Figure 1

The tree of life and major steps in cell evolution. Archaebacteria are sisters to eukaryotes and, contrary to widespread assumptions, the youngest bacterial phylum [6, 13]. This tree topology, coupled with extensive losses of posibacterial properties by the ancestral archaebacterium, explains (without lateral gene transfer) how eukaryotes possess a unique combination of properties now seen in archaebacteria, posibacteria and α-proteobacteria. Eukaryote origins in three stages indicated by asterisks probably immediately followed divergence of archaebacteria and eukaryote precursors from the ancestral neomuran. This ancestor arose from a stem actinobacterial posibacterium by a quantum evolutionary shake-up of bacterial organization - the neomuran revolution [6, 12]: surface N-linked glycoproteins replaced murein; ribosomes evolved the signal recognition particle's translational arrest domain; histones replaced DNA gyrase, radically changing DNA replication, repair, and transcription enzymes. The eukaryote depicted is a hypothetical early stage after the origin of nucleus, mitochondrion, cilium, and microtubular skeleton but before distinct anterior and posterior cilia and centriolar and ciliary transformation (anterior cilium young, posterior old: [3]) evolved (probably in the cenancestral eukaryote [9]). Kingdom Chromista was recently expanded to include not only the original groups Heterokonta, Cryptista and Haptophyta, but also Alveolata, Rhizaria and Heliozoa [9], making the name chromalveolates now unnecessary. Excavata now exclude Euglenozoa and comprise just three phyla: the ancestrally aerobic Percolozoa and Loukozoa and the ancestrally anaerobic Metamonada (e.g. Giardia, Trichomonas), which evolved from an aerobic Malawimonas-related loukozoan. Sterols and phosphatidylinositol (PI) probably evolved in the ancestral stem actinobacterium but the ancestral hyperthermophilic archaebacterium lost them when isoprenoid ethers replaced acyl ester lipids.