribosomal subunit RNA sequences yield a new perspective on eukaryote evolution. In agreement with classical views regarding evolution, animals, green plants, and fungi form monophyletic groups which seem to have originated nearly simultaneously. The evolution of these organisms took place in a relatively short time interval and is characterized by a massive diversification of life forms. In contrast, the dissimilarity among protoctist small ribosomal subunit RNA sequences is huge and exceeds the diversity seen in the entire prokaryotic world. Furthermore, some Protoctista branch off very soon in eukaryote evolution, while others diverge much later. Based on these ribosomal RNA data, Protoctista should be regarded as a collection of independent evolutionary lineages. Because the evolutionary distance between the different groups of Protoctista is, in several cases, larger than the evolutionary distance between plants, fungi and animals, the classification of eukaryotes into four kingdoms seems to be artificial and may not reflect true evolutionary relationships. Generally, eukaryotes are considered to be a relatively recently diverged lineage. Based on ribosomal RNA, however, they seem to be as old as the prokaryote lineages one distinguishes nowadays, namely eubacteria and archaebacteria.