The Evolution of Cells

The origin of cells was crucial to the evolution of life: by creating the first discrete organisms it vastly strengthened organismic selection, at the expense of genie and population (group) selection which would have dominated early evolution. There are four major sorts of cell, two bacterial (kingdoms Eubacteria and Archaebacteria) and two eukaryotic (superkingdoms Archezoa and Meta- karyota): their origins are the most significant examples of quantum evolution in the history of life. Ultrastructural, molecular and fossil evidence suggest that the first cells were eubacteria, and that archaebacteria and the primitively amito- chondrial archezoa both evolved from a wall-less mutant eubacterium. Subsequently a tetraciliated archezoan metamonad phagocytosed purple non-sulphur and Gram-positive bacteria (both eubacteria), converting them into mitochondria and peroxisomes respectively, creating the first metakaryote. Metakaryota comprise five kingdoms: Protozoa, Chromista, Fungi, Animalia, Plantae. Chloroplasts evolved from symbiotic cyanobacteria, probably once only, either in the ancestral metakaryote or in the common ancestor of dinoflagellates, green and red algae. A separate symbiosis involving a proteromonad–like protozoan host and an aberrantly pigmented dinoflagellate symbiont created the kingdom Chromista, with chloroplasts located inside the rough endoplasmic reticulum. Euglenoids also may have obtained their chloroplasts secondarily from a eukaryotic symbiont, but a direct inheritance from the ancestral metakaryote cannot yet be ruled out. There is no evidence for a symbiotic origin of cilia or eukaryotes; the most primitive eukaryotes may be the monociliated archezoan Archamoebae. The first eubacteria were probably Gram-negative photoheterotrophs with an outer membrane outside their plasma membrane: this outer membrane probably evolved into the outer membrane of mitochondria and chloroplasts, but was lost in the ancestor (s) of Posibacteria and Thermotoga. The first cell perhaps evolved by “gastrulation” of an inside-out-cell bearing ribosomes and chromosomes on its outer surface. Archaebacteria and eukaryotes probably evolved from a Thermotoga-like eubacterium after it lost the murein cell wall: the archaebacterial ancestor evolved a new wall and isoprenoidal ether lipids, whilst the eukaryote ancestor instead evolved an actin/microtubulule endoskeleton allowing phagotrophy, endomembranes, nuclei, and sex (i.e., syngamy and meiosis) to evolve. The mechanisms and significance of these megaevolutionary events are discussed.